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Electronic Circuits I
Instructor:
Dr. Alaa Mahmoud
alaa_y_emam@hotmail.com
Chapter 6
CAPACITORS AND
INDUCTORS
Outline:
— Introduction
— Capacitors
— Series and Parallel Capacitors
— Inductors
— Series and Parallel Inductors
Introduction
— So far we studied resistive circuits
Now, we introduce two new important passive linear
elements: capacitor & inductor
— Resistors dissipate energy, while capacitors & inductors
store energy
à they are called storage elements
— The application of resistive circuits is limited
However, capacitors & inductors provide important &
practical circuits
— The circuit analysis techniques covered in Chapters 3 & 4
are equally applicable to circuits with capacitors &
inductors
Capacitor
Inductor
Capacitor
— Capacitor: is a passive element that stores energy in its electric field
— Capacitors are used in electronics, communications, computers, &
power systems. e.g., tuning circuits & memory elements
— The plates may be aluminum foil
The dielectric may be air, ceramic, paper, or mica
— When voltage source v is connected to capacitor
à it deposits +q on one plate & −q on the other
— The capacitor is storing the electric charge q
— q is proportional to the applied voltage v as:
C: capacitance of the capacitor
— The unit of capacitance is the farad (F)
1 farad = 1coulomb/volt
6.2 CAPACITORS
A capacitor is a passive element designed to store energy in its electric
field. Besides resistors, capacitors are the most common electrical com-
ponents. Capacitors are used extensively in electronics, communications,
computers, and power systems. For example, they are used in the tuning
circuits of radio receivers and as dynamic memory elements in computer
systems.
A capacitor is typically constructed as depicted in Fig. 6.1.
or.
A capacitor consists of two conducting plates separated
by an insulator (or dielectric).
In many practical applications, the plates may be aluminum foil while the
dielectric may be air, ceramic, paper, or mica.
When a voltage source v is connected to the capacitor, as in Fig.
6.2, the source deposits a positive charge q on one plate and a negative
charge −q on the other. The capacitor is said to store the electric charge.
The amount of charge stored, represented by q, is directly proportional
to the applied voltage v so that
q = Cv (6.1)
where C, the constant of proportionality, is known as the capacitance
of the capacitor. The unit of capacitance is the farad (F), in honor of
the English physicist Michael Faraday (1791–1867). From Eq. (6.1), we
may derive the following definition.
harge
e in a
q
capac
retrie
called
In contrast to a resistor, which spends or dis-
sipates energy irreversibly, an inductor or ca-
pacitor stores or releases energy (i.e., has a
memory).
trodu
analy
analy
circu
them
typic
amps
6.2
A ca
field.
pone
comp
circu
syste
Metal plates,
each with area A
d
Dielectric with permittivity e
Figure6.1 A typical capacitor.
In ma
diele
6.2, t
charg
The a
to the
wher
of th
the E
!
!
!
!q
+q
+
+
+
+
+
+
!
+
v
Figure6.2 A capacitor
with applied voltage v.
A capacitor is a passive element designed to store energy in its electric
field. Besides resistors, capacitors are the most common electrical com-
ponents. Capacitors are used extensively in electronics, communications,
computers, and power systems. For example, they are used in the tuning
circuits of radio receivers and as dynamic memory elements in computer
systems.
A capacitor is typically constructed as depicted in Fig. 6.1.
A capacitor consists of two conducting plates separated
by an insulator (or dielectric).
In many practical applications, the plates may be aluminum foil while the
dielectric may be air, ceramic, paper, or mica.
When a voltage source v is connected to the capacitor, as in Fig.
6.2, the source deposits a positive charge q on one plate and a negative
charge −q on the other. The capacitor is said to store the electric charge.
The amount of charge stored, represented by q, is directly proportional
to the applied voltage v so that
q = Cv (6.1)
where C, the constant of proportionality, is known as the capacitance
of the capacitor. The unit of capacitance is the farad (F), in honor of
the English physicist Michael Faraday (1791–1867). From Eq. (6.1), we
may derive the following definition.
— C does not depend on q or v
C depends on physical dimensions of capacitor
— Three factors affect capacitance:
1. Surface area of plates (A): ⬆ A à ⬆ C
2. Spacing between plates (d): ⬇ d à ⬆ C
3. Permittivity (dielectric) of material (ε): ⬆ ε à ⬆ C
— Capacitors are commercially available in different types & values
from pF (10–12 ) to μF (10–6)
— Capacitors are described by the dielectric material
Capacitors are described by their kinds (fixed or variable)
— When capacitor is charging
à current flows into +ve terminal à +i
— When capacitor is discharging
à current flows out of +ve terminal à -i
of the capacitor. The unit of capacitance is the farad (F), in honor of
the English physicist Michael Faraday (1791–1867). From Eq. (6.1), we
may derive the following definition.
theamountofcharge
oltage difference in a
Capacitance is the ratio of the charge on one plate of a capacitor to the voltage
difference between the two plates, measured in farads (F).
Note from Eq. (6.1) that 1 farad = 1 coulomb/volt.
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Text Main Menu TextbookTable of Contents Problem Solving Workbook Contents
CHAPTER 6 Capacitors and Inductors
Although the capacitance C of a capacitor is the ratio of the charge
q per plate to the applied voltage v, it does not depend on q or v. It
depends on the physical dimensions of the capacitor. For example, for
the parallel-plate capacitor shown in Fig. 6.1, the capacitance is given by
C =
ϵA
d
(6.2)
where A is the surface area of each plate, d is the distance between the
plates, and ϵ is the permittivity of the dielectric material between the
plates. Although Eq. (6.2) applies to only parallel-plate capacitors, we
may infer from it that, in general, three factors determine the value of the
capacitance:
Cap
icall
Eqs
and
1. The surface area of the plates—the larger the area, the greater
the capacitance.
2. The spacing between the plates—the smaller the spacing, the
greater the capacitance.
3. The permittivity of the material—the higher the permittivity,
the greater the capacitance.
Capacitors are commercially available in different values and types.
Typically, capacitors have values in the picofarad (pF) to microfarad (µF)
range. They are described by the dielectric material they are made of and
by whether they are of fixed or variable type. Figure 6.3 shows the circuit
symbols for fixed and variable capacitors. Note that according to the
i C
C =
ϵA
d
(6.2)
e A is the surface area of each plate, d is the distance between the
, and ϵ is the permittivity of the dielectric material between the
. Although Eq. (6.2) applies to only parallel-plate capacitors, we
nfer from it that, in general, three factors determine the value of the
itance:
Capacitorvoltageratingandcapacitancearetyp-
ically inversely rated due to the relationships in
Eqs. (6.1) and (6.2). Arcing occurs if d is smal
and V is high.
1. The surface area of the plates—the larger the area, the greater
the capacitance.
2. The spacing between the plates—the smaller the spacing, the
greater the capacitance.
3. The permittivity of the material—the higher the permittivity,
the greater the capacitance.
Capacitors are commercially available in different values and types.
ally, capacitors have values in the picofarad (pF) to microfarad (µF)
. They are described by the dielectric material they are made of and
ether they are of fixed or variable type. Figure 6.3 shows the circuit
ols for fixed and variable capacitors. Note that according to the
ve sign convention, current is considered to flow into the positive
nal of the capacitor when the capacitor is being charged, and out of
ositive terminal when the capacitor is discharging.
i i
C
v
+ !
C
v
+ !
Figure6.3 Circuit symbols for capacitors:
(a) fixed capacitor, (b) variable capacitor.
Figure6.4showscommontypesoffixed-valuecapacitors. Polyester
itors are light in weight, stable, and their change with temperature is
ctable. Instead of polyester, other dielectric materials such as mica
voltage v, it does not depend on q or v. It
mensions of the capacitor. For example, for
shown in Fig. 6.1, the capacitance is given by
C =
ϵA
d
(6.2)
of each plate, d is the distance between the
tivity of the dielectric material between the
applies to only parallel-plate capacitors, we
neral, three factors determine the value of the
Capacitorvoltageratingandcapacitancearetyp-
ically inversely rated due to the relationships in
Eqs. (6.1) and (6.2). Arcing occurs if d is small
and V is high.
f the plates—the larger the area, the greater
een the plates—the smaller the spacing, the
ance.
f the material—the higher the permittivity,
acitance.
ercially available in different values and types.
alues in the picofarad (pF) to microfarad (µF)
y the dielectric material they are made of and
or variable type. Figure 6.3 shows the circuit
able capacitors. Note that according to the
urrent is considered to flow into the positive
hen the capacitor is being charged, and out of
i i
C
v
+ !
C
v
+ !
Fixed capacitor
Variable capacitor
to the applied voltage v so that
q = Cv (6.1)
where C, the constant of proportionality, is known as the capacitance
of the capacitor. The unit of capacitance is the farad (F), in honor of
the English physicist Michael Faraday (1791–1867). From Eq. (6.1), we
may derive the following definition.
charge
ce in a
Capacitance is the ratio of the charge on one plate of a capacitor to the voltage
difference between the two plates, measured in farads (F).
Note from Eq. (6.1) that 1 farad = 1 coulomb/volt.
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n Menu TextbookTable of Contents Problem Solving Workbook Contents
Common types of fixed-value capacitors:
1. Polyester capacitors: light in weight, stable,
their change with temperature is predictable
— Mica & polystyrene are used as dielectric materials
2. Film capacitors: rolled and housed in metal or plastic films
3. Electrolytic capacitors: produce very high capacitance
Common types of variable -value capacitors:
1. A trimmer (or padder) capacitor or a glass piston capacitor:
is varied by turning the screw
2. The capacitance of the variable air capacitor (meshed plates):
is varied by turning the shaft
— Variable capacitors are used in radio receivers
— Capacitors are used to block dc, pass ac, shift phase, store energy,
start motors, and suppress noise
(a) (b)
Figure 6.4 Fixed capacitors: (a) polyester capacitor, (b) ceramic capacitor, (c)
(Courtesy of Tech America.)
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e-Text Main Menu TextbookTable of Contents Problem S
Ceramic capacitor
plates) is varied by turning the shaft. Variable capacitors are used in radio
(a) (b) (c)
Figure 6.4 Fixed capacitors: (a) polyester capacitor, (b) ceramic capacitor, (c) electrolytic capacitor.
(Courtesy of Tech America.)
| | | |
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e-Text Main Menu Textbook Table of Contents Problem Solving Workbook Conte
Electrolytic capacitor
(a) (b) (c
Figure 6.4 Fixed capacitors: (a) polyester capacitor, (b) ceramic capacitor, (c) el
(Courtesy of Tech America.)
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e-Text Main Menu TextbookTable of Contents Problem Sol
Polyester capacitor
204 PART 1 DC C
receivers allowi
are used to bloc
suppress noise.
(a)
(b)
Figure6.5 Variable capacitors:
(a) trimmer capacitor, (b) filmtrim
capacitor.
(Courtesy of Johanson.)
To obtain
the derivative o
differentiating b
According to Eq. (6.4), for a capacitor to carry
current, its voltage must vary with time. Hence,
for constant voltage, i = 0 .
This is the curre
itive sign conve
capacitor whose
satisfy Eq. (6.4
plot of the curr
some capacitor
capacitors in th
The volta
integrating both
or
Trimmer capacitor
Capacitors are commercially available in different values and types.
Typically, capacitors have values in the picofarad (pF) to microfarad (µF)
range. They are described by the dielectric material they are made of and
by whether they are of fixed or variable type. Figure 6.3 shows the circuit
symbols for fixed and variable capacitors. Note that according to the
passive sign convention, current is considered to flow into the positive
terminal of the capacitor when the capacitor is being charged, and out of
the positive terminal when the capacitor is discharging.
i i
C
v
+ !
C
v
+ !
Figure6.3 Circuit symbols for capacitors:
(a) fixed capacitor, (b) variable capacitor.
Figure6.4showscommontypesoffixed-valuecapacitors. Polyester
capacitors are light in weight, stable, and their change with temperature is
predictable. Instead of polyester, other dielectric materials such as mica
and polystyrene may be used. Film capacitors are rolled and housed in
metal or plastic films. Electrolytic capacitors produce very high capaci-
tance. Figure 6.5 shows the most common types of variable capacitors.
The capacitance of a trimmer (or padder) capacitor or a glass piston capac-
itor is varied by turning the screw. The trimmer capacitor is often placed
in parallel with another capacitor so that the equivalent capacitance can
be varied slightly. The capacitance of the variable air capacitor (meshed
plates) is varied by turning the shaft. Variable capacitors are used in radio
(a) (b) (c)
Figure 6.4 Fixed capacitors: (a) polyester capacitor, (b) ceramic capacitor, (c) electrolytic capacitor.
(Courtesy of Tech America.)
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e-Text Main Menu Textbook Table of Contents Problem Solving Workbook Contents
C =
ϵA
d
(6.2)
urface area of each plate, d is the distance between the
the permittivity of the dielectric material between the
h Eq. (6.2) applies to only parallel-plate capacitors, we
that, in general, three factors determine the value of the
Capacitorvoltageratingandcapacitancearetyp-
ically inversely rated due to the relationships in
Eqs. (6.1) and (6.2). Arcing occurs if d is small
and V is high.
face area of the plates—the larger the area, the greater
acitance.
cing between the plates—the smaller the spacing, the
the capacitance.
mittivity of the material—the higher the permittivity,
ter the capacitance.
are commercially available in different values and types.
tors have values in the picofarad (pF) to microfarad (µF)
described by the dielectric material they are made of and
are of fixed or variable type. Figure 6.3 shows the circuit
d and variable capacitors. Note that according to the
vention, current is considered to flow into the positive
apacitor when the capacitor is being charged, and out of
inal when the capacitor is discharging.
i i
C
v
+ !
C
v
+ !
Figure6.3 Circuit symbols for capacitors:
(a) fixed capacitor, (b) variable capacitor.
showscommontypesoffixed-valuecapacitors. Polyester
ht in weight, stable, and their change with temperature is
ead of polyester, other dielectric materials such as mica
may be used. Film capacitors are rolled and housed in
films. Electrolytic capacitors produce very high capaci-
5 shows the most common types of variable capacitors.
of a trimmer (or padder) capacitor or a glass piston capac-
urning the screw. The trimmer capacitor is often placed
nother capacitor so that the equivalent capacitance can
y. The capacitance of the variable air capacitor (meshed
by turning the shaft. Variable capacitors are used in radio Filmtrim capacitor
204 PART 1 D
receivers all
are used to b
suppress no
(a)
(b)
Figure6.5 Variable capacitors:
(a) trimmer capacitor, (b) filmtrim
capacitor.
To obt
the derivativ
differentiati
This is the c
itive sign co
capacitor w
satisfy Eq.
plot of the c
some capac
capacitors in
The i-v relationship of the capacitor:
à
1. Linear capacitor: capacitance is independent of voltage
— This figure shows linear relationship for a capacitor
2. Nonlinear capacitor: the i-v plot is not straight line
—
where v(t0) = q(t0)/C is the voltage across the capacitor at time t0
à for capacitor, v depends on the past history of i
à capacitor has memory
charge −q on the other. The capacitor is said to store the electric charge.
The amount of charge stored, represented by q, is directly proportional
to the applied voltage v so that
q = Cv (6.1)
where C, the constant of proportionality, is known as the capacitance
of the capacitor. The unit of capacitance is the farad (F), in honor of
the English physicist Michael Faraday (1791–1867). From Eq. (6.1), we
may derive the following definition.
Capacitance is the ratio of the charge on one plate of a capacitor to the voltage
difference between the two plates, measured in farads (F).
Note from Eq. (6.1) that 1 farad = 1 coulomb/volt.
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TextbookTable of Contents Problem Solving Workbook Contents
rs allowing one to tune to various stations. In addition, capacitors
d to block dc, pass ac, shift phase, store energy, start motors, and
ss noise.
To obtain the current-voltage relationship of the capacitor, we take
ivative of both sides of Eq. (6.1). Since
i =
dq
dt
(6.3)
ntiating both sides of Eq. (6.1) gives
i = C
dv
dt
(6.4)
the current-voltage relationship for a capacitor, assuming the pos-
gn convention. The relationship is illustrated in Fig. 6.6 for a
or whose capacitance is independent of voltage. Capacitors that
Eq. (6.4) are said to be linear. For a nonlinear capacitor, the
the current-voltage relationship is not a straight line. Although
apacitors are nonlinear, most are linear. We will assume linear
ors in this book.
The voltage-current relation of the capacitor can be obtained by
ting both sides of Eq. (6.4). We get
v =
1
C
! t
−∞
i dt (6.5)
!
(Courtesy of Johanson.)
According to Eq. (6.4), for a capacitor to carry
current, its voltage must vary with time. Hence,
for constant voltage, i = 0 .
Slope = C
dv⁄dt
0
i
Figure6.6 Current-voltage
relationship of a capacitor.
integr
or
where
Equat
of the
that is
The e
w =
We no
−∞.
Using
(b)
Figure6.5 Variable capacitors:
(a) trimmer capacitor, (b) filmtrim
capacitor.
(Courtesy of Johanson.)
According to Eq. (6.4), for a capacitor to carry
current, its voltage must vary with time. Hence,
for constant voltage, i = 0 .
Slope = C
dv⁄dt
0
i
Figure6.6 Current-voltage
relationship of a capacitor.
204 PART 1 DC Circuits
receivers allowing one to tune to various stations. In addition, capacitors
are used to block dc, pass ac, shift phase, store energy, start motors, and
suppress noise.
(a)
(b)
Figure6.5 Variable capacitors:
(a) trimmer capacitor, (b) filmtrim
capacitor.
(Courtesy of Johanson.)
To obtain the current-voltage relationship of the capacitor, we take
the derivative of both sides of Eq. (6.1). Since
i =
dq
dt
(6.3)
differentiating both sides of Eq. (6.1) gives
i = C
dv
dt
(6.4)
According to Eq. (6.4), for a capacitor to carry
current, its voltage must vary with time. Hence,
for constant voltage, i = 0 .
This is the current-voltage relationship for a capacitor, assuming the pos-
itive sign convention. The relationship is illustrated in Fig. 6.6 for a
capacitor whose capacitance is independent of voltage. Capacitors that
satisfy Eq. (6.4) are said to be linear. For a nonlinear capacitor, the
plot of the current-voltage relationship is not a straight line. Although
some capacitors are nonlinear, most are linear. We will assume linear
capacitors in this book.
Slope = C
dv⁄dt
0
i
Figure6.6 Current-voltage
relationship of a capacitor.
The voltage-current relation of the capacitor can be obtained by
integrating both sides of Eq. (6.4). We get
v =
1
C
! t
−∞
i dt (6.5)
or
v =
1
C
! t
t0
i dt + v(t0) (6.6)
where v(t0) = q(t0)/C is the voltage across the capacitor at time t0.
Equation (6.6) shows that capacitor voltage depends on the past history
of the capacitor current. Hence, the capacitor has memory—a property
that is often exploited.
The instantaneous power delivered to the capacitor is
p = vi = Cv
dv
dt
(6.7)
The energy stored in the capacitor is therefore
w =
! t
−∞
p dt = C
! t
−∞
v
dv
dt
dt = C
! t
−∞
v dv =
1
2
Cv2
"
"
"
"
t
t=−∞
(6.8)
We note that v(−∞) = 0, because the capacitor was uncharged at t =
−∞. Thus,
i =
dq
dt
(6.3)
ng both sides of Eq. (6.1) gives
i = C
dv
dt
(6.4)
urrent-voltage relationship for a capacitor, assuming the pos-
onvention. The relationship is illustrated in Fig. 6.6 for a
hose capacitance is independent of voltage. Capacitors that
(6.4) are said to be linear. For a nonlinear capacitor, the
current-voltage relationship is not a straight line. Although
itors are nonlinear, most are linear. We will assume linear
n this book.
oltage-current relation of the capacitor can be obtained by
both sides of Eq. (6.4). We get
v =
1
C
! t
−∞
i dt (6.5)
v =
1
C
! t
t0
i dt + v(t0) (6.6)
= q(t0)/C is the voltage across the capacitor at time t0.
.6) shows that capacitor voltage depends on the past history
à
à
The instantaneous power delivered to the capacitor is:
since
The energy stored in the capacitor is:
v(−∞) = 0, because at t = −∞ capacitor is uncharged
à
Since q = Cv , à
— last 2 equations represent the energy stored in E between capacitor plates
— This energy can be re-trieved, since an ideal capacitor cannot dissipate energy
Equation (6.6) shows that capacitor voltage depends on the past history
of the capacitor current. Hence, the capacitor has memory—a property
that is often exploited.
The instantaneous power delivered to the capacitor is
p = vi = Cv
dv
dt
(6.7)
The energy stored in the capacitor is therefore
w =
! t
−∞
p dt = C
! t
−∞
v
dv
dt
dt = C
! t
−∞
v dv =
1
2
Cv2
"
"
"
"
t
t=−∞
(6.8)
We note that v(−∞) = 0, because the capacitor was uncharged at t =
−∞. Thus,
w =
1
2
Cv2
(6.9)
Using Eq. (6.1), we may rewrite Eq. (6.9) as
w =
q2
2C
(6.10)
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u TextbookTable of Contents Problem Solving Workbook Contents
Slope = C
dv⁄dt
ent-voltage
capacitor.
where v(t0) = q(t0)/C is the voltage across the capacitor at time t0.
Equation (6.6) shows that capacitor voltage depends on the past history
of the capacitor current. Hence, the capacitor has memory—a property
that is often exploited.
The instantaneous power delivered to the capacitor is
p = vi = Cv
dv
dt
(6.7)
The energy stored in the capacitor is therefore
w =
! t
−∞
p dt = C
! t
−∞
v
dv
dt
dt = C
! t
−∞
v dv =
1
2
Cv2
"
"
"
"
t
t=−∞
(6.8)
We note that v(−∞) = 0, because the capacitor was uncharged at t =
−∞. Thus,
w =
1
2
Cv2
(6.9)
Using Eq. (6.1), we may rewrite Eq. (6.9) as
w =
q2
2C
(6.10)
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Text Main Menu Textbook Table of Contents Problem Solving Workbook Contents
of the capacitor current. Hence, the capacitor has memory—a property
that is often exploited.
The instantaneous power delivered to the capacitor is
p = vi = Cv
dv
dt
(6.7)
The energy stored in the capacitor is therefore
w =
! t
−∞
p dt = C
! t
−∞
v
dv
dt
dt = C
! t
−∞
v dv =
1
2
Cv2
"
"
"
"
t
t=−∞
(6.8)
We note that v(−∞) = 0, because the capacitor was uncharged at t =
−∞. Thus,
w =
1
2
Cv2
(6.9)
Using Eq. (6.1), we may rewrite Eq. (6.9) as
w =
q2
2C
(6.10)
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u Textbook Table of Contents Problem Solving Workbook Contents
Important properties of a capacitor:
1. From i = Cdv/dt , when v is not changing with time (dc voltage)à i = zero
à
— If a battery (dc voltage) is connected across a capacitor à capacitor charging
2. The voltage on the capacitor must be continuous
— The capacitor resists an abrupt change
in the voltage across it
— From i = Cdv/dt, a discontinuous change
in voltage requires an infinite current
— Conversely, the current through a capacitor can
change instantaneously
capacitor is derived from this element’s capacity to store energy in an
electric field.
We should note the following important properties of a capacitor:
1. Note from Eq. (6.4) that when the voltage across a capacitor is
not changing with time (i.e., dc voltage), the current through
the capacitor is zero. Thus,
A capacitor is an open circuit to dc.
However, if a battery (dc voltage) is connected across a
capacitor, the capacitor charges.
2. The voltage on the capacitor must be continuous.
The voltage on a capacitor cannot change abruptly.
The capacitor resists an abrupt change in the voltage across it.
According to Eq. (6.4), a discontinuous change in voltage
requires an infinite current, which is physically impossible.
For example, the voltage across a capacitor may take the form
shown in Fig. 6.7(a), whereas it is not physically possible for
the capacitor voltage to take the form shown in Fig. 6.7(b)
because of the abrupt change. Conversely, the current through
a capacitor can change instantaneously.
3. The ideal capacitor does not dissipate energy. It takes power
from the circuit when storing energy in its field and returns
previously stored energy when delivering power to the circuit.
4. A real, nonideal capacitor has a parallel-model leakage
resistance, as shown in Fig. 6.8. The leakage resistance may be
An alternative way of looking at this is using E
(6.9), whichindicatesthatenergyisproportion
to voltage squared. Since injecting or extracti
energy can only be done over some finite tim
voltage cannot change instantaneously across
capacitor.
v
t
(a)
v
t
(b)
Figure6.7 Voltage across a capacitor:
(a) allowed, (b) not allowable; an abrupt
change is not possible.
Leakage resistance
Capacitance
electric field.
We should note the following important properties of a capacitor:
1. Note from Eq. (6.4) that when the voltage across a capacitor is
not changing with time (i.e., dc voltage), the current through
the capacitor is zero. Thus,
A capacitor is an open circuit to dc.
However, if a battery (dc voltage) is connected across a
capacitor, the capacitor charges.
2. The voltage on the capacitor must be continuous.
The voltage on a capacitor cannot change abruptly.
The capacitor resists an abrupt change in the voltage across it.
According to Eq. (6.4), a discontinuous change in voltage
requires an infinite current, which is physically impossible.
For example, the voltage across a capacitor may take the form
shown in Fig. 6.7(a), whereas it is not physically possible for
the capacitor voltage to take the form shown in Fig. 6.7(b)
because of the abrupt change. Conversely, the current through
a capacitor can change instantaneously.
3. The ideal capacitor does not dissipate energy. It takes power
from the circuit when storing energy in its field and returns
previously stored energy when delivering power to the circuit.
4. A real, nonideal capacitor has a parallel-model leakage
resistance, as shown in Fig. 6.8. The leakage resistance may be
An alternative way of looking at this is using E
(6.9),whichindicatesthatenergyisproportio
to voltage squared. Since injecting or extract
energy can only be done over some finite tim
voltage cannot change instantaneously across
capacitor.
v
t
(a)
v
t
(b)
Figure6.7 Voltage across a capacitor:
(a) allowed, (b) not allowable; an abrupt
change is not possible.
Leakage resistance
Capacitance
CHAPTER 6 Capacitors and Inductors 205
.9) or (6.10) represents the energy stored in the electric field
between the plates of the capacitor. This energy can be re-
e an ideal capacitor cannot dissipate energy. In fact, the word
derived from this element’s capacity to store energy in an
d.
ould note the following important properties of a capacitor:
te from Eq. (6.4) that when the voltage across a capacitor is
t changing with time (i.e., dc voltage), the current through
capacitor is zero. Thus,
A capacitor is an open circuit to dc.
wever, if a battery (dc voltage) is connected across a
pacitor, the capacitor charges.
e voltage on the capacitor must be continuous.
The voltage on a capacitor cannot change abruptly.
v
t
(a)
v
t
(b)
Figure6.7 Voltage across a capacitor:
(a) allowed, (b) not allowable; an abrupt
change is not possible.
Important properties of a capacitor:
3. The ideal capacitor does not dissipate energy
— It takes power from the circuit
when storing energy in its field (charging)
— It returns stored energy
when delivering power to the circuit (discharging)
4. A nonideal (real) capacitor has a parallel-model
leakage resistance
— The leakage resistance may be as high as 100 MΩ
— It can be neglected for most practical applications
A capacitor is an open circuit to dc.
However, if a battery (dc voltage) is connected across a
capacitor, the capacitor charges.
2. The voltage on the capacitor must be continuous.
The voltage on a capacitor cannot change abruptly.
The capacitor resists an abrupt change in the voltage across it.
According to Eq. (6.4), a discontinuous change in voltage
requires an infinite current, which is physically impossible.
For example, the voltage across a capacitor may take the form
shown in Fig. 6.7(a), whereas it is not physically possible for
the capacitor voltage to take the form shown in Fig. 6.7(b)
because of the abrupt change. Conversely, the current through
a capacitor can change instantaneously.
3. The ideal capacitor does not dissipate energy. It takes power
from the circuit when storing energy in its field and returns
previously stored energy when delivering power to the circuit.
4. A real, nonideal capacitor has a parallel-model leakage
resistance, as shown in Fig. 6.8. The leakage resistance may be
as high as 100 M! and can be neglected for most practical
applications. For this reason, we will assume ideal capacitors
in this book.
An alternative way of looking at this is using Eq.
(6.9), whichindicatesthatenergyisproportional
to voltage squared. Since injecting or extracting
energy can only be done over some finite time,
voltage cannot change instantaneously across a
capacitor.
v
t
(a)
v
t
(b)
Figure6.7 Voltage across a capacitor:
(a) allowed, (b) not allowable; an abrupt
change is not possible.
Leakage resistance
Capacitance
Figure 6.8 Circuit model of a
nonideal capacitor.
E X A M P L E 6 . 1
(a) Calculate the charge stored on a 3-pF capacitor with 20 V across it.
takes power delivers power
as high as 100 M! and can be neglected for most practical
applications. For this reason, we will assume ideal capacitors
in this book.
Figure 6.8 Circuit model of a
nonideal capacitor.
E X A M P L E 6 . 1
(a) Calculate the charge stored on a 3-pF capacitor with 20 V across it.
(b) Find the energy stored in the capacitor.
Solution:
(a) Since q = Cv,
q = 3 × 10−12
× 20 = 60 pC
(b) The energy stored is
w =
1
2
Cv2
=
1
2
× 3 × 10−12
× 400 = 600 pJ
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206 PART 1 DC Circuits
P R A C T I C E P R O B L E M 6 . 1
What is the voltage across a 3-µF capacitor if the charge on one plate is
0.12 mC? How much energy is stored?
Answer: 40 V, 2.4 mJ.
E X A M P L E 6 . 2
The voltage across a 5-µF capacitor is
v(t) = 10 cos 6000t V
Calculate the current through it.
Solution:
By definition, the current is
i(t) = C
dv
dt
= 5 × 10−6 d
dt
(10 cos 6000t)
= −5 × 10−6
× 6000 × 10 sin 6000t = −0.3 sin 6000t A
P R A C T I C E P R O B L E M 6 . 2
If a 10-µF capacitor is connected to a voltage source with
v(t) = 50 sin 2000t V
determine the current through the capacitor.
Answer: cos 2000t A.
E X A M P L E 6 . 3
Determine the voltage across a 2-µF capacitor if the current through it is
i(t) = 6e−3000t
mA
Assume that the initial capacitor voltage is zero.
Solution:
Since v =
1
C
! t
0
i dt + v(0) and v(0) = 0,
v =
1
2 × 10−6
! t
0
6e−3000t
dt·10−3
=
3 × 103
−3000
e−3000t
"
"
"
"
t
0
= (1 − e−3000t
) V
P R A C T I C E P R O B L E M 6 . 3
The current through a 100-µF capacitor is i(t) = 50 sin 120πt mA. Cal-
culate the voltage across it at t = 1 ms and t = 5 ms. Take v(0) = 0.
Answer: −93.137 V, −1.736 V.
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CHAPTER 6 Capacitors and Inductors 207
E X A M P L E 6 . 4
Determine the current through a 200-µFcapacitor whose voltage is shown
in Fig. 6.9.
v(t)
0
4
3
2
1
50
!50
t
Figure6.9 For Example 6.4.
Solution:
The voltage waveform can be described mathematically as
v(t) =
⎧
⎪
⎪
⎨
⎪
⎪
⎩
50t V 0 < t < 1
100 − 50t V 1 < t < 3
−200 + 50t V 3 < t < 4
0 otherwise
Since i = C dv/dt and C = 200 µF, we take the derivative of v to obtain
i(t) = 200 × 10−6
×
⎧
⎪
⎪
⎨
⎪
⎪
⎩
50 0 < t < 1
−50 1 < t < 3
50 3 < t < 4
0 otherwise
=
⎧
⎪
⎪
⎨
⎪
⎪
⎩
10 mA 0 < t < 1
−10 mA 1 < t < 3
10 mA 3 < t < 4
0 otherwise
Thus the current waveform is as shown in Fig. 6.10.
i (mA)
0
4
3
2
1
10
!10
t
Figure6.10 For Example 6.4.
P R A C T I C E P R O B L E M 6 . 4
An initially uncharged 1-mF capacitor has the current shown in Fig. 6.11 i (mA)
Figure6.11 For Practice Prob. 6.4.
E X A M P L E 6 . 5
Obtain the energy stored in each capacitor in Fig. 6.12(a) under dc con-
ditions.
Solution:
Under dc conditions, we replace each capacitor with an open circuit, as
shown in Fig. 6.12(b). The current through the series combination of the
2-k! and 4-k! resistors is obtained by current division as
i =
3
3 + 2 + 4
(6 mA) = 2 mA
Hence, the voltages v1 and v2 across the capacitors are
v1 = 2000i = 4 V v2 = 4000i = 8 V
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208 PART 1 DC Circuits
v1
+ !
v2
+
!
6 mA 3 k"
5 k"
4 k"
2 k"
2 mF
4 mF
(a)
6 mA 3 k"
5 k"
4 k"
2 k"
(b)
i
Figure6.12 For Example 6.5.
PART 1 DC Circuits
v1
+ !
v2
+
!
3 k"
5 k"
4 k"
2 k"
2 mF
4 mF
(a)
6 mA 3 k"
5 k"
4 k"
2 k"
(b)
i
Example 6.5.
and the energies stored in them are
w1 =
1
2
C1v2
1 =
1
2
(2 × 10−3
)(4)2
= 16 mJ
w2 =
1
2
C2v2
2 =
1
2
(4 × 10−3
)(8)2
= 128 mJ
M 6 . 5
Under dc conditions, find the energy stored in the capacitors in Fig. 6.13.
Answer: 405 µJ, 90 µJ.
208 PART 1 DC Circuits
6 mA 3 k"
5 k"
4 k"
2 k"
2 mF
4 mF
(a)
6 mA
Figure6.12 For Example 6.5.
and the energies stored
w1 =
1
2
C
w2 =
1
2
C
P R A C T I C E P R O B L E M 6 . 5
Under dc conditions, fi
3 k"
à Same i through 2 & 4kΩ
0
6
4
2 t (ms)
Figure6.11 For Practice Prob. 6.4.
E X A M P L E 6 . 5
Obtain the energy stored in each capacitor in Fig. 6.12(a) under dc con-
ditions.
Solution:
Under dc conditions, we replace each capacitor with an open circuit, as
shown in Fig. 6.12(b). The current through the series combination of the
2-k! and 4-k! resistors is obtained by current division as
i =
3
3 + 2 + 4
(6 mA) = 2 mA
Hence, the voltages v1 and v2 across the capacitors are
v1 = 2000i = 4 V v2 = 4000i = 8 V
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e-Text Main Menu TextbookTable of Contents Problem Solving Workbook Contents
v = iR
206 PART 1 DC Circuits
P R A C T I C E P R O B L E M 6 . 1
What is the voltage across a 3-µF capacitor if the charge on one plate is
0.12 mC? How much energy is stored?
Answer: 40 V, 2.4 mJ.
E X A M P L E 6 . 2
The voltage across a 5-µF capacitor is
v(t) = 10 cos 6000t V
Calculate the current through it.
Solution:
By definition, the current is
i(t) = C
dv
dt
= 5 × 10−6 d
dt
(10 cos 6000t)
= −5 × 10−6
× 6000 × 10 sin 6000t = −0.3 sin 6000t A
P R A C T I C E P R O B L E M 6 . 2
If a 10-µF capacitor is connected to a voltage source with
v(t) = 50 sin 2000t V
determine the current through the capacitor.
Answer: cos 2000t A.
E X A M P L E 6 . 3
The voltage across a 5-µF capacitor is
v(t) = 10 cos 6000t V
Calculate the current through it.
Solution:
By definition, the current is
i(t) = C
dv
dt
= 5 × 10−6 d
dt
(10 cos 6000t)
= −5 × 10−6
× 6000 × 10 sin 6000t = −0.3 sin 6000t A
P R A C T I C E P R O B L E M 6 . 2
If a 10-µF capacitor is connected to a voltage source with
v(t) = 50 sin 2000t V
determine the current through the capacitor.
Answer: cos 2000t A.
E X A M P L E 6 . 3
Determine the voltage across a 2-µF capacitor if the current through it is
i(t) = 6e−3000t
mA
Assume that the initial capacitor voltage is zero.
Solution:
Since v =
1
C
! t
0
i dt + v(0) and v(0) = 0,
v =
1
! t
6e−3000t
dt·10−3
If a 10-µF capacitor is connected to a voltage source with
v(t) = 50 sin 2000t V
determine the current through the capacitor.
Answer: cos 2000t A.
E X A M P L E 6 . 3
Determine the voltage across a 2-µF capacitor if the current through it is
i(t) = 6e−3000t
mA
Assume that the initial capacitor voltage is zero.
Solution:
Since v =
1
C
! t
0
i dt + v(0) and v(0) = 0,
v =
1
2 × 10−6
! t
0
6e−3000t
dt·10−3
=
3 × 103
−3000
e−3000t
"
"
"
"
t
0
= (1 − e−3000t
) V
P R A C T I C E P R O B L E M 6 . 3
The current through a 100-µF capacitor is i(t) = 50 sin 120πt mA. Cal-
culate the voltage across it at t = 1 ms and t = 5 ms. Take v(0) = 0.
Answer: −93.137 V, −1.736 V.
▲
▲
Thus the current waveform is as shown in Fig. 6.10.
Figure6.10 For Example 6.4.
P R A C T I C E P R O B L E M 6 . 4
An initially uncharged 1-mF capacitor has the current shown in Fig. 6.11
across it. Calculate the voltage across it at t = 2 ms and t = 5 ms.
i (mA)
0
6
4
2
100
t (ms)
Figure6.11 For Practice Prob. 6.4.
Answer: 100 mV, 400 mV.
E X A M P L E 6 . 5
Obtain the energy stored in each capacitor in Fig. 6.12(a) under dc con-
ditions.
Solution:
Under dc conditions, we replace each capacitor with an open circuit, as
shown in Fig. 6.12(b). The current through the series combination of the
2-k! and 4-k! resistors is obtained by current division as
i =
3
3 + 2 + 4
(6 mA) = 2 mA
Hence, the voltages v1 and v2 across the capacitors are
v1 = 2000i = 4 V v2 = 4000i = 8 V
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e-Text Main Menu TextbookTable of Contents Problem Solving Workbook Contents
v2
+
!
6 mA 3 k"
5 k"
4 k"
2 k"
4 mF
(a)
6 mA 3 k"
5 k"
4 k"
(b)
Figure6.12 For Example 6.5.
and the energies stored in them are
w1 =
1
2
C1v2
1 =
1
2
(2 × 10−3
)(4)2
= 16 mJ
w2 =
1
2
C2v2
2 =
1
2
(4 × 10−3
)(8)2
= 128 mJ
P R A C T I C E P R O B L E M 6 . 5
Under dc conditions, find the energy stored in the capacitors in Fig. 6.13.
10 V +
! 6 k"
1 k"
20 mF
10 mF
3 k"
Figure6.13 For Practice Prob. 6.5.
Answer: 405 µJ, 90 µJ.
6.3 SERIES AND PARALLEL CAPACITORS
We know from resistive circuits that series-parallel combination is a pow-
PARALLEL CAPACITORS
The equivalent capacitor Ceq of N capacitors in parallel
— Capacitors have same v across them
— Applying KCL
(capacitors in parallel combine in the same manner as resistors in series)
Figure6.13 For Practice Prob. 6.5.
6.3 S
We kno
erful to
parallel
desire t
i C1
(a)
i1
C2 C3 CN
iN
v
i
(b)
Ceq v
+
!
+
!
i2 i3
Figure6.14 (a) Parallel-connected N
capacitors, (b) equivalent circuit for the parallel
capacitors.
In
parallel
Fig. 6.1
them. A
But ik =
| | |
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e-Text Main Menu Textbook
now from resistive circuits that series-parallel combination is a pow-
tool for reducing circuits. This technique can be extended to series-
lel connections of capacitors, which are sometimes encountered. We
e to replace these capacitors by a single equivalent capacitor Ceq.
In order to obtain the equivalent capacitor Ceq of N capacitors in
lel, consider the circuit in Fig. 6.14(a). The equivalent circuit is in
6.14(b). Note that the capacitors have the same voltage v across
. Applying KCL to Fig. 6.14(a),
i = i1 + i2 + i3 + · · · + iN (6.11)
k = Ck dv/dt. Hence,
i = C1
dv
dt
+ C2
dv
dt
+ C3
dv
dt
+ · · · + CN
dv
dt
=
!
N
"
k=1
Ck
#
dv
dt
= Ceq
dv
dt
(6.12)
|
okTable of Contents Problem Solving Workbook Contents
6.3 SERIES AND PARALLEL CAPACITORS
We know from resistive circuits that series-parallel combination is a pow-
erful tool for reducing circuits. This technique can be extended to series-
parallel connections of capacitors, which are sometimes encountered. We
desire to replace these capacitors by a single equivalent capacitor Ceq.
CN
iN
v
v
+
!
+
!
ected N
it for the parallel
In order to obtain the equivalent capacitor Ceq of N capacitors in
parallel, consider the circuit in Fig. 6.14(a). The equivalent circuit is in
Fig. 6.14(b). Note that the capacitors have the same voltage v across
them. Applying KCL to Fig. 6.14(a),
i = i1 + i2 + i3 + · · · + iN (6.11)
But ik = Ck dv/dt. Hence,
i = C1
dv
dt
+ C2
dv
dt
+ C3
dv
dt
+ · · · + CN
dv
dt
=
!
N
"
k=1
Ck
#
dv
dt
= Ceq
dv
dt
(6.12)
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ain Menu TextbookTable of Contents Problem Solving Workbook Contents
CHAPTER 6 Capacitors and Inductors 209
where
Ceq = C1 + C2 + C3 + · · · + CN (6.13)
The equivalent capacitance of N parallel-connected capacitors is the
sum of the individual capacitances.
We observe that capacitors in parallel combine in the same manner as
resistors in series.
CHAPTER 6 Capacitors and Inductors 209
where
Ceq = C1 + C2 + C3 + · · · + CN (6.13)
The equivalent capacitance of N parallel-connected capacitors is the
sum of the individual capacitances.
We observe that capacitors in parallel combine in the same manner as
resistors in series.
We now obtain Ceq of N capacitors connected in series by compar-
The equivalent capacitor Ceq of N capacitors in series:
— Same i flows through the capacitors
— Applying KVL to the loop
For N = 2
(capacitors in series combine in the same manner as resistors in parallel)
SERIES CAPACITORS
We observe that capacitors in parallel combine in the same manner as
resistors in series.
v
C1
(a)
C2 C3 CN
v
(b)
Ceq v
v1 v2 v3 vN
+
!
+
!
i
i
+ !
+ !
+ ! + !
+
!
Figure6.15 (a) Series-connected N
capacitors, (b) equivalent circuit for the series
capacitor.
We now obtain Ceq of N capacitors connected in series by compar-
ing the circuit in Fig. 6.15(a) with the equivalent circuit in Fig. 6.15(b).
Note that the same current i flows (and consequently the same charge)
through the capacitors. Applying KVL to the loop in Fig. 6.15(a),
v = v1 + v2 + v3 + · · · + vN (6.14)
But vk =
1
Ck
! t
t0
i(t) dt + vk(t0). Therefore,
v =
1
C1
! t
t0
i(t) dt + v1(t0) +
1
C2
! t
t0
i(t) dt + v2(t0)
+ · · · +
1
CN
! t
t0
i(t) dt + vN (t0)
=
"
1
C1
+
1
C2
+ · · · +
1
CN
# ! t
t0
i(t) dt + v1(t0) + v2(t0)
+ · · · + vN (t0)
=
1
Ceq
! t
t0
i(t) dt + v(t0)
(6.15)
where
1
Ceq
=
1
C1
+
1
C2
+
1
C3
+ · · · +
1
CN
(6.16)
The initial voltage v(t0) across Ceq is required by KVL to be the sum of
the capacitor voltages at t0. Or according to Eq. (6.15),
v(t0) = v1(t0) + v2(t0) + · · · + vN (t0)
Thus, according to Eq. (6.16),
The equivalent capacitance of N parallel-connected capacitors is the
sum of the individual capacitances.
We observe that capacitors in parallel combine in the same manner as
resistors in series.
v
C1
(a)
C2 C3 CN
v
(b)
Ceq v
v1 v2 v3 vN
+
!
+
!
i
i
+ !
+ !
+ ! + !
+
!
Figure6.15 (a) Series-connected N
capacitors, (b) equivalent circuit for the series
capacitor.
We now obtain Ceq of N capacitors connected in series by compar-
ing the circuit in Fig. 6.15(a) with the equivalent circuit in Fig. 6.15(b).
Note that the same current i flows (and consequently the same charge)
through the capacitors. Applying KVL to the loop in Fig. 6.15(a),
v = v1 + v2 + v3 + · · · + vN (6.14)
But vk =
1
Ck
! t
t0
i(t) dt + vk(t0). Therefore,
v =
1
C1
! t
t0
i(t) dt + v1(t0) +
1
C2
! t
t0
i(t) dt + v2(t0)
+ · · · +
1
CN
! t
t0
i(t) dt + vN (t0)
=
"
1
C1
+
1
C2
+ · · · +
1
CN
# ! t
t0
i(t) dt + v1(t0) + v2(t0)
+ · · · + vN (t0)
=
1
Ceq
! t
t0
i(t) dt + v(t0)
(6.15)
where
1
Ceq
=
1
C1
+
1
C2
+
1
C3
+ · · · +
1
CN
(6.16)
The initial voltage v(t0) across Ceq is required by KVL to be the sum of
the capacitor voltages at t0. Or according to Eq. (6.15),
v(t0) = v1(t0) + v2(t0) + · · · + vN (t0)
Thus, according to Eq. (6.16),
v
C1
(a)
C2 C3 CN
v
(b)
Ceq v
v1 v2 v3 vN
+
!
+
!
i
i
+ !
+ !
+ ! + !
+
!
Figure6.15 (a) Series-connected N
capacitors, (b) equivalent circuit for the series
capacitor.
eq
g the circuit in Fig. 6.15(a) with the equivalent circuit in Fig. 6.15(b).
ote that the same current i flows (and consequently the same charge)
ough the capacitors. Applying KVL to the loop in Fig. 6.15(a),
v = v1 + v2 + v3 + · · · + vN (6.14)
t vk =
1
Ck
! t
t0
i(t) dt + vk(t0). Therefore,
v =
1
C1
! t
t0
i(t) dt + v1(t0) +
1
C2
! t
t0
i(t) dt + v2(t0)
+ · · · +
1
CN
! t
t0
i(t) dt + vN (t0)
=
"
1
C1
+
1
C2
+ · · · +
1
CN
# ! t
t0
i(t) dt + v1(t0) + v2(t0)
+ · · · + vN (t0)
=
1
Ceq
! t
t0
i(t) dt + v(t0)
(6.15)
here
1
Ceq
=
1
C1
+
1
C2
+
1
C3
+ · · · +
1
CN
(6.16)
e initial voltage v(t0) across Ceq is required by KVL to be the sum of
e capacitor voltages at t0. Or according to Eq. (6.15),
v(t0) = v1(t0) + v2(t0) + · · · + vN (t0)
us, according to Eq. (6.16),
The equivalent capacitance of series-connected capacitors is the reciprocal of the
v
(b)
Ceq v
+
!
i
+
!
Figure6.15 (a) Series-connected N
capacitors, (b) equivalent circuit for the series
capacitor.
v =
1
C1
! t
t0
i(t) dt + v1(t0) +
1
C2
! t
t0
i(t) dt + v2(t0)
+ · · · +
1
CN
! t
t0
i(t) dt + vN (t0)
=
"
1
C1
+
1
C2
+ · · · +
1
CN
# ! t
t0
i(t) dt + v1(t0) + v2(t0)
+ · · · + vN (t0)
=
1
Ceq
! t
t0
i(t) dt + v(t0)
(6.15)
where
1
Ceq
=
1
C1
+
1
C2
+
1
C3
+ · · · +
1
CN
(6.16)
The initial voltage v(t0) across Ceq is required by KVL to be the sum of
the capacitor voltages at t0. Or according to Eq. (6.15),
v(t0) = v1(t0) + v2(t0) + · · · + vN (t0)
Thus, according to Eq. (6.16),
The equivalent capacitance of series-connected capacitors is the reciprocal of the
sum of the reciprocals of the individual capacitances.
Note that capacitors in series combine in the same manner as resistors in
parallel. For N = 2 (i.e., two capacitors in series), Eq. (6.16) becomes
0 PART 1 DC Circuits
or
Ceq =
C1C2
C1 + C2
(6.17)
X A M P L E 6 . 6
Find the equivalent capacitance seen between terminals a and b of the
circuit in Fig. 6.16.
5 mF 60 mF
sum of the individual capacitances.
We observe that capacitors in parallel combine in the same manner as
resistors in series.
v
C1
(a)
C2 C3 CN
v
(b)
Ceq v
v1 v2 v3 vN
+
!
+
!
i
i
+
+ !
+ ! + !
+
!
Figure6.15 (a) Series-connected N
capacitors, (b) equivalent circuit for the se
capacitor.
We now obtain Ceq of N capacitors connected in series by compar-
ing the circuit in Fig. 6.15(a) with the equivalent circuit in Fig. 6.15(b).
Note that the same current i flows (and consequently the same charge)
through the capacitors. Applying KVL to the loop in Fig. 6.15(a),
v = v1 + v2 + v3 + · · · + vN (6.14)
But vk =
1
Ck
! t
t0
i(t) dt + vk(t0). Therefore,
v =
1
C1
! t
t0
i(t) dt + v1(t0) +
1
C2
! t
t0
i(t) dt + v2(t0)
+ · · · +
1
CN
! t
t0
i(t) dt + vN (t0)
=
"
1
C1
+
1
C2
+ · · · +
1
CN
# ! t
t0
i(t) dt + v1(t0) + v2(t0)
+ · · · + vN (t0)
=
1
Ceq
! t
t0
i(t) dt + v(t0)
(6.15)
where
1
Ceq
=
1
C1
+
1
C2
+
1
C3
+ · · · +
1
CN
(6.16)
The initial voltage v(t0) across Ceq is required by KVL to be the sum of
the capacitor voltages at t0. Or according to Eq. (6.15),
v(t0) = v1(t0) + v2(t0) + · · · + vN (t0)
Thus, according to Eq. (6.16),
The equivalent capacitance of series-connected capacitors is the reciprocal of the
sum of the reciprocals of the individual capacitances.
sum of the individual capacitances.
We observe that capacitors in parallel combine in the same manner as
resistors in series.
v
C1
(a)
C2 C3 CN
v
(b)
Ceq v
v1 v2 v3 vN
+
!
+
!
i
i
+ !
+ !
+ ! + !
+
!
Figure6.15 (a) Series-connected N
capacitors, (b) equivalent circuit for the series
capacitor.
We now obtain Ceq of N capacitors connected in series by compar-
ing the circuit in Fig. 6.15(a) with the equivalent circuit in Fig. 6.15(b).
Note that the same current i flows (and consequently the same charge)
through the capacitors. Applying KVL to the loop in Fig. 6.15(a),
v = v1 + v2 + v3 + · · · + vN (6.14)
But vk =
1
Ck
! t
t0
i(t) dt + vk(t0). Therefore,
v =
1
C1
! t
t0
i(t) dt + v1(t0) +
1
C2
! t
t0
i(t) dt + v2(t0)
+ · · · +
1
CN
! t
t0
i(t) dt + vN (t0)
=
"
1
C1
+
1
C2
+ · · · +
1
CN
# ! t
t0
i(t) dt + v1(t0) + v2(t0)
+ · · · + vN (t0)
=
1
Ceq
! t
t0
i(t) dt + v(t0)
(6.15)
where
1
Ceq
=
1
C1
+
1
C2
+
1
C3
+ · · · +
1
CN
(6.16)
The initial voltage v(t0) across Ceq is required by KVL to be the sum of
the capacitor voltages at t0. Or according to Eq. (6.15),
v(t0) = v1(t0) + v2(t0) + · · · + vN (t0)
Thus, according to Eq. (6.16),
The equivalent capacitance of series-connected capacitors is the reciprocal of the
sum of the reciprocals of the individual capacitances.
We observe that capacitors in parallel combine in the same manner as
resistors in series.
v
C1
(a)
C2 C3 CN
v
(b)
Ceq v
v1 v2 v3 vN
+
!
+
!
i
i
+ !
+ !
+ ! + !
+
!
Figure6.15 (a) Series-connected N
capacitors, (b) equivalent circuit for the series
capacitor.
We now obtain Ceq of N capacitors connected in series by compar-
ing the circuit in Fig. 6.15(a) with the equivalent circuit in Fig. 6.15(b).
Note that the same current i flows (and consequently the same charge)
through the capacitors. Applying KVL to the loop in Fig. 6.15(a),
v = v1 + v2 + v3 + · · · + vN (6.14)
But vk =
1
Ck
! t
t0
i(t) dt + vk(t0). Therefore,
v =
1
C1
! t
t0
i(t) dt + v1(t0) +
1
C2
! t
t0
i(t) dt + v2(t0)
+ · · · +
1
CN
! t
t0
i(t) dt + vN (t0)
=
"
1
C1
+
1
C2
+ · · · +
1
CN
# ! t
t0
i(t) dt + v1(t0) + v2(t0)
+ · · · + vN (t0)
=
1
Ceq
! t
t0
i(t) dt + v(t0)
(6.15)
where
1
Ceq
=
1
C1
+
1
C2
+
1
C3
+ · · · +
1
CN
(6.16)
The initial voltage v(t0) across Ceq is required by KVL to be the sum of
the capacitor voltages at t0. Or according to Eq. (6.15),
v(t0) = v1(t0) + v2(t0) + · · · + vN (t0)
Thus, according to Eq. (6.16),
The equivalent capacitance of series-connected capacitors is the reciprocal of the
sum of the reciprocals of the individual capacitances.
We observe that capacitors in parallel combine in the same manner as
resistors in series.
v
C1
(a)
C2 C3 CN
v
(b)
Ceq v
v1 v2 v3 vN
+
!
+
!
i
i
+ !
+ !
+ ! + !
+
!
Figure6.15 (a) Series-connected N
capacitors, (b) equivalent circuit for the series
capacitor.
We now obtain Ceq of N capacitors connected in series by compar-
ing the circuit in Fig. 6.15(a) with the equivalent circuit in Fig. 6.15(b).
Note that the same current i flows (and consequently the same charge)
through the capacitors. Applying KVL to the loop in Fig. 6.15(a),
v = v1 + v2 + v3 + · · · + vN (6.14)
But vk =
1
Ck
! t
t0
i(t) dt + vk(t0). Therefore,
v =
1
C1
! t
t0
i(t) dt + v1(t0) +
1
C2
! t
t0
i(t) dt + v2(t0)
+ · · · +
1
CN
! t
t0
i(t) dt + vN (t0)
=
"
1
C1
+
1
C2
+ · · · +
1
CN
# ! t
t0
i(t) dt + v1(t0) + v2(t0)
+ · · · + vN (t0)
=
1
Ceq
! t
t0
i(t) dt + v(t0)
(6.15)
where
1
Ceq
=
1
C1
+
1
C2
+
1
C3
+ · · · +
1
CN
(6.16)
The initial voltage v(t0) across Ceq is required by KVL to be the sum of
the capacitor voltages at t0. Or according to Eq. (6.15),
v(t0) = v1(t0) + v2(t0) + · · · + vN (t0)
Thus, according to Eq. (6.16),
The equivalent capacitance of series-connected capacitors is the reciprocal of the
sum of the reciprocals of the individual capacitances.
E X A M P L E 6 . 6
Find the equivalent capacitance seen between terminals a and b of the
circuit in Fig. 6.16.
a
b
Ceq
5 mF
20 mF 20 mF
6 mF
60 mF
Figure6.16 For Example 6.6.
Solution:
The 20-µF and 5-µF capacitors are in series; their equivalent capacitance
is
20 × 5
20 + 5
= 4 µF
This 4-µF capacitor is in parallel with the 6-µF and 20-µF capacitors;
their combined capacitance is
4 + 6 + 20 = 30 µF
This 30-µF capacitor is in series with the 60-µF capacitor. Hence, the
equivalent capacitance for the entire circuit is
Ceq =
30 × 60
30 + 60
= 20 µF
P R A C T I C E P R O B L E M 6 . 6
50 mF
L E 6 . 6
Find the equivalent capacitance seen between terminals a and b of the
circuit in Fig. 6.16.
a
b
Ceq
5 mF
20 mF 20 mF
6 mF
60 mF
Figure6.16 For Example 6.6.
Solution:
The 20-µF and 5-µF capacitors are in series; their equivalent capacitance
is
20 × 5
20 + 5
= 4 µF
This 4-µF capacitor is in parallel with the 6-µF and 20-µF capacitors;
their combined capacitance is
4 + 6 + 20 = 30 µF
This 30-µF capacitor is in series with the 60-µF capacitor. Hence, the
equivalent capacitance for the entire circuit is
Ceq =
30 × 60
30 + 60
= 20 µF
CHAPTER 6 Capacitors and Inductors 211
Solution:
We first find the equivalent capacitance Ceq, shown in Fig. 6.19. The two
parallel capacitors in Fig. 6.18 can be combined to get 40+20 = 60 mF.
This 60-mF capacitor is in series with the 20-mF and 30-mF capacitors.
Thus,
Ceq =
1
1
60
+ 1
30
+ 1
20
mF = 10 mF
The total charge is
q = Ceqv = 10 × 10−3
× 30 = 0.3 C
This is the charge on the 20-mF and 30-mF capacitors, because they are
in series with the 30-V source. (A crude way to see this is to imagine that
charge acts like current, since i = dq/dt.) Therefore,
v1 =
q
C1
=
0.3
20 × 10−3
= 15 V v2 =
q
C2
=
0.3
30 × 10−3
= 10 V
Having determined v1 and v2, we now use KVL to determine v3 by
v3 = 30 − v1 − v2 = 5 V
20 mF
40 mF
30 mF
20 mF
30 V +
!
v1 v2
v3
+
!
+ ! + !
Figure6.18 For Example 6.7.
Ceq
30 V +
!
q
+
!
Figure6.19 Equivalent
circuit for Fig. 6.18.
Alternatively, since the 40-mF and 20-mF capacitors are in parallel,
they have the same voltage v3 and their combined capacitance is 40 +
20 = 60 mF. This combined capacitance is in series with the 20-mF and
30-mF capacitors and consequently has the same charge on it. Hence,
v3 =
q
60 mF
=
0.3
60 × 10−3
= 5 V
E X A M P L E 6 . 7
For the circuit in Fig. 6.18, find the voltage across each capacitor.
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e-Text Main Menu TextbookTable of Contents Problem Solving Workbook Contents
For the circuit in Fig. 6.18, find the voltage across each capacitor.
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Ceq =
30 × 60
30 + 60
= 20 µF
P R A C T I C E P R O B L E M 6 . 6
Find the equivalent capacitance seen at the terminals of the circuit in Fig.
6.17.
Answer: 40 µF.
Ceq
120 mF
20 mF
70 F
60 mF
50 mF
Figure6.17 For Practice Prob. 6.6.
E X A M P L E 6 . 7
For the circuit in Fig. 6.18, find the voltage across each capacitor.
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e-Text Main Menu TextbookTable of Contents Problem Solving Workbook Contents
This is the charge on the 20-mF and 30-mF capacitors, because they are
in series with the 30-V source. (A crude way to see this is to imagine that
charge acts like current, since i = dq/dt.) Therefore,
v1 =
q
C1
=
0.3
20 × 10−3
= 15 V v2 =
q
C2
=
0.3
30 × 10−3
= 10 V
Having determined v1 and v2, we now use KVL to determine v3 by
v3 = 30 − v1 − v2 = 5 V
Ceq
30 V +
!
q
!
Figure6.19 Equivalent
circuit for Fig. 6.18.
Alternatively, since the 40-mF and 20-mF capacitors are in parallel,
they have the same voltage v3 and their combined capacitance is 40 +
20 = 60 mF. This combined capacitance is in series with the 20-mF and
30-mF capacitors and consequently has the same charge on it. Hence,
v3 =
q
60 mF
=
0.3
60 × 10−3
= 5 V
P R A C T I C E P R O B L E M 6 . 7
Find the voltage across each of the capacitors in Fig. 6.20.
30 mF
20 mF
60 mF
40 mF
60 V +
!
v1 v3
v2 v4
+ ! + !
+
!
+
!
Figure6.20 For Practice Prob. 6.7.
Answer: v1 = 30 V, v2 = 30 V, v3 = 10 V, v4 = 20 V.
6.4 INDUCTORS
An inductor is a passive element designed to store energy in its magnetic
INDUCTORS
— Inductor: is a passive element designed to store energy in its magnetic field
— Inductors applications in electronic & power systems.
e.g., in power supplies, transformers, radios,
TVs, radars, & electric motors
— When current passes through an inductor
à v across the inductor is directly proportional to
the time rate of change in i
where L: the constant of proportionality, called inductance of inductor
— The SI unit of inductance is henry (H)
— 1 henry =1 volt-second per ampere.
212 PART
If cur
acros
the c
wher
induc
Ame
that 1
Length, l
Cross-sectional area, A
Core material
Number of turns, N
Figure6.21 Typical form of an inductor.
In view of Eq. (6.18), for an inductor to have
voltageacrossitsterminals,itscurrentmustvary
with time. Hence, v = 0 for constant current
through the inductor.
In
PART 1 DC Circuits
An inductor consists of a coil of conducting wire.
If current is allowed to pass through an inductor, it is found that the voltage
across the inductor is directly proportional to the time rate of change of
the current. Using the passive sign convention,
v = L
di
dt
(6.18)
where L is the constant of proportionality called the inductance of the
inductor. The unit of inductance is the henry (H), named in honor of the
American inventor Joseph Henry (1797–1878). It is clear from Eq. (6.18)
that 1 henry equals 1 volt-second per ampere.
onal area, A
material
f turns, N
ductor.
ave
ary
ent
Inductance is the property whereby an inductor exhibits opposition to the change
of current flowing through it, measured in henrys (H).
PART 1 DC Circuits
An inductor consists of a coil of conducting wire.
If current is allowed to pass through an inductor, it is found that the voltage
across the inductor is directly proportional to the time rate of change of
the current. Using the passive sign convention,
v = L
di
dt
(6.18)
where L is the constant of proportionality called the inductance of the
inductor. The unit of inductance is the henry (H), named in honor of the
American inventor Joseph Henry (1797–1878). It is clear from Eq. (6.18)
that 1 henry equals 1 volt-second per ampere.
nal area, A
material
turns, N
uctor.
ve
ry
nt
Inductance is the property whereby an inductor exhibits opposition to the change
— The inductance depends on the inductor dimension & construction
where N: number of turns, l: length, A: cross-sectional
area, μ: permeability of the core
— Four factors increase the value L:
1. Increasing number of turns of coil N
2. Using core material with higher permeability μ
3. Increasing cross-sectional area A
4. Reducing the length of coil l
— Commercially available inductors have different values & types
— Typical practical inductors have inductance values ranging
μH to tens of H
— Inductors may be fixed or variable
— The core may be made of iron, steel, plastic, or air
— The terms coil and choke are also used for inductors
ound in standard electrical engineering handbooks. For example, for the
nductor (solenoid) shown in Fig. 6.21,
L =
N2
µA
ℓ
(6.19)
where N is the number of turns, ℓ is the length, A is the cross-sectional
area, and µ is the permeability of the core. We can see from Eq. (6.19)
hat inductance can be increased by increasing the number of turns of
coil, using material with higher permeability as the core, increasing the
cross-sectional area, or reducing the length of the coil.
Like capacitors, commercially available inductors come in different
values and types. Typical practical inductors have inductance values
anging from a few microhenrys (µH), as in communication systems,
o tens of henrys (H) as in power systems. Inductors may be fixed or
variable. The core may be made of iron, steel, plastic, or air. The terms
coil and choke are also used for inductors. Common inductors are shown
n Fig. 6.22. The circuit symbols for inductors are shown in Fig. 6.23,
ollowing the passive sign convention.
Equation (6.18) is the voltage-current relationship for an inductor.
Figure 6.24 shows this relationship graphically for an inductor whose
nductance is independent of current. Such an inductor is known as a
inear inductor. For a nonlinear inductor, the plot of Eq. (6.18) will not
be a straight line because its inductance varies with current. We will
Core material
Number of turns, N
Figure6.21 Typical form of an inductor.
In view of Eq. (6.18), for an inductor to have
voltageacrossitsterminals,itscurrentmustvary
with time. Hence, v = 0 for constant current
through the inductor.
(a)
212 PART 1
If current
across the
the curren
where L
inductor.
American
that 1 hen
Length, l
Cross-sectional area, A
Core material
Number of turns, N
Figure6.21 Typical form of an inductor.
In view of Eq. (6.18), for an inductor to have
voltageacrossitsterminals,itscurrentmustvary
with time. Hence, v = 0 for constant current
through the inductor.
Inducta
The
and const
of differe
found in s
inductor (
(a)
212 PART 1 DC Circuits
An inductor consists of a coil of conducting wire.
If current is allowed to pass through an inductor, it is found that the voltage
across the inductor is directly proportional to the time rate of change of
the current. Using the passive sign convention,
v = L
di
dt
(6.18)
where L is the constant of proportionality called the inductance of the
inductor. The unit of inductance is the henry (H), named in honor of the
American inventor Joseph Henry (1797–1878). It is clear from Eq. (6.18)
that 1 henry equals 1 volt-second per ampere.
Length, l
Cross-sectional area, A
Core material
Number of turns, N
Figure6.21 Typical form of an inductor.
In view of Eq. (6.18), for an inductor to have
voltageacrossitsterminals,itscurrentmustvary
with time. Hence, v = 0 for constant current
through the inductor.
Inductance is the property whereby an inductor exhibits opposition to the change
of current flowing through it, measured in henrys (H).
The inductance of an inductor depends on its physical dimension
and construction. Formulas for calculating the inductance of inductors
of different shapes are derived from electromagnetic theory and can be
found in standard electrical engineering handbooks. For example, for the
inductor (solenoid) shown in Fig. 6.21,
L =
N2
µA
ℓ
(6.19)
where N is the number of turns, ℓ is the length, A is the cross-sectional
area, and µ is the permeability of the core. We can see from Eq. (6.19)
that inductance can be increased by increasing the number of turns of
coil, using material with higher permeability as the core, increasing the
cross-sectional area, or reducing the length of the coil.
Like capacitors, commercially available inductors come in different
values and types. Typical practical inductors have inductance values
(a)
(b)
212 PART 1 D
If current is
across the i
the current.
where L is
inductor. Th
American in
that 1 henry
Length, l
Cross-sectional area, A
Core material
Number of turns, N
Figure6.21 Typical form of an inductor.
In view of Eq. (6.18), for an inductor to have
voltageacrossitsterminals,itscurrentmustvary
with time. Hence, v = 0 for constant current
through the inductor.
Inductance
The i
and constru
of different
found in sta
inductor (so
where N is
area, and µ
that inducta
coil, using m
cross-sectio
Like c
values and
(a)
212 PART
If cu
acro
the
whe
indu
Am
that
Length, l
Cross-sectional area, A
Core material
Number of turns, N
Figure6.21 Typical form of an inductor.
In view of Eq. (6.18), for an inductor to have
voltageacrossitsterminals,itscurrentmustvary
with time. Hence, v = 0 for constant current
through the inductor.
I
and
of d
foun
indu
whe
area
that
coil
cros
valu
rang
to te
vari
(a)
(b)
212 PART 1 DC Circuits
An inductor consists o
If current is allowed to pass through
across the inductor is directly prop
the current. Using the passive sign
v =
where L is the constant of propor
inductor. The unit of inductance is
American inventor Joseph Henry (1
that 1 henry equals 1 volt-second p
Length, l
Cross-sectional area, A
Core material
Number of turns, N
Figure6.21 Typical form of an inductor.
In view of Eq. (6.18), for an inductor to have
voltageacrossitsterminals,itscurrentmustvary
with time. Hence, v = 0 for constant current
through the inductor.
Inductance is the property whereby an
of current flowing throug
The inductance of an induct
and construction. Formulas for ca
of different shapes are derived fro
found in standard electrical engine
inductor (solenoid) shown in Fig. 6
L =
where N is the number of turns, ℓ
area, and µ is the permeability of
that inductance can be increased
coil, using material with higher pe
cross-sectional area, or reducing th
Like capacitors, commerciall
values and types. Typical practic
ranging from a few microhenrys
to tens of henrys (H) as in power
variable. The core may be made o
coil and choke are also used for ind
in Fig. 6.22. The circuit symbols
following the passive sign convent
(a)
(b)
(c)
Figure6.22 Various types of inductors:
Equation (6.18) is the voltag
Figure 6.24 shows this relationsh
inductance is independent of curr
linear inductor. For a nonlinear in
be a straight line because its indu
Solenoidal wound inductor
Chip inductor
Toroidal inductor
Circuit symbols for inductors is shown in figure
i-v relationship for an inductor:
— Linear inductor: inductance is independent of current
i-v relationship is shown in the figure
— Nonlinear inductor, inductance varies with current
the plot of the equation is not a straight line
à
where i(t0) is the total current for −∞ < t < t0 and i(−∞) = 0
— i(−∞) = 0 is reasonable
because there must be a time in the past when there was no current in the inductor
Integrating gives
i =
1
L
! t
−∞
v(t) dt (6.20)
i i i
(a)
v L
+
!
(b)
v L
+
!
(c)
v L
+
!
Figure6.23 Circuit symbols for inductors:
(a) air-core, (b) iron-core, (c) variable
iron-core.
or
i =
1
L
! t
t0
v(t) dt + i(t0) (6.21)
where i(t0) is the total current for −∞ < t < t0 and i(−∞) = 0. The
idea of making i(−∞) = 0 is practical and reasonable, because there
must be a time in the past when there was no current in the inductor.
The inductor is designed to store energy in its magnetic field. The
energy stored can be obtained from Eqs. (6.18) and (6.20). The power
delivered to the inductor is
p = vi =
"
L
di
dt
#
i (6.22)
The energy stored is
w =
! t
−∞
p dt =
! t
−∞
"
L
di
dt
#
i dt
= L
! t
−∞
i di =
1
2
Li2
(t) −
1
2
Li2
(−∞)
(6.23)
Since i(−∞) = 0,
w =
1
2
Li2
(6.24)
Slope = L
di ⁄dt
0
v
Figure6.24 Voltage-current
relationship of an inductor.
We should note the following important properties of an inductor.
1. Note from Eq. (6.18) that the voltage across an inductor is zero
when the current is constant. Thus,
An inductor acts like a short circuit to dc.
ypes. Typical practical inductors have inductance values
m a few microhenrys (µH), as in communication systems,
nrys (H) as in power systems. Inductors may be fixed or
e core may be made of iron, steel, plastic, or air. The terms
e are also used for inductors. Common inductors are shown
The circuit symbols for inductors are shown in Fig. 6.23,
e passive sign convention.
on (6.18) is the voltage-current relationship for an inductor.
shows this relationship graphically for an inductor whose
s independent of current. Such an inductor is known as a
or. For a nonlinear inductor, the plot of Eq. (6.18) will not
line because its inductance varies with current. We will
r inductors in this textbook unless stated otherwise.
rrent-voltage relationship is obtained from Eq. (6.18) as
di =
1
L
v dt
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e of Contents Problem Solving Workbook Contents
An inductor consists of a coil of conducting wire.
If current is allowed to pass through an inductor, it is found that the voltage
across the inductor is directly proportional to the time rate of change of
the current. Using the passive sign convention,
v = L
di
dt
(6.18)
where L is the constant of proportionality called the inductance of the
inductor. The unit of inductance is the henry (H), named in honor of the
American inventor Joseph Henry (1797–1878). It is clear from Eq. (6.18)
that 1 henry equals 1 volt-second per ampere.
onal area, A
material
f turns, N
ductor.
ave
ary
ent
Inductance is the property whereby an inductor exhibits opposition to the change
of current flowing through it, measured in henrys (H).
The inductance of an inductor depends on its physical dimension
and construction. Formulas for calculating the inductance of inductors
of different shapes are derived from electromagnetic theory and can be
found in standard electrical engineering handbooks. For example, for the
inductor (solenoid) shown in Fig. 6.21,
CHAPTER 6 Capacitors and Inductors 213
Integrating gives
i =
1
L
! t
−∞
v(t) dt (6.20)
i i i
(a)
v L
+
!
(b)
v L
+
!
(c)
v L
+
!
Figure6.23 Circuit symbols for inductors:
(a) air-core, (b) iron-core, (c) variable
iron-core.
or
i =
1
L
! t
t0
v(t) dt + i(t0) (6.21)
where i(t0) is the total current for −∞ < t < t0 and i(−∞) = 0. The
idea of making i(−∞) = 0 is practical and reasonable, because there
must be a time in the past when there was no current in the inductor.
The inductor is designed to store energy in its magnetic field. The
energy stored can be obtained from Eqs. (6.18) and (6.20). The power
delivered to the inductor is
p = vi =
"
L
di
dt
#
i (6.22)
The energy stored is
w =
! t
−∞
p dt =
! t
−∞
"
L
di
dt
#
i dt
= L
! t
−∞
i di =
1
2
Li2
(t) −
1
2
Li2
(−∞)
(6.23)
Since i(−∞) = 0,
w =
1
2
Li2
(6.24)
Slope = L
di ⁄dt
0
v
Figure6.24 Voltage-current
relationship of an inductor.
CHAPTER 6 Capacitors and Inductors
Integrating gives
i =
1
L
! t
−∞
v(t) dt (6.20)
i i i
(a)
v L
+
!
(b)
v L
+
!
(
v
+
!
Figure6.23 Circuit symbols for induc
(a) air-core, (b) iron-core, (c) variable
iron-core.
or
i =
1
L
! t
t0
v(t) dt + i(t0) (6.21)
where i(t0) is the total current for −∞ < t < t0 and i(−∞) = 0. The
idea of making i(−∞) = 0 is practical and reasonable, because there
must be a time in the past when there was no current in the inductor.
The inductor is designed to store energy in its magnetic field. The
energy stored can be obtained from Eqs. (6.18) and (6.20). The power
delivered to the inductor is v
CHAPTER 6 Capacitors and Inductors 213
Integrating gives
i =
1
L
! t
−∞
v(t) dt (6.20)
i i i
(a)
v L
+
!
(b)
v L
+
!
(c)
v L
+
!
Figure6.23 Circuit symbols for inductors:
(a) air-core, (b) iron-core, (c) variable
iron-core.
or
i =
1
L
! t
t0
v(t) dt + i(t0) (6.21)
where i(t0) is the total current for −∞ < t < t0 and i(−∞) = 0. The
idea of making i(−∞) = 0 is practical and reasonable, because there
must be a time in the past when there was no current in the inductor.
The inductor is designed to store energy in its magnetic field. The
energy stored can be obtained from Eqs. (6.18) and (6.20). The power
delivered to the inductor is
" #
v
i-v relationship for linear inductor
— The power delivered to the inductor is
— The energy stored in the inductor is
iron-core.
must be a time in the past when there was no current in the inductor.
The inductor is designed to store energy in its magnetic field. The
energy stored can be obtained from Eqs. (6.18) and (6.20). The power
delivered to the inductor is
p = vi =
"
L
di
dt
#
i (6.22)
The energy stored is
w =
! t
−∞
p dt =
! t
−∞
"
L
di
dt
#
i dt
= L
! t
−∞
i di =
1
2
Li2
(t) −
1
2
Li2
(−∞)
(6.23)
Since i(−∞) = 0,
w =
1
2
Li2
(6.24)
0
v
Figure6.24 Vol
relationship of an
We should note the following important properties of an inductor.
1. Note from Eq. (6.18) that the voltage across an inductor is zero
when the current is constant. Thus,
An inductor acts like a short circuit to dc.
(a) (b)
Figure6.23 Circuit symbols f
(a) air-core, (b) iron-core, (c) v
iron-core.
where i(t0) is the total current for −∞ < t < t0 and i(−∞) = 0. The
idea of making i(−∞) = 0 is practical and reasonable, because there
must be a time in the past when there was no current in the inductor.
The inductor is designed to store energy in its magnetic field. The
energy stored can be obtained from Eqs. (6.18) and (6.20). The power
delivered to the inductor is
p = vi =
"
L
di
dt
#
i (6.22)
The energy stored is
w =
! t
−∞
p dt =
! t
−∞
"
L
di
dt
#
i dt
= L
! t
−∞
i di =
1
2
Li2
(t) −
1
2
Li2
(−∞)
(6.23)
Since i(−∞) = 0,
w =
1
2
Li2
(6.24)
Slo
0
v
Figure6.24 Voltage-curre
relationship of an inductor.
We should note the following important properties of an inductor.
1. Note from Eq. (6.18) that the voltage across an inductor is zero
when the current is constant. Thus,
An inductor acts like a short circuit to dc.
(a) (b) (c)
Figure6.23 Circuit symbols for inductor
(a) air-core, (b) iron-core, (c) variable
iron-core.
t0
where i(t0) is the total current for −∞ < t < t0 and i(−∞) = 0. The
idea of making i(−∞) = 0 is practical and reasonable, because there
must be a time in the past when there was no current in the inductor.
The inductor is designed to store energy in its magnetic field. The
energy stored can be obtained from Eqs. (6.18) and (6.20). The power
delivered to the inductor is
p = vi =
"
L
di
dt
#
i (6.22)
The energy stored is
w =
! t
−∞
p dt =
! t
−∞
"
L
di
dt
#
i dt
= L
! t
−∞
i di =
1
2
Li2
(t) −
1
2
Li2
(−∞)
(6.23)
Since i(−∞) = 0,
w =
1
2
Li2
(6.24)
Slope = L
di⁄dt
0
v
Figure6.24 Voltage-current
relationship of an inductor.
We should note the following important properties of an inductor.
1. Note from Eq. (6.18) that the voltage across an inductor is zero
when the current is constant. Thus,
An inductor acts like a short circuit to dc.
Important properties of an inductor:
1. From v = L di/dt , when i is constant à v = zero
2. Inductor opposes the change in current flowing through it
— An inductor opposes an abrupt change in current
through it
— From v = L di/dt, a discontinuous change in i
through an inductor requires an infinite v,
which is not physically possible
— Voltage across an inductor can change abruptly
w =
1
2
Li2
(6.24)
We should note the following important properties of an inductor.
i
t
(a)
i
(b)
Figure6.25 Current through an inductor:
(a) allowed, (b) not allowable; an abrupt
change is not possible.
1. Note from Eq. (6.18) that the voltage across an inductor is zero
when the current is constant. Thus,
An inductor acts like a short circuit to dc.
2. An important property of the inductor is its opposition to the
change in current flowing through it.
The current through an inductor cannot change instantaneously.
According to Eq. (6.18), a discontinuous change in the current
through an inductor requires an infinite voltage, which is not
physically possible. Thus, an inductor opposes an abrupt
change in the current through it. For example, the current
through an inductor may take the form shown in Fig. 6.25(a),
whereas the inductor current cannot take the form shown in
Fig. 6.25(b) in real-life situations due to the discontinuities.
However, the voltage across an inductor can change abruptly.
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w =
2
Li (6.24)
We should note the following important properties of an inductor.
i
t
(a)
i
(b)
Figure6.25 Current through an inductor:
(a) allowed, (b) not allowable; an abrupt
change is not possible.
1. Note from Eq. (6.18) that the voltage across an inductor is zero
when the current is constant. Thus,
An inductor acts like a short circuit to dc.
2. An important property of the inductor is its opposition to the
change in current flowing through it.
The current through an inductor cannot change instantaneously.
According to Eq. (6.18), a discontinuous change in the current
through an inductor requires an infinite voltage, which is not
physically possible. Thus, an inductor opposes an abrupt
change in the current through it. For example, the current
through an inductor may take the form shown in Fig. 6.25(a),
whereas the inductor current cannot take the form shown in
Fig. 6.25(b) in real-life situations due to the discontinuities.
However, the voltage across an inductor can change abruptly.
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Since i(−∞) = 0,
w =
1
2
Li2
(6.24)
We should note the following important properties of an inductor.
i
t
(a)
i
t
(b)
Figure6.25 Current through an inductor:
(a) allowed, (b) not allowable; an abrupt
change is not possible.
1. Note from Eq. (6.18) that the voltage across an inductor is zero
when the current is constant. Thus,
An inductor acts like a short circuit to dc.
2. An important property of the inductor is its opposition to the
change in current flowing through it.
The current through an inductor cannot change instantaneously.
According to Eq. (6.18), a discontinuous change in the current
through an inductor requires an infinite voltage, which is not
physically possible. Thus, an inductor opposes an abrupt
change in the current through it. For example, the current
through an inductor may take the form shown in Fig. 6.25(a),
whereas the inductor current cannot take the form shown in
Fig. 6.25(b) in real-life situations due to the discontinuities.
However, the voltage across an inductor can change abruptly.
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Important properties of an inductor:
3. Like the ideal capacitor, the ideal inductor does not dissipate energy
— Inductor takes power from circuit when storing energy
(switch a is closed)
— Inductor delivers power to circuit when returning energy
(switch b is closed)
4. A practical, nonideal inductor has a significant resistive component
— This resistance is called the winding resistance Rw
— Rw due to the resistance of the inductor material
— Rw appears in series with the inductance
— Rw makes inductor an energy storage & dissipation device
— Rw is usually very small (ignorable)
5. The nonideal inductor has a winding capacitance Cw
— Cw due to capacitive coupling between conducting coils
— Cw is very small & can be ignored in most cases, except
at high frequencies
214 PAR
Since an inductor is often made of a highly con-
ducting wire, it has a very small resistance.
L Rw
Cw
Figure6.26 Circuit model
for a practical inductor.
E X A M P L E 6 . 8
The
214
Since an inductor is often made of a highly con-
ducting wire, it has a very small resistance.
The time constant tC describes the fall of the charge as well as its rise.
An analogous slowing of the rise (or fall) of the current occurs if we introduce
n emf ! into (or remove it from) a single-loop circuit containing a resistor R and
n inductor L.When the switch S in Fig. 30-15 is closed on a, for example, the cur-
ent in the resistor starts to rise. If the inductor were not present, the current
would rise rapidly to a steady value !/R. Because of the inductor, however, a self-
nduced emf !L appears in the circuit; from Lenz’s law, this emf opposes the rise of
he current, which means that it opposes the battery emf ! in polarity. Thus, the
urrent in the resistor responds to the difference between two emfs, a constant !
ue to the battery and a variable !L (! "L di/dt) due to self-induction.As long as
!L is present,the current will be less than !/R.
As time goes on, the rate at which the current increases becomes less rapid
nd the magnitude of the self-induced emf, which is proportional to di/dt,
ecomes smaller. Thus, the current in the circuit approaches !/R asymptotically.
Fig. 30-15 An RL circuit.When switch
S is closed on a, the current rises and ap-
proaches a limiting value !/R.
S
a
b R
L
–
+
very small and can be ignored in most cases, except at high
frequencies. We will assume ideal inductors in this book.
E X A M P L E 6 . 8
The current through a 0.1-H inductor is i(t) = 10te−5t
A. Find the voltage
across the inductor and the energy stored in it.
Solution:
Since v = L di/dt and L = 0.1 H,
v = 0.1
d
dt
(10te−5t
) = e−5t
+ t(−5)e−5t
= e−5t
(1 − 5t) V
The energy stored is
w =
1
2
Li2
=
1
2
(0.1)100t2
e−10t
= 5t2
e−10t
J
P R A C T I C E P R O B L E M 6 . 8
If the current through a 1-mH inductor is i(t) = 20 cos 100t mA, find the
terminal voltage and the energy stored.
Answer: −2 sin 100t mV, 0.2 cos2
100t µJ.
E X A M P L E 6 . 9
Find the current through a 5-H inductor if the voltage across it is
v(t) =
!
30t2
, t > 0
0, t < 0
Also find the energy stored within 0 < t < 5 s.
Solution:
Since i =
1
L
" t
t0
v(t) dt + i(t0) and L = 5 H,
i =
1
5
" t
0
30t2
dt + 0 = 6 ×
t3
3
= 2t3
A
Since v = L di/dt and L = 0.1 H,
v = 0.1
d
dt
(10te−5t
) = e−5t
+ t(−5)e−5t
= e−5t
(1 − 5t) V
The energy stored is
w =
1
2
Li2
=
1
2
(0.1)100t2
e−10t
= 5t2
e−10t
J
P R A C T I C E P R O B L E M 6 . 8
If the current through a 1-mH inductor is i(t) = 20 cos 100t mA, find the
terminal voltage and the energy stored.
Answer: −2 sin 100t mV, 0.2 cos2
100t µJ.
E X A M P L E 6 . 9
Find the current through a 5-H inductor if the voltage across it is
v(t) =
!
30t2
, t > 0
0, t < 0
Also find the energy stored within 0 < t < 5 s.
Solution:
Since i =
1
L
" t
t0
v(t) dt + i(t0) and L = 5 H,
i =
1
5
" t
0
30t2
dt + 0 = 6 ×
t3
3
= 2t3
A
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e-Text Main Menu TextbookTable of Contents Problem Solving Workbook Contents
CHAPTER 6 Capacitors and Inductors 215
The power p = vi = 60t5
, and the energy stored is then
w =
!
p dt =
! 5
0
60t5
dt = 60
t6
6
"
"
"
"
5
0
= 156.25 kJ
Alternatively, we can obtain the energy stored using Eq. (6.13), by writing
w
"
"5
0
=
1
2
Li2
(5) −
1
2
Li(0) =
1
2
(5)(2 × 53
)2
− 0 = 156.25 kJ
as obtained before.
P R A C T I C E P R O B L E M 6 . 9
The terminal voltage of a 2-H inductor is v = 10(1−t) V. Find the current
flowing through it at t = 4 s and the energy stored in it within 0 < t <
4 s. Assume i(0) = 2 A.
Answer: −18 A, 320 J.
E X A M P L E 6 . 1 0
Consider the circuit in Fig. 6.27(a). Under dc conditions, find: (a) i, vC,
and iL, (b) the energy stored in the capacitor and inductor.
12 V
1 F
+
!
4 "
5 "
1 "
2 H
i
iL
vC
+
!
(a)
(a)
Solution:
(a) Under dc conditions, we replace the capacitor with an open circuit
and the inductor with a short circuit, as in Fig. 6.27(b). It is evident from
Fig. 6.27(b) that
i = iL =
12
= 2 A
Answer: −18 A, 320 J.
E X A M P L E 6 . 1 0
Consider the circuit in Fig. 6.27(a). Under dc conditions, find: (a) i, vC,
and iL, (b) the energy stored in the capacitor and inductor.
12 V
1 F
+
!
4 "
5 "
1 "
2 H
i
iL
vC
+
!
vC
+
!
(a)
(a)
12 V +
!
4 "
5 "
1 "
i
iL
(b)
Figure6.27 For Example 6.10.
Solution:
(a) Under dc conditions, we replace the capacitor with an open circuit
and the inductor with a short circuit, as in Fig. 6.27(b). It is evident from
Fig. 6.27(b) that
i = iL =
12
1 + 5
= 2 A
The voltage vC is the same as the voltage across the 5-! resistor. Hence,
vC = 5i = 10 V
(b) The energy in the capacitor is
wC =
1
2
Cv2
C =
1
2
(1)(102
) = 50 J
and that in the inductor is
wL =
1
2
Li2
L =
1
2
(2)(22
) = 4 J
P R A C T I C E P R O B L E M 6 . 1 0
Determine vC, iL, and the energy stored in the capacitor and inductor in
the circuit of Fig. 6.28 under dc conditions.
4 A 2 F
3 " 1 "
0.25 H
iL
vC
+
!
Figure6.28 For Practice Prob. 6.10.
Answer: 3 V, 3 A, 9 J, 1.125 J.
@
!" =
5
1 + 5
12 = 10)
w =
1
2
Li2
=
1
2
(0.1)100t2
e−10t
= 5t2
e−10t
J
P R A C T I C E P R O B L E M 6 . 8
If the current through a 1-mH inductor is i(t) = 20 cos 100t mA, find the
terminal voltage and the energy stored.
Answer: −2 sin 100t mV, 0.2 cos2
100t µJ.
E X A M P L E 6 . 9
Find the current through a 5-H inductor if the voltage across it is
v(t) =
!
30t2
, t > 0
0, t < 0
Also find the energy stored within 0 < t < 5 s.
Solution:
Since i =
1
L
" t
t0
v(t) dt + i(t0) and L = 5 H,
i =
1
5
" t
0
30t2
dt + 0 = 6 ×
t3
3
= 2t3
A
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The power p = vi = 60t5
, and the energy stored is then
w =
!
p dt =
! 5
0
60t5
dt = 60
t6
6
"
"
"
"
5
0
= 156.25 kJ
Alternatively, we can obtain the energy stored using Eq. (6.13), by writing
w
"
"5
0
=
1
2
Li2
(5) −
1
2
Li(0) =
1
2
(5)(2 × 53
)2
− 0 = 156.25 kJ
as obtained before.
P R A C T I C E P R O B L E M 6 . 9
The terminal voltage of a 2-H inductor is v = 10(1−t) V. Find the current
flowing through it at t = 4 s and the energy stored in it within 0 < t <
4 s. Assume i(0) = 2 A.
Answer: −18 A, 320 J.
E X A M P L E 6 . 1 0
Consider the circuit in Fig. 6.27(a). Under dc conditions, find: (a) i, vC,
and iL, (b) the energy stored in the capacitor and inductor.
12 V
1 F
+
!
4 "
5 "
1 "
2 H
i
iL
vC
+
!
(a)
(a)
5 "
1 "
i
Solution:
(a) Under dc conditions, we replace the capacitor with an open circuit
and the inductor with a short circuit, as in Fig. 6.27(b). It is evident from
Fig. 6.27(b) that
i = iL =
12
1 + 5
= 2 A
The voltage vC is the same as the voltage across the 5-! resistor. Hence,
Consider the circuit in Fig. 6.27(a). Under dc conditions, find: (a) i, vC,
and iL, (b) the energy stored in the capacitor and inductor.
12 V
1 F
+
!
4 "
5 "
1 "
2 H
i
iL
vC
+
!
vC
+
!
(a)
(a)
12 V +
!
4 "
5 "
1 "
i
iL
(b)
Figure6.27 For Example 6.10.
Solution:
(a) Under dc conditions, we replace the capacitor with an open circuit
and the inductor with a short circuit, as in Fig. 6.27(b). It is evident from
Fig. 6.27(b) that
i = iL =
12
1 + 5
= 2 A
The voltage vC is the same as the voltage across the 5-! resistor. Hence,
vC = 5i = 10 V
(b) The energy in the capacitor is
wC =
1
2
Cv2
C =
1
2
(1)(102
) = 50 J
and that in the inductor is
wL =
1
2
Li2
L =
1
2
(2)(22
) = 4 J
P R A C T I C E P R O B L E M 6 . 1 0
Determine vC, iL, and the energy stored in the capacitor and inductor in
the circuit of Fig. 6.28 under dc conditions.
4 A 2 F
3 " 1 "
0.25 H
iL
vC
+
!
Figure6.28 For Practice Prob. 6.10.
Answer: 3 V, 3 A, 9 J, 1.125 J.
@
SERIES INDUCTORS
The equivalent inductance Leq of N inductors in series:
— Same i flows through the inductors
— Applying KVL to the loop
(Inductors in series are combined in exactly the same way as resistors in series)
6.5 S
Now tha
necessar
need to k
or parall
Co
6.29(a),
have the
L1
(a)
L2 L3 LN
(b)
Leq
i
i
v
+
!
v
+
!
+ !
v1
+ !
v2
+ !
v3
+ !
vN
. . .
Figure6.29 (a) A series connection of N
inductors, (b) equivalent circuit for the
series inductors.
Substitu
where
Thus,
Inductor
series.
PART 1 DC Circuits
6.5 SERIES AND PARALLEL INDUCTORS
Now that the inductor has been added to our list of passive elements, it is
necessary to extend the powerful tool of series-parallel combination. We
need to know how to find the equivalent inductance of a series-connected
or parallel-connected set of inductors found in practical circuits.
Consider a series connection of N inductors, as shown in Fig.
6.29(a), with the equivalent circuit shown in Fig. 6.29(b). The inductors
have the same current through them. Applying KVL to the loop,
v = v1 + v2 + v3 + · · · + vN (6.25)
LN
Leq
+ !
vN
. .
ection of N
for the
Substituting vk = Lk di/dt results in
v = L1
di
dt
+ L2
di
dt
+ L3
di
dt
+ · · · + LN
di
dt
= (L1 + L2 + L3 + · · · + LN )
di
dt
=
!
N
"
k=1
Lk
#
di
dt
= Leq
di
dt
(6.26)
where
Leq = L1 + L2 + L3 + · · · + LN (6.27)
Thus,
The equivalent inductance of series-connected inductors is the
sum of the individual inductances.
Inductors in series are combined in exactly the same way as resistors in
series.
to know how to find the equivalent inductance of a series-connected
rallel-connected set of inductors found in practical circuits.
Consider a series connection of N inductors, as shown in Fig.
a), with the equivalent circuit shown in Fig. 6.29(b). The inductors
the same current through them. Applying KVL to the loop,
v = v1 + v2 + v3 + · · · + vN (6.25)
tituting vk = Lk di/dt results in
v = L1
di
dt
+ L2
di
dt
+ L3
di
dt
+ · · · + LN
di
dt
= (L1 + L2 + L3 + · · · + LN )
di
dt
=
!
N
"
k=1
Lk
#
di
dt
= Leq
di
dt
(6.26)
e
Leq = L1 + L2 + L3 + · · · + LN (6.27)
,
The equivalent inductance of series-connected inductors is the
sum of the individual inductances.
need to know how to find the equivalent inductance of a series-connected
or parallel-connected set of inductors found in practical circuits.
Consider a series connection of N inductors, as shown in Fig.
6.29(a), with the equivalent circuit shown in Fig. 6.29(b). The inductors
have the same current through them. Applying KVL to the loop,
v = v1 + v2 + v3 + · · · + vN (6.25)
LN
+ !
vN
n of N
he
Substituting vk = Lk di/dt results in
v = L1
di
dt
+ L2
di
dt
+ L3
di
dt
+ · · · + LN
di
dt
= (L1 + L2 + L3 + · · · + LN )
di
dt
=
!
N
"
k=1
Lk
#
di
dt
= Leq
di
dt
(6.26)
where
Leq = L1 + L2 + L3 + · · · + LN (6.27)
Thus,
The equivalent inductance of series-connected inductors is the
sum of the individual inductances.
Inductors in series are combined in exactly the same way as resistors in
series.
PARALLEL INDUCTORS
The equivalent inductance Leq of N inductors in parallel:
— Inductors have same v across them
— Applying KCL
For N = 2
(Inductors in parallel are combined in the same way as resistors in parallel)
Induct
series.
(a)
(b)
Leq
i
v
+
!
v
+
!
L1 L2 L3 LN
i
i1 i2 i3 iN
Figure6.30 (a) A parallel connection of N
inductors, (b) equivalent circuit for the parallel
inductors.
in Fig.
have th
But ik
i
| | |
▲
▲
e-Text Main Menu Textboo
CHAPTER 6 Capacitors and Inductors 217
where
1
Leq
=
1
L1
+
1
L2
+
1
L3
+ · · · +
1
LN
(6.30)
he initial current i(t0) through Leq at t = t0 is expected by KCL to be
he sum of the inductor currents at t0. Thus, according to Eq. (6.29),
i(t0) = i1(t0) + i2(t0) + · · · + iN (t0)
According to Eq. (6.30),
The equivalent inductance of series-connected inductors is the
sum of the individual inductances.
Inductors in series are combined in exactly the same way as resistors in
series.
LN
iN
n of N
e parallel
We now consider a parallel connection of N inductors, as shown
in Fig. 6.30(a), with the equivalent circuit in Fig. 6.30(b). The inductors
have the same voltage across them. Using KCL,
i = i1 + i2 + i3 + · · · + iN (6.28)
But ik =
1
Lk
$ t
t0
v dt + ik(t0); hence,
i =
1
L1
$ t
t0
v dt + i1(t0) +
1
L2
$ t
t0
v dt + i2(t0)
+ · · · +
1
LN
$ t
t0
v dt + iN (t0)
=
%
1
L1
+
1
L2
+ · · · +
1
LN
& $ t
t0
v dt + i1(t0) + i2(t0)
+ · · · + iN (t0)
=
!
N
"
k=1
1
Lk
# $ t
t0
v dt +
N
"
k=1
ik(t0) =
1
Leq
$ t
t0
v dt + i(t0)
(6.29)
| |
Menu TextbookTable of Contents Problem Solving Workbook Contents
Inductors in series are combined in exactly the same way as resistors in
series.
(a)
(b)
Leq
i
v
+
!
v
+
!
L1 L2 L3 LN
i
i1 i2 i3 iN
Figure6.30 (a) A parallel connection of N
inductors, (b) equivalent circuit for the parallel
inductors.
We now consider a parallel connection of N inductors, as shown
in Fig. 6.30(a), with the equivalent circuit in Fig. 6.30(b). The inductors
have the same voltage across them. Using KCL,
i = i1 + i2 + i3 + · · · + iN (6.28)
But ik =
1
Lk
$ t
t0
v dt + ik(t0); hence,
i =
1
L1
$ t
t0
v dt + i1(t0) +
1
L2
$ t
t0
v dt + i2(t0)
+ · · · +
1
LN
$ t
t0
v dt + iN (t0)
=
%
1
L1
+
1
L2
+ · · · +
1
LN
& $ t
t0
v dt + i1(t0) + i2(t0)
+ · · · + iN (t0)
=
!
N
"
k=1
1
Lk
# $ t
t0
v dt +
N
"
k=1
ik(t0) =
1
Leq
$ t
t0
v dt + i(t0)
(6.29)
| | | |
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▲
e-Text Main Menu TextbookTable of Contents Problem Solving Workbook Contents
sum of the individual inductances.
Inductors in series are combined in exactly the same way as resistors in
series.
LN
iN
on of N
he parallel
We now consider a parallel connection of N inductors, as shown
in Fig. 6.30(a), with the equivalent circuit in Fig. 6.30(b). The inductors
have the same voltage across them. Using KCL,
i = i1 + i2 + i3 + · · · + iN (6.28)
But ik =
1
Lk
$ t
t0
v dt + ik(t0); hence,
i =
1
L1
$ t
t0
v dt + i1(t0) +
1
L2
$ t
t0
v dt + i2(t0)
+ · · · +
1
LN
$ t
t0
v dt + iN (t0)
=
%
1
L1
+
1
L2
+ · · · +
1
LN
& $ t
t0
v dt + i1(t0) + i2(t0)
+ · · · + iN (t0)
=
!
N
"
k=1
1
Lk
# $ t
t0
v dt +
N
"
k=1
ik(t0) =
1
Leq
$ t
t0
v dt + i(t0)
(6.29)
| |
Menu TextbookTable of Contents Problem Solving Workbook Contents
Leq
L3 LN
iN
connection of N
cuit for the parallel
We now consider a parallel connection of N inductors, as shown
in Fig. 6.30(a), with the equivalent circuit in Fig. 6.30(b). The inductors
have the same voltage across them. Using KCL,
i = i1 + i2 + i3 + · · · + iN (6.28)
But ik =
1
Lk
$ t
t0
v dt + ik(t0); hence,
i =
1
L1
$ t
t0
v dt + i1(t0) +
1
L2
$ t
t0
v dt + i2(t0)
+ · · · +
1
LN
$ t
t0
v dt + iN (t0)
=
%
1
L1
+
1
L2
+ · · · +
1
LN
& $ t
t0
v dt + i1(t0) + i2(t0)
+ · · · + iN (t0)
=
!
N
"
k=1
1
Lk
# $ t
t0
v dt +
N
"
k=1
ik(t0) =
1
Leq
$ t
t0
v dt + i(t0)
(6.29)
| |
Main Menu TextbookTable of Contents Problem Solving Workbook Contents
series.
LN
N
of N
parallel
We now consider a parallel connection of N inductors, as shown
in Fig. 6.30(a), with the equivalent circuit in Fig. 6.30(b). The inductors
have the same voltage across them. Using KCL,
i = i1 + i2 + i3 + · · · + iN (6.28)
But ik =
1
Lk
$ t
t0
v dt + ik(t0); hence,
i =
1
L1
$ t
t0
v dt + i1(t0) +
1
L2
$ t
t0
v dt + i2(t0)
+ · · · +
1
LN
$ t
t0
v dt + iN (t0)
=
%
1
L1
+
1
L2
+ · · · +
1
LN
& $ t
t0
v dt + i1(t0) + i2(t0)
+ · · · + iN (t0)
=
!
N
"
k=1
1
Lk
# $ t
t0
v dt +
N
"
k=1
ik(t0) =
1
Leq
$ t
t0
v dt + i(t0)
(6.29)
| |
enu TextbookTable of Contents Problem Solving Workbook Contents
CHAPTER 6 Capacitors and Inductors 217
here
1
Leq
=
1
L1
+
1
L2
+
1
L3
+ · · · +
1
LN
(6.30)
e initial current i(t0) through Leq at t = t0 is expected by KCL to be
e sum of the inductor currents at t0. Thus, according to Eq. (6.29),
i(t0) = i1(t0) + i2(t0) + · · · + iN (t0)
cording to Eq. (6.30),
The equivalent inductance of parallel inductors is the reciprocal of the sum of the
reciprocals of the individual inductances.
ote that the inductors in parallel are combined in the same way as resis-
s in parallel.
CHAPTER 6 Capacitors and Inductors 217
where
1
Leq
=
1
L1
+
1
L2
+
1
L3
+ · · · +
1
LN
(6.30)
The initial current i(t0) through Leq at t = t0 is expected by KCL to be
the sum of the inductor currents at t0. Thus, according to Eq. (6.29),
i(t0) = i1(t0) + i2(t0) + · · · + iN (t0)
According to Eq. (6.30),
The equivalent inductance of parallel inductors is the reciprocal of the sum of the
reciprocals of the individual inductances.
Note that the inductors in parallel are combined in the same way as resis-
CHAPTER 6 Capacitors and Inductors
where
1
Leq
=
1
L1
+
1
L2
+
1
L3
+ · · · +
1
LN
(6.30)
The initial current i(t0) through Leq at t = t0 is expected by KCL to be
the sum of the inductor currents at t0. Thus, according to Eq. (6.29),
i(t0) = i1(t0) + i2(t0) + · · · + iN (t0)
According to Eq. (6.30),
The equivalent inductance of parallel inductors is the reciprocal of the sum of the
reciprocals of the individual inductances.
Note that the inductors in parallel are combined in the same way as resis-
tors in parallel.
For two inductors in parallel (N = 2), Eq. (6.30) becomes
1
Leq
=
1
L1
+
1
L2
or Leq =
L1L2
L1 + L2
(6.31)
It is appropriate at this point to summarize the most important character-
istics of the three basic circuit elements we have studied. The summary
is given in Table 6.1.
TABLE 6.1 Important characteristics of the basic elements.†
E X A M P L E 6 . 1 1
Find the equivalent inductance of the circuit shown in Fig. 6.31.
| | | |
▲
▲
e-Text Main Menu Textbook Table of Contents Problem Solving Workbook Contents
218 PART 1 DC Circuits
4 H 20 H
8 H 10 H
12 H
7 H
Leq
Figure6.31 For Example 6.11.
Solution:
The 10-H, 12-H, and 20-H inductors are in series; thus, combining them
gives a 42-H inductance. This 42-H inductor is in parallel with the 7-H
inductor so that they are combined, to give
7 × 42
7 + 42
= 6 H
This 6-H inductor is in series with the 4-H and 8-H inductors. Hence,
Leq = 4 + 6 + 8 = 18 H
P R A C T I C E P R O B L E M 6 . 1 1
Calculate the equivalent inductance for the inductive ladder network in
Fig. 6.32.
20 mH 100 mH 40 mH
30 mH 20 mH
40 mH
50 mH
Leq
Figure6.32 For Practice Prob. 6.11.
Leq = 4 + 6 + 8 = 18 H
P R A C T I C E P R O B L E M 6 . 1 1
Calculate the equivalent inductance for the inductive ladder network in
Fig. 6.32.
20 mH 100 mH 40 mH
30 mH 20 mH
40 mH
50 mH
Leq
Figure6.32 For Practice Prob. 6.11.
Answer: 25 mH.
E X A M P L E 6 . 1 2
For the circuit in Fig. 6.33, i(t) = 4(2 − e−10t
) mA. If i2(0) = −1 mA,
find: (a) i1(0); (b) v(t), v1(t), and v2(t); (c) i1(t) and i2(t).
4 H
12 H
4 H
v
+
!
v2
v1
+
+ !
!
i
i1 i2
Figure6.33 For Example 6.12.
Solution:
(a) From i(t) = 4(2 − e−10t
) mA, i(0) = 4(2 − 1) = 4 mA. Since i =
i1 + i2,
i1(0) = i(0) − i2(0) = 4 − (−1) = 5 mA
(b) The equivalent inductance is
Leq = 2 + 4 ∥ 12 = 2 + 3 = 5 H
Thus,
2 H
(b) The equivalent inductance is
Leq = 2 + 4 ∥ 12 = 2 + 3 = 5 H
Thus,
v(t) = Leq
di
dt
= 5(4)(−1)(−10)e−10t
mV = 200e−10t
mV
and
v1(t) = 2
di
dt
= 2(−4)(−10)e−10t
mV = 80e−10t
mV
Since v = v1 + v2,
v2(t) = v(t) − v1(t) = 120e−10t
mV
| |
ain Menu TextbookTable of Contents Problem Solving Workbook Contents
CHAPTER 6 Capacitors and Inductors
(c) The current i1 is obtained as
i1(t) =
1
4
! t
0
v2 dt + i1(0) =
120
4
! t
0
e−10t
dt + 5 mA
= −3e−10t
"
"t
0
+ 5 mA = −3e−10t
+ 3 + 5 = 8 − 3e−10t
mA
Similarly,
i2(t) =
1
12
! t
0
v2 dt + i2(0) =
120
12
! t
0
e−10t
dt − 1 mA
= −e−10t
"
"t
0
− 1 mA = −e−10t
+ 1 − 1 = −e−10t
mA
Note that i1(t) + i2(t) = i(t).
P R A C T I C E P R O B L E M 6 . 1 2
For the circuit in Fig. 6.33, i(t) = 4(2 − e−10t
) mA. If i2(0) = −1 mA,
find: (a) i1(0); (b) v(t), v1(t), and v2(t); (c) i1(t) and i2(t).
2 H
2
Solution:
(a) From i(t) = 4(2 − e−10t
) mA, i(0) = 4(2 − 1) = 4 mA. Since i =
i1 + i2,
i1(0) = i(0) − i2(0) = 4 − (−1) = 5 mA
(b) The equivalent inductance is
Leq = 2 + 4 ∥ 12 = 2 + 3 = 5 H
Thus,
v(t) = Leq
di
dt
= 5(4)(−1)(−10)e−10t
mV = 200e−10t
mV
and
v1(t) = 2
di
dt
= 2(−4)(−10)e−10t
mV = 80e−10t
mV
Since v = v1 + v2,
v2(t) = v(t) − v1(t) = 120e−10t
mV
| |
nu TextbookTable of Contents Problem Solving Workbook Contents
E X A M P L E 6 . 1 2
For
find
4 H
12 H
4 H
v
+
!
v2
v1
+
+ !
!
i
i1 i2
Figure6.33 For Example 6.12.
Solu
(a) F
i1 +
(b) T
Thu
and
Sinc
| | |
▲
▲
e-Text Main Menu Textbo
2 H
212 PART 1 DC Circuits
An inductor consists of a coil of conducting wire.
If current is allowed to pass through an inductor, it is found that the
across the inductor is directly proportional to the time rate of ch
the current. Using the passive sign convention,
v = L
di
dt
where L is the constant of proportionality called the inductance
inductor. The unit of inductance is the henry (H), named in hono
American inventor Joseph Henry (1797–1878). It is clear from Eq
that 1 henry equals 1 volt-second per ampere.
Length, l
Cross-sectional area, A
Core material
Number of turns, N
Figure6.21 Typical form of an inductor.
In view of Eq. (6.18), for an inductor to have
voltageacrossitsterminals,itscurrentmustvary
with time. Hence, v = 0 for constant current
through the inductor.
Inductance is the property whereby an inductor exhibits opposition to the ch
of current flowing through it, measured in henrys (H).
The inductance of an inductor depends on its physical dim
and construction. Formulas for calculating the inductance of in
of different shapes are derived from electromagnetic theory and
found in standard electrical engineering handbooks. For example
inductor (solenoid) shown in Fig. 6.21,
L =
N2
µA
ℓ
where N is the number of turns, ℓ is the length, A is the cross-se
area, and µ is the permeability of the core. We can see from Eq
that inductance can be increased by increasing the number of t
coil, using material with higher permeability as the core, increas
cross-sectional area, or reducing the length of the coil.
Like capacitors, commercially available inductors come in d
values and types. Typical practical inductors have inductance
(a)
CHAPTER 6 Capacitors and
Integrating gives
i =
1
L
! t
−∞
v(t) dt
or
i =
1
L
! t
t0
v(t) dt + i(t0)
where i(t0) is the total current for −∞ < t < t0 and i(−∞) =
idea of making i(−∞) = 0 is practical and reasonable, becaus
must be a time in the past when there was no current in the induc
The inductor is designed to store energy in its magnetic fie
energy stored can be obtained from Eqs. (6.18) and (6.20). The
delivered to the inductor is
p = vi =
"
L
di
dt
#
i
The energy stored is
w =
! t
−∞
p dt =
! t
−∞
"
L
di
dt
#
i dt
= L
! t
−∞
i di =
1
2
Li2
(t) −
1
2
Li2
(−∞)
Since i(−∞) = 0,
w =
1
2
Li2
We should note the following important properties of an in
1. Note from Eq. (6.18) that the voltage across an inductor
Leq = 4 + 6 + 8 = 18 H
P R A C T I C E P R O B L E M 6 . 1 1
Calculate the equivalent inductance for the inductive ladder network in
Fig. 6.32.
20 mH 100 mH 40 mH
30 mH 20 mH
40 mH
50 mH
Leq
Figure6.32 For Practice Prob. 6.11.
Answer: 25 mH.
E X A M P L E 6 . 1 2
For the circuit in Fig. 6.33, i(t) = 4(2 − e−10t
) mA. If i2(0) = −1 mA,
find: (a) i1(0); (b) v(t), v1(t), and v2(t); (c) i1(t) and i2(t).
4 H
12 H
4 H
v
+
!
v2
v1
+
+ !
!
i
i1 i2
Figure6.33 For Example 6.12.
Solution:
(a) From i(t) = 4(2 − e−10t
) mA, i(0) = 4(2 − 1) = 4 mA. Since i =
i1 + i2,
i1(0) = i(0) − i2(0) = 4 − (−1) = 5 mA
(b) The equivalent inductance is
Leq = 2 + 4 ∥ 12 = 2 + 3 = 5 H
Thus,
312-chapter 6.pdf gsjsjdkkdkkkkkkkkkkkkkkkk
312-chapter 6.pdf gsjsjdkkdkkkkkkkkkkkkkkkk
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312-chapter 6.pdf gsjsjdkkdkkkkkkkkkkkkkkkk

  • 1. Electronic Circuits I Instructor: Dr. Alaa Mahmoud alaa_y_emam@hotmail.com
  • 3. Outline: — Introduction — Capacitors — Series and Parallel Capacitors — Inductors — Series and Parallel Inductors
  • 4. Introduction — So far we studied resistive circuits Now, we introduce two new important passive linear elements: capacitor & inductor — Resistors dissipate energy, while capacitors & inductors store energy à they are called storage elements — The application of resistive circuits is limited However, capacitors & inductors provide important & practical circuits — The circuit analysis techniques covered in Chapters 3 & 4 are equally applicable to circuits with capacitors & inductors Capacitor Inductor
  • 5. Capacitor — Capacitor: is a passive element that stores energy in its electric field — Capacitors are used in electronics, communications, computers, & power systems. e.g., tuning circuits & memory elements — The plates may be aluminum foil The dielectric may be air, ceramic, paper, or mica — When voltage source v is connected to capacitor à it deposits +q on one plate & −q on the other — The capacitor is storing the electric charge q — q is proportional to the applied voltage v as: C: capacitance of the capacitor — The unit of capacitance is the farad (F) 1 farad = 1coulomb/volt 6.2 CAPACITORS A capacitor is a passive element designed to store energy in its electric field. Besides resistors, capacitors are the most common electrical com- ponents. Capacitors are used extensively in electronics, communications, computers, and power systems. For example, they are used in the tuning circuits of radio receivers and as dynamic memory elements in computer systems. A capacitor is typically constructed as depicted in Fig. 6.1. or. A capacitor consists of two conducting plates separated by an insulator (or dielectric). In many practical applications, the plates may be aluminum foil while the dielectric may be air, ceramic, paper, or mica. When a voltage source v is connected to the capacitor, as in Fig. 6.2, the source deposits a positive charge q on one plate and a negative charge −q on the other. The capacitor is said to store the electric charge. The amount of charge stored, represented by q, is directly proportional to the applied voltage v so that q = Cv (6.1) where C, the constant of proportionality, is known as the capacitance of the capacitor. The unit of capacitance is the farad (F), in honor of the English physicist Michael Faraday (1791–1867). From Eq. (6.1), we may derive the following definition. harge e in a q capac retrie called In contrast to a resistor, which spends or dis- sipates energy irreversibly, an inductor or ca- pacitor stores or releases energy (i.e., has a memory). trodu analy analy circu them typic amps 6.2 A ca field. pone comp circu syste Metal plates, each with area A d Dielectric with permittivity e Figure6.1 A typical capacitor. In ma diele 6.2, t charg The a to the wher of th the E ! ! ! !q +q + + + + + + ! + v Figure6.2 A capacitor with applied voltage v. A capacitor is a passive element designed to store energy in its electric field. Besides resistors, capacitors are the most common electrical com- ponents. Capacitors are used extensively in electronics, communications, computers, and power systems. For example, they are used in the tuning circuits of radio receivers and as dynamic memory elements in computer systems. A capacitor is typically constructed as depicted in Fig. 6.1. A capacitor consists of two conducting plates separated by an insulator (or dielectric). In many practical applications, the plates may be aluminum foil while the dielectric may be air, ceramic, paper, or mica. When a voltage source v is connected to the capacitor, as in Fig. 6.2, the source deposits a positive charge q on one plate and a negative charge −q on the other. The capacitor is said to store the electric charge. The amount of charge stored, represented by q, is directly proportional to the applied voltage v so that q = Cv (6.1) where C, the constant of proportionality, is known as the capacitance of the capacitor. The unit of capacitance is the farad (F), in honor of the English physicist Michael Faraday (1791–1867). From Eq. (6.1), we may derive the following definition.
  • 6. — C does not depend on q or v C depends on physical dimensions of capacitor — Three factors affect capacitance: 1. Surface area of plates (A): ⬆ A à ⬆ C 2. Spacing between plates (d): ⬇ d à ⬆ C 3. Permittivity (dielectric) of material (ε): ⬆ ε à ⬆ C — Capacitors are commercially available in different types & values from pF (10–12 ) to μF (10–6) — Capacitors are described by the dielectric material Capacitors are described by their kinds (fixed or variable) — When capacitor is charging à current flows into +ve terminal à +i — When capacitor is discharging à current flows out of +ve terminal à -i of the capacitor. The unit of capacitance is the farad (F), in honor of the English physicist Michael Faraday (1791–1867). From Eq. (6.1), we may derive the following definition. theamountofcharge oltage difference in a Capacitance is the ratio of the charge on one plate of a capacitor to the voltage difference between the two plates, measured in farads (F). Note from Eq. (6.1) that 1 farad = 1 coulomb/volt. | | Text Main Menu TextbookTable of Contents Problem Solving Workbook Contents CHAPTER 6 Capacitors and Inductors Although the capacitance C of a capacitor is the ratio of the charge q per plate to the applied voltage v, it does not depend on q or v. It depends on the physical dimensions of the capacitor. For example, for the parallel-plate capacitor shown in Fig. 6.1, the capacitance is given by C = ϵA d (6.2) where A is the surface area of each plate, d is the distance between the plates, and ϵ is the permittivity of the dielectric material between the plates. Although Eq. (6.2) applies to only parallel-plate capacitors, we may infer from it that, in general, three factors determine the value of the capacitance: Cap icall Eqs and 1. The surface area of the plates—the larger the area, the greater the capacitance. 2. The spacing between the plates—the smaller the spacing, the greater the capacitance. 3. The permittivity of the material—the higher the permittivity, the greater the capacitance. Capacitors are commercially available in different values and types. Typically, capacitors have values in the picofarad (pF) to microfarad (µF) range. They are described by the dielectric material they are made of and by whether they are of fixed or variable type. Figure 6.3 shows the circuit symbols for fixed and variable capacitors. Note that according to the i C C = ϵA d (6.2) e A is the surface area of each plate, d is the distance between the , and ϵ is the permittivity of the dielectric material between the . Although Eq. (6.2) applies to only parallel-plate capacitors, we nfer from it that, in general, three factors determine the value of the itance: Capacitorvoltageratingandcapacitancearetyp- ically inversely rated due to the relationships in Eqs. (6.1) and (6.2). Arcing occurs if d is smal and V is high. 1. The surface area of the plates—the larger the area, the greater the capacitance. 2. The spacing between the plates—the smaller the spacing, the greater the capacitance. 3. The permittivity of the material—the higher the permittivity, the greater the capacitance. Capacitors are commercially available in different values and types. ally, capacitors have values in the picofarad (pF) to microfarad (µF) . They are described by the dielectric material they are made of and ether they are of fixed or variable type. Figure 6.3 shows the circuit ols for fixed and variable capacitors. Note that according to the ve sign convention, current is considered to flow into the positive nal of the capacitor when the capacitor is being charged, and out of ositive terminal when the capacitor is discharging. i i C v + ! C v + ! Figure6.3 Circuit symbols for capacitors: (a) fixed capacitor, (b) variable capacitor. Figure6.4showscommontypesoffixed-valuecapacitors. Polyester itors are light in weight, stable, and their change with temperature is ctable. Instead of polyester, other dielectric materials such as mica voltage v, it does not depend on q or v. It mensions of the capacitor. For example, for shown in Fig. 6.1, the capacitance is given by C = ϵA d (6.2) of each plate, d is the distance between the tivity of the dielectric material between the applies to only parallel-plate capacitors, we neral, three factors determine the value of the Capacitorvoltageratingandcapacitancearetyp- ically inversely rated due to the relationships in Eqs. (6.1) and (6.2). Arcing occurs if d is small and V is high. f the plates—the larger the area, the greater een the plates—the smaller the spacing, the ance. f the material—the higher the permittivity, acitance. ercially available in different values and types. alues in the picofarad (pF) to microfarad (µF) y the dielectric material they are made of and or variable type. Figure 6.3 shows the circuit able capacitors. Note that according to the urrent is considered to flow into the positive hen the capacitor is being charged, and out of i i C v + ! C v + ! Fixed capacitor Variable capacitor to the applied voltage v so that q = Cv (6.1) where C, the constant of proportionality, is known as the capacitance of the capacitor. The unit of capacitance is the farad (F), in honor of the English physicist Michael Faraday (1791–1867). From Eq. (6.1), we may derive the following definition. charge ce in a Capacitance is the ratio of the charge on one plate of a capacitor to the voltage difference between the two plates, measured in farads (F). Note from Eq. (6.1) that 1 farad = 1 coulomb/volt. | | n Menu TextbookTable of Contents Problem Solving Workbook Contents
  • 7. Common types of fixed-value capacitors: 1. Polyester capacitors: light in weight, stable, their change with temperature is predictable — Mica & polystyrene are used as dielectric materials 2. Film capacitors: rolled and housed in metal or plastic films 3. Electrolytic capacitors: produce very high capacitance Common types of variable -value capacitors: 1. A trimmer (or padder) capacitor or a glass piston capacitor: is varied by turning the screw 2. The capacitance of the variable air capacitor (meshed plates): is varied by turning the shaft — Variable capacitors are used in radio receivers — Capacitors are used to block dc, pass ac, shift phase, store energy, start motors, and suppress noise (a) (b) Figure 6.4 Fixed capacitors: (a) polyester capacitor, (b) ceramic capacitor, (c) (Courtesy of Tech America.) | | | | ▲ ▲ e-Text Main Menu TextbookTable of Contents Problem S Ceramic capacitor plates) is varied by turning the shaft. Variable capacitors are used in radio (a) (b) (c) Figure 6.4 Fixed capacitors: (a) polyester capacitor, (b) ceramic capacitor, (c) electrolytic capacitor. (Courtesy of Tech America.) | | | | ▲ ▲ e-Text Main Menu Textbook Table of Contents Problem Solving Workbook Conte Electrolytic capacitor (a) (b) (c Figure 6.4 Fixed capacitors: (a) polyester capacitor, (b) ceramic capacitor, (c) el (Courtesy of Tech America.) | | | | ▲ ▲ e-Text Main Menu TextbookTable of Contents Problem Sol Polyester capacitor 204 PART 1 DC C receivers allowi are used to bloc suppress noise. (a) (b) Figure6.5 Variable capacitors: (a) trimmer capacitor, (b) filmtrim capacitor. (Courtesy of Johanson.) To obtain the derivative o differentiating b According to Eq. (6.4), for a capacitor to carry current, its voltage must vary with time. Hence, for constant voltage, i = 0 . This is the curre itive sign conve capacitor whose satisfy Eq. (6.4 plot of the curr some capacitor capacitors in th The volta integrating both or Trimmer capacitor Capacitors are commercially available in different values and types. Typically, capacitors have values in the picofarad (pF) to microfarad (µF) range. They are described by the dielectric material they are made of and by whether they are of fixed or variable type. Figure 6.3 shows the circuit symbols for fixed and variable capacitors. Note that according to the passive sign convention, current is considered to flow into the positive terminal of the capacitor when the capacitor is being charged, and out of the positive terminal when the capacitor is discharging. i i C v + ! C v + ! Figure6.3 Circuit symbols for capacitors: (a) fixed capacitor, (b) variable capacitor. Figure6.4showscommontypesoffixed-valuecapacitors. Polyester capacitors are light in weight, stable, and their change with temperature is predictable. Instead of polyester, other dielectric materials such as mica and polystyrene may be used. Film capacitors are rolled and housed in metal or plastic films. Electrolytic capacitors produce very high capaci- tance. Figure 6.5 shows the most common types of variable capacitors. The capacitance of a trimmer (or padder) capacitor or a glass piston capac- itor is varied by turning the screw. The trimmer capacitor is often placed in parallel with another capacitor so that the equivalent capacitance can be varied slightly. The capacitance of the variable air capacitor (meshed plates) is varied by turning the shaft. Variable capacitors are used in radio (a) (b) (c) Figure 6.4 Fixed capacitors: (a) polyester capacitor, (b) ceramic capacitor, (c) electrolytic capacitor. (Courtesy of Tech America.) | | | | ▲ ▲ e-Text Main Menu Textbook Table of Contents Problem Solving Workbook Contents C = ϵA d (6.2) urface area of each plate, d is the distance between the the permittivity of the dielectric material between the h Eq. (6.2) applies to only parallel-plate capacitors, we that, in general, three factors determine the value of the Capacitorvoltageratingandcapacitancearetyp- ically inversely rated due to the relationships in Eqs. (6.1) and (6.2). Arcing occurs if d is small and V is high. face area of the plates—the larger the area, the greater acitance. cing between the plates—the smaller the spacing, the the capacitance. mittivity of the material—the higher the permittivity, ter the capacitance. are commercially available in different values and types. tors have values in the picofarad (pF) to microfarad (µF) described by the dielectric material they are made of and are of fixed or variable type. Figure 6.3 shows the circuit d and variable capacitors. Note that according to the vention, current is considered to flow into the positive apacitor when the capacitor is being charged, and out of inal when the capacitor is discharging. i i C v + ! C v + ! Figure6.3 Circuit symbols for capacitors: (a) fixed capacitor, (b) variable capacitor. showscommontypesoffixed-valuecapacitors. Polyester ht in weight, stable, and their change with temperature is ead of polyester, other dielectric materials such as mica may be used. Film capacitors are rolled and housed in films. Electrolytic capacitors produce very high capaci- 5 shows the most common types of variable capacitors. of a trimmer (or padder) capacitor or a glass piston capac- urning the screw. The trimmer capacitor is often placed nother capacitor so that the equivalent capacitance can y. The capacitance of the variable air capacitor (meshed by turning the shaft. Variable capacitors are used in radio Filmtrim capacitor 204 PART 1 D receivers all are used to b suppress no (a) (b) Figure6.5 Variable capacitors: (a) trimmer capacitor, (b) filmtrim capacitor. To obt the derivativ differentiati This is the c itive sign co capacitor w satisfy Eq. plot of the c some capac capacitors in
  • 8. The i-v relationship of the capacitor: à 1. Linear capacitor: capacitance is independent of voltage — This figure shows linear relationship for a capacitor 2. Nonlinear capacitor: the i-v plot is not straight line — where v(t0) = q(t0)/C is the voltage across the capacitor at time t0 à for capacitor, v depends on the past history of i à capacitor has memory charge −q on the other. The capacitor is said to store the electric charge. The amount of charge stored, represented by q, is directly proportional to the applied voltage v so that q = Cv (6.1) where C, the constant of proportionality, is known as the capacitance of the capacitor. The unit of capacitance is the farad (F), in honor of the English physicist Michael Faraday (1791–1867). From Eq. (6.1), we may derive the following definition. Capacitance is the ratio of the charge on one plate of a capacitor to the voltage difference between the two plates, measured in farads (F). Note from Eq. (6.1) that 1 farad = 1 coulomb/volt. | TextbookTable of Contents Problem Solving Workbook Contents rs allowing one to tune to various stations. In addition, capacitors d to block dc, pass ac, shift phase, store energy, start motors, and ss noise. To obtain the current-voltage relationship of the capacitor, we take ivative of both sides of Eq. (6.1). Since i = dq dt (6.3) ntiating both sides of Eq. (6.1) gives i = C dv dt (6.4) the current-voltage relationship for a capacitor, assuming the pos- gn convention. The relationship is illustrated in Fig. 6.6 for a or whose capacitance is independent of voltage. Capacitors that Eq. (6.4) are said to be linear. For a nonlinear capacitor, the the current-voltage relationship is not a straight line. Although apacitors are nonlinear, most are linear. We will assume linear ors in this book. The voltage-current relation of the capacitor can be obtained by ting both sides of Eq. (6.4). We get v = 1 C ! t −∞ i dt (6.5) ! (Courtesy of Johanson.) According to Eq. (6.4), for a capacitor to carry current, its voltage must vary with time. Hence, for constant voltage, i = 0 . Slope = C dv⁄dt 0 i Figure6.6 Current-voltage relationship of a capacitor. integr or where Equat of the that is The e w = We no −∞. Using (b) Figure6.5 Variable capacitors: (a) trimmer capacitor, (b) filmtrim capacitor. (Courtesy of Johanson.) According to Eq. (6.4), for a capacitor to carry current, its voltage must vary with time. Hence, for constant voltage, i = 0 . Slope = C dv⁄dt 0 i Figure6.6 Current-voltage relationship of a capacitor. 204 PART 1 DC Circuits receivers allowing one to tune to various stations. In addition, capacitors are used to block dc, pass ac, shift phase, store energy, start motors, and suppress noise. (a) (b) Figure6.5 Variable capacitors: (a) trimmer capacitor, (b) filmtrim capacitor. (Courtesy of Johanson.) To obtain the current-voltage relationship of the capacitor, we take the derivative of both sides of Eq. (6.1). Since i = dq dt (6.3) differentiating both sides of Eq. (6.1) gives i = C dv dt (6.4) According to Eq. (6.4), for a capacitor to carry current, its voltage must vary with time. Hence, for constant voltage, i = 0 . This is the current-voltage relationship for a capacitor, assuming the pos- itive sign convention. The relationship is illustrated in Fig. 6.6 for a capacitor whose capacitance is independent of voltage. Capacitors that satisfy Eq. (6.4) are said to be linear. For a nonlinear capacitor, the plot of the current-voltage relationship is not a straight line. Although some capacitors are nonlinear, most are linear. We will assume linear capacitors in this book. Slope = C dv⁄dt 0 i Figure6.6 Current-voltage relationship of a capacitor. The voltage-current relation of the capacitor can be obtained by integrating both sides of Eq. (6.4). We get v = 1 C ! t −∞ i dt (6.5) or v = 1 C ! t t0 i dt + v(t0) (6.6) where v(t0) = q(t0)/C is the voltage across the capacitor at time t0. Equation (6.6) shows that capacitor voltage depends on the past history of the capacitor current. Hence, the capacitor has memory—a property that is often exploited. The instantaneous power delivered to the capacitor is p = vi = Cv dv dt (6.7) The energy stored in the capacitor is therefore w = ! t −∞ p dt = C ! t −∞ v dv dt dt = C ! t −∞ v dv = 1 2 Cv2 " " " " t t=−∞ (6.8) We note that v(−∞) = 0, because the capacitor was uncharged at t = −∞. Thus, i = dq dt (6.3) ng both sides of Eq. (6.1) gives i = C dv dt (6.4) urrent-voltage relationship for a capacitor, assuming the pos- onvention. The relationship is illustrated in Fig. 6.6 for a hose capacitance is independent of voltage. Capacitors that (6.4) are said to be linear. For a nonlinear capacitor, the current-voltage relationship is not a straight line. Although itors are nonlinear, most are linear. We will assume linear n this book. oltage-current relation of the capacitor can be obtained by both sides of Eq. (6.4). We get v = 1 C ! t −∞ i dt (6.5) v = 1 C ! t t0 i dt + v(t0) (6.6) = q(t0)/C is the voltage across the capacitor at time t0. .6) shows that capacitor voltage depends on the past history à à
  • 9. The instantaneous power delivered to the capacitor is: since The energy stored in the capacitor is: v(−∞) = 0, because at t = −∞ capacitor is uncharged à Since q = Cv , à — last 2 equations represent the energy stored in E between capacitor plates — This energy can be re-trieved, since an ideal capacitor cannot dissipate energy Equation (6.6) shows that capacitor voltage depends on the past history of the capacitor current. Hence, the capacitor has memory—a property that is often exploited. The instantaneous power delivered to the capacitor is p = vi = Cv dv dt (6.7) The energy stored in the capacitor is therefore w = ! t −∞ p dt = C ! t −∞ v dv dt dt = C ! t −∞ v dv = 1 2 Cv2 " " " " t t=−∞ (6.8) We note that v(−∞) = 0, because the capacitor was uncharged at t = −∞. Thus, w = 1 2 Cv2 (6.9) Using Eq. (6.1), we may rewrite Eq. (6.9) as w = q2 2C (6.10) | | u TextbookTable of Contents Problem Solving Workbook Contents Slope = C dv⁄dt ent-voltage capacitor. where v(t0) = q(t0)/C is the voltage across the capacitor at time t0. Equation (6.6) shows that capacitor voltage depends on the past history of the capacitor current. Hence, the capacitor has memory—a property that is often exploited. The instantaneous power delivered to the capacitor is p = vi = Cv dv dt (6.7) The energy stored in the capacitor is therefore w = ! t −∞ p dt = C ! t −∞ v dv dt dt = C ! t −∞ v dv = 1 2 Cv2 " " " " t t=−∞ (6.8) We note that v(−∞) = 0, because the capacitor was uncharged at t = −∞. Thus, w = 1 2 Cv2 (6.9) Using Eq. (6.1), we may rewrite Eq. (6.9) as w = q2 2C (6.10) | | Text Main Menu Textbook Table of Contents Problem Solving Workbook Contents of the capacitor current. Hence, the capacitor has memory—a property that is often exploited. The instantaneous power delivered to the capacitor is p = vi = Cv dv dt (6.7) The energy stored in the capacitor is therefore w = ! t −∞ p dt = C ! t −∞ v dv dt dt = C ! t −∞ v dv = 1 2 Cv2 " " " " t t=−∞ (6.8) We note that v(−∞) = 0, because the capacitor was uncharged at t = −∞. Thus, w = 1 2 Cv2 (6.9) Using Eq. (6.1), we may rewrite Eq. (6.9) as w = q2 2C (6.10) | u Textbook Table of Contents Problem Solving Workbook Contents
  • 10. Important properties of a capacitor: 1. From i = Cdv/dt , when v is not changing with time (dc voltage)à i = zero à — If a battery (dc voltage) is connected across a capacitor à capacitor charging 2. The voltage on the capacitor must be continuous — The capacitor resists an abrupt change in the voltage across it — From i = Cdv/dt, a discontinuous change in voltage requires an infinite current — Conversely, the current through a capacitor can change instantaneously capacitor is derived from this element’s capacity to store energy in an electric field. We should note the following important properties of a capacitor: 1. Note from Eq. (6.4) that when the voltage across a capacitor is not changing with time (i.e., dc voltage), the current through the capacitor is zero. Thus, A capacitor is an open circuit to dc. However, if a battery (dc voltage) is connected across a capacitor, the capacitor charges. 2. The voltage on the capacitor must be continuous. The voltage on a capacitor cannot change abruptly. The capacitor resists an abrupt change in the voltage across it. According to Eq. (6.4), a discontinuous change in voltage requires an infinite current, which is physically impossible. For example, the voltage across a capacitor may take the form shown in Fig. 6.7(a), whereas it is not physically possible for the capacitor voltage to take the form shown in Fig. 6.7(b) because of the abrupt change. Conversely, the current through a capacitor can change instantaneously. 3. The ideal capacitor does not dissipate energy. It takes power from the circuit when storing energy in its field and returns previously stored energy when delivering power to the circuit. 4. A real, nonideal capacitor has a parallel-model leakage resistance, as shown in Fig. 6.8. The leakage resistance may be An alternative way of looking at this is using E (6.9), whichindicatesthatenergyisproportion to voltage squared. Since injecting or extracti energy can only be done over some finite tim voltage cannot change instantaneously across capacitor. v t (a) v t (b) Figure6.7 Voltage across a capacitor: (a) allowed, (b) not allowable; an abrupt change is not possible. Leakage resistance Capacitance electric field. We should note the following important properties of a capacitor: 1. Note from Eq. (6.4) that when the voltage across a capacitor is not changing with time (i.e., dc voltage), the current through the capacitor is zero. Thus, A capacitor is an open circuit to dc. However, if a battery (dc voltage) is connected across a capacitor, the capacitor charges. 2. The voltage on the capacitor must be continuous. The voltage on a capacitor cannot change abruptly. The capacitor resists an abrupt change in the voltage across it. According to Eq. (6.4), a discontinuous change in voltage requires an infinite current, which is physically impossible. For example, the voltage across a capacitor may take the form shown in Fig. 6.7(a), whereas it is not physically possible for the capacitor voltage to take the form shown in Fig. 6.7(b) because of the abrupt change. Conversely, the current through a capacitor can change instantaneously. 3. The ideal capacitor does not dissipate energy. It takes power from the circuit when storing energy in its field and returns previously stored energy when delivering power to the circuit. 4. A real, nonideal capacitor has a parallel-model leakage resistance, as shown in Fig. 6.8. The leakage resistance may be An alternative way of looking at this is using E (6.9),whichindicatesthatenergyisproportio to voltage squared. Since injecting or extract energy can only be done over some finite tim voltage cannot change instantaneously across capacitor. v t (a) v t (b) Figure6.7 Voltage across a capacitor: (a) allowed, (b) not allowable; an abrupt change is not possible. Leakage resistance Capacitance CHAPTER 6 Capacitors and Inductors 205 .9) or (6.10) represents the energy stored in the electric field between the plates of the capacitor. This energy can be re- e an ideal capacitor cannot dissipate energy. In fact, the word derived from this element’s capacity to store energy in an d. ould note the following important properties of a capacitor: te from Eq. (6.4) that when the voltage across a capacitor is t changing with time (i.e., dc voltage), the current through capacitor is zero. Thus, A capacitor is an open circuit to dc. wever, if a battery (dc voltage) is connected across a pacitor, the capacitor charges. e voltage on the capacitor must be continuous. The voltage on a capacitor cannot change abruptly. v t (a) v t (b) Figure6.7 Voltage across a capacitor: (a) allowed, (b) not allowable; an abrupt change is not possible.
  • 11. Important properties of a capacitor: 3. The ideal capacitor does not dissipate energy — It takes power from the circuit when storing energy in its field (charging) — It returns stored energy when delivering power to the circuit (discharging) 4. A nonideal (real) capacitor has a parallel-model leakage resistance — The leakage resistance may be as high as 100 MΩ — It can be neglected for most practical applications A capacitor is an open circuit to dc. However, if a battery (dc voltage) is connected across a capacitor, the capacitor charges. 2. The voltage on the capacitor must be continuous. The voltage on a capacitor cannot change abruptly. The capacitor resists an abrupt change in the voltage across it. According to Eq. (6.4), a discontinuous change in voltage requires an infinite current, which is physically impossible. For example, the voltage across a capacitor may take the form shown in Fig. 6.7(a), whereas it is not physically possible for the capacitor voltage to take the form shown in Fig. 6.7(b) because of the abrupt change. Conversely, the current through a capacitor can change instantaneously. 3. The ideal capacitor does not dissipate energy. It takes power from the circuit when storing energy in its field and returns previously stored energy when delivering power to the circuit. 4. A real, nonideal capacitor has a parallel-model leakage resistance, as shown in Fig. 6.8. The leakage resistance may be as high as 100 M! and can be neglected for most practical applications. For this reason, we will assume ideal capacitors in this book. An alternative way of looking at this is using Eq. (6.9), whichindicatesthatenergyisproportional to voltage squared. Since injecting or extracting energy can only be done over some finite time, voltage cannot change instantaneously across a capacitor. v t (a) v t (b) Figure6.7 Voltage across a capacitor: (a) allowed, (b) not allowable; an abrupt change is not possible. Leakage resistance Capacitance Figure 6.8 Circuit model of a nonideal capacitor. E X A M P L E 6 . 1 (a) Calculate the charge stored on a 3-pF capacitor with 20 V across it. takes power delivers power
  • 12. as high as 100 M! and can be neglected for most practical applications. For this reason, we will assume ideal capacitors in this book. Figure 6.8 Circuit model of a nonideal capacitor. E X A M P L E 6 . 1 (a) Calculate the charge stored on a 3-pF capacitor with 20 V across it. (b) Find the energy stored in the capacitor. Solution: (a) Since q = Cv, q = 3 × 10−12 × 20 = 60 pC (b) The energy stored is w = 1 2 Cv2 = 1 2 × 3 × 10−12 × 400 = 600 pJ | | | | ▲ ▲ e-Text Main Menu TextbookTable of Contents Problem Solving Workbook Contents 206 PART 1 DC Circuits P R A C T I C E P R O B L E M 6 . 1 What is the voltage across a 3-µF capacitor if the charge on one plate is 0.12 mC? How much energy is stored? Answer: 40 V, 2.4 mJ. E X A M P L E 6 . 2 The voltage across a 5-µF capacitor is v(t) = 10 cos 6000t V Calculate the current through it. Solution: By definition, the current is i(t) = C dv dt = 5 × 10−6 d dt (10 cos 6000t) = −5 × 10−6 × 6000 × 10 sin 6000t = −0.3 sin 6000t A P R A C T I C E P R O B L E M 6 . 2 If a 10-µF capacitor is connected to a voltage source with v(t) = 50 sin 2000t V
  • 13. determine the current through the capacitor. Answer: cos 2000t A. E X A M P L E 6 . 3 Determine the voltage across a 2-µF capacitor if the current through it is i(t) = 6e−3000t mA Assume that the initial capacitor voltage is zero. Solution: Since v = 1 C ! t 0 i dt + v(0) and v(0) = 0, v = 1 2 × 10−6 ! t 0 6e−3000t dt·10−3 = 3 × 103 −3000 e−3000t " " " " t 0 = (1 − e−3000t ) V P R A C T I C E P R O B L E M 6 . 3 The current through a 100-µF capacitor is i(t) = 50 sin 120πt mA. Cal- culate the voltage across it at t = 1 ms and t = 5 ms. Take v(0) = 0. Answer: −93.137 V, −1.736 V. | | | | ▲ ▲ e-Text Main Menu TextbookTable of Contents Problem Solving Workbook Contents
  • 14. CHAPTER 6 Capacitors and Inductors 207 E X A M P L E 6 . 4 Determine the current through a 200-µFcapacitor whose voltage is shown in Fig. 6.9. v(t) 0 4 3 2 1 50 !50 t Figure6.9 For Example 6.4. Solution: The voltage waveform can be described mathematically as v(t) = ⎧ ⎪ ⎪ ⎨ ⎪ ⎪ ⎩ 50t V 0 < t < 1 100 − 50t V 1 < t < 3 −200 + 50t V 3 < t < 4 0 otherwise Since i = C dv/dt and C = 200 µF, we take the derivative of v to obtain i(t) = 200 × 10−6 × ⎧ ⎪ ⎪ ⎨ ⎪ ⎪ ⎩ 50 0 < t < 1 −50 1 < t < 3 50 3 < t < 4 0 otherwise = ⎧ ⎪ ⎪ ⎨ ⎪ ⎪ ⎩ 10 mA 0 < t < 1 −10 mA 1 < t < 3 10 mA 3 < t < 4 0 otherwise Thus the current waveform is as shown in Fig. 6.10. i (mA) 0 4 3 2 1 10 !10 t Figure6.10 For Example 6.4. P R A C T I C E P R O B L E M 6 . 4 An initially uncharged 1-mF capacitor has the current shown in Fig. 6.11 i (mA)
  • 15. Figure6.11 For Practice Prob. 6.4. E X A M P L E 6 . 5 Obtain the energy stored in each capacitor in Fig. 6.12(a) under dc con- ditions. Solution: Under dc conditions, we replace each capacitor with an open circuit, as shown in Fig. 6.12(b). The current through the series combination of the 2-k! and 4-k! resistors is obtained by current division as i = 3 3 + 2 + 4 (6 mA) = 2 mA Hence, the voltages v1 and v2 across the capacitors are v1 = 2000i = 4 V v2 = 4000i = 8 V | | | | ▲ ▲ e-Text Main Menu TextbookTable of Contents Problem Solving Workbook Contents 208 PART 1 DC Circuits v1 + ! v2 + ! 6 mA 3 k" 5 k" 4 k" 2 k" 2 mF 4 mF (a) 6 mA 3 k" 5 k" 4 k" 2 k" (b) i Figure6.12 For Example 6.5. PART 1 DC Circuits v1 + ! v2 + ! 3 k" 5 k" 4 k" 2 k" 2 mF 4 mF (a) 6 mA 3 k" 5 k" 4 k" 2 k" (b) i Example 6.5. and the energies stored in them are w1 = 1 2 C1v2 1 = 1 2 (2 × 10−3 )(4)2 = 16 mJ w2 = 1 2 C2v2 2 = 1 2 (4 × 10−3 )(8)2 = 128 mJ M 6 . 5 Under dc conditions, find the energy stored in the capacitors in Fig. 6.13. Answer: 405 µJ, 90 µJ. 208 PART 1 DC Circuits 6 mA 3 k" 5 k" 4 k" 2 k" 2 mF 4 mF (a) 6 mA Figure6.12 For Example 6.5. and the energies stored w1 = 1 2 C w2 = 1 2 C P R A C T I C E P R O B L E M 6 . 5 Under dc conditions, fi 3 k" à Same i through 2 & 4kΩ 0 6 4 2 t (ms) Figure6.11 For Practice Prob. 6.4. E X A M P L E 6 . 5 Obtain the energy stored in each capacitor in Fig. 6.12(a) under dc con- ditions. Solution: Under dc conditions, we replace each capacitor with an open circuit, as shown in Fig. 6.12(b). The current through the series combination of the 2-k! and 4-k! resistors is obtained by current division as i = 3 3 + 2 + 4 (6 mA) = 2 mA Hence, the voltages v1 and v2 across the capacitors are v1 = 2000i = 4 V v2 = 4000i = 8 V | | | | ▲ ▲ e-Text Main Menu TextbookTable of Contents Problem Solving Workbook Contents v = iR
  • 16. 206 PART 1 DC Circuits P R A C T I C E P R O B L E M 6 . 1 What is the voltage across a 3-µF capacitor if the charge on one plate is 0.12 mC? How much energy is stored? Answer: 40 V, 2.4 mJ. E X A M P L E 6 . 2 The voltage across a 5-µF capacitor is v(t) = 10 cos 6000t V Calculate the current through it. Solution: By definition, the current is i(t) = C dv dt = 5 × 10−6 d dt (10 cos 6000t) = −5 × 10−6 × 6000 × 10 sin 6000t = −0.3 sin 6000t A P R A C T I C E P R O B L E M 6 . 2 If a 10-µF capacitor is connected to a voltage source with v(t) = 50 sin 2000t V determine the current through the capacitor. Answer: cos 2000t A. E X A M P L E 6 . 3 The voltage across a 5-µF capacitor is v(t) = 10 cos 6000t V Calculate the current through it. Solution: By definition, the current is i(t) = C dv dt = 5 × 10−6 d dt (10 cos 6000t) = −5 × 10−6 × 6000 × 10 sin 6000t = −0.3 sin 6000t A P R A C T I C E P R O B L E M 6 . 2 If a 10-µF capacitor is connected to a voltage source with v(t) = 50 sin 2000t V determine the current through the capacitor. Answer: cos 2000t A. E X A M P L E 6 . 3 Determine the voltage across a 2-µF capacitor if the current through it is i(t) = 6e−3000t mA Assume that the initial capacitor voltage is zero. Solution: Since v = 1 C ! t 0 i dt + v(0) and v(0) = 0, v = 1 ! t 6e−3000t dt·10−3 If a 10-µF capacitor is connected to a voltage source with v(t) = 50 sin 2000t V determine the current through the capacitor. Answer: cos 2000t A. E X A M P L E 6 . 3 Determine the voltage across a 2-µF capacitor if the current through it is i(t) = 6e−3000t mA Assume that the initial capacitor voltage is zero. Solution: Since v = 1 C ! t 0 i dt + v(0) and v(0) = 0, v = 1 2 × 10−6 ! t 0 6e−3000t dt·10−3 = 3 × 103 −3000 e−3000t " " " " t 0 = (1 − e−3000t ) V P R A C T I C E P R O B L E M 6 . 3 The current through a 100-µF capacitor is i(t) = 50 sin 120πt mA. Cal- culate the voltage across it at t = 1 ms and t = 5 ms. Take v(0) = 0. Answer: −93.137 V, −1.736 V. ▲ ▲
  • 17. Thus the current waveform is as shown in Fig. 6.10. Figure6.10 For Example 6.4. P R A C T I C E P R O B L E M 6 . 4 An initially uncharged 1-mF capacitor has the current shown in Fig. 6.11 across it. Calculate the voltage across it at t = 2 ms and t = 5 ms. i (mA) 0 6 4 2 100 t (ms) Figure6.11 For Practice Prob. 6.4. Answer: 100 mV, 400 mV. E X A M P L E 6 . 5 Obtain the energy stored in each capacitor in Fig. 6.12(a) under dc con- ditions. Solution: Under dc conditions, we replace each capacitor with an open circuit, as shown in Fig. 6.12(b). The current through the series combination of the 2-k! and 4-k! resistors is obtained by current division as i = 3 3 + 2 + 4 (6 mA) = 2 mA Hence, the voltages v1 and v2 across the capacitors are v1 = 2000i = 4 V v2 = 4000i = 8 V | | | | ▲ ▲ e-Text Main Menu TextbookTable of Contents Problem Solving Workbook Contents v2 + ! 6 mA 3 k" 5 k" 4 k" 2 k" 4 mF (a) 6 mA 3 k" 5 k" 4 k" (b) Figure6.12 For Example 6.5. and the energies stored in them are w1 = 1 2 C1v2 1 = 1 2 (2 × 10−3 )(4)2 = 16 mJ w2 = 1 2 C2v2 2 = 1 2 (4 × 10−3 )(8)2 = 128 mJ P R A C T I C E P R O B L E M 6 . 5 Under dc conditions, find the energy stored in the capacitors in Fig. 6.13. 10 V + ! 6 k" 1 k" 20 mF 10 mF 3 k" Figure6.13 For Practice Prob. 6.5. Answer: 405 µJ, 90 µJ. 6.3 SERIES AND PARALLEL CAPACITORS We know from resistive circuits that series-parallel combination is a pow-
  • 18. PARALLEL CAPACITORS The equivalent capacitor Ceq of N capacitors in parallel — Capacitors have same v across them — Applying KCL (capacitors in parallel combine in the same manner as resistors in series) Figure6.13 For Practice Prob. 6.5. 6.3 S We kno erful to parallel desire t i C1 (a) i1 C2 C3 CN iN v i (b) Ceq v + ! + ! i2 i3 Figure6.14 (a) Parallel-connected N capacitors, (b) equivalent circuit for the parallel capacitors. In parallel Fig. 6.1 them. A But ik = | | | ▲ ▲ e-Text Main Menu Textbook now from resistive circuits that series-parallel combination is a pow- tool for reducing circuits. This technique can be extended to series- lel connections of capacitors, which are sometimes encountered. We e to replace these capacitors by a single equivalent capacitor Ceq. In order to obtain the equivalent capacitor Ceq of N capacitors in lel, consider the circuit in Fig. 6.14(a). The equivalent circuit is in 6.14(b). Note that the capacitors have the same voltage v across . Applying KCL to Fig. 6.14(a), i = i1 + i2 + i3 + · · · + iN (6.11) k = Ck dv/dt. Hence, i = C1 dv dt + C2 dv dt + C3 dv dt + · · · + CN dv dt = ! N " k=1 Ck # dv dt = Ceq dv dt (6.12) | okTable of Contents Problem Solving Workbook Contents 6.3 SERIES AND PARALLEL CAPACITORS We know from resistive circuits that series-parallel combination is a pow- erful tool for reducing circuits. This technique can be extended to series- parallel connections of capacitors, which are sometimes encountered. We desire to replace these capacitors by a single equivalent capacitor Ceq. CN iN v v + ! + ! ected N it for the parallel In order to obtain the equivalent capacitor Ceq of N capacitors in parallel, consider the circuit in Fig. 6.14(a). The equivalent circuit is in Fig. 6.14(b). Note that the capacitors have the same voltage v across them. Applying KCL to Fig. 6.14(a), i = i1 + i2 + i3 + · · · + iN (6.11) But ik = Ck dv/dt. Hence, i = C1 dv dt + C2 dv dt + C3 dv dt + · · · + CN dv dt = ! N " k=1 Ck # dv dt = Ceq dv dt (6.12) | | ain Menu TextbookTable of Contents Problem Solving Workbook Contents CHAPTER 6 Capacitors and Inductors 209 where Ceq = C1 + C2 + C3 + · · · + CN (6.13) The equivalent capacitance of N parallel-connected capacitors is the sum of the individual capacitances. We observe that capacitors in parallel combine in the same manner as resistors in series. CHAPTER 6 Capacitors and Inductors 209 where Ceq = C1 + C2 + C3 + · · · + CN (6.13) The equivalent capacitance of N parallel-connected capacitors is the sum of the individual capacitances. We observe that capacitors in parallel combine in the same manner as resistors in series. We now obtain Ceq of N capacitors connected in series by compar-
  • 19. The equivalent capacitor Ceq of N capacitors in series: — Same i flows through the capacitors — Applying KVL to the loop For N = 2 (capacitors in series combine in the same manner as resistors in parallel) SERIES CAPACITORS We observe that capacitors in parallel combine in the same manner as resistors in series. v C1 (a) C2 C3 CN v (b) Ceq v v1 v2 v3 vN + ! + ! i i + ! + ! + ! + ! + ! Figure6.15 (a) Series-connected N capacitors, (b) equivalent circuit for the series capacitor. We now obtain Ceq of N capacitors connected in series by compar- ing the circuit in Fig. 6.15(a) with the equivalent circuit in Fig. 6.15(b). Note that the same current i flows (and consequently the same charge) through the capacitors. Applying KVL to the loop in Fig. 6.15(a), v = v1 + v2 + v3 + · · · + vN (6.14) But vk = 1 Ck ! t t0 i(t) dt + vk(t0). Therefore, v = 1 C1 ! t t0 i(t) dt + v1(t0) + 1 C2 ! t t0 i(t) dt + v2(t0) + · · · + 1 CN ! t t0 i(t) dt + vN (t0) = " 1 C1 + 1 C2 + · · · + 1 CN # ! t t0 i(t) dt + v1(t0) + v2(t0) + · · · + vN (t0) = 1 Ceq ! t t0 i(t) dt + v(t0) (6.15) where 1 Ceq = 1 C1 + 1 C2 + 1 C3 + · · · + 1 CN (6.16) The initial voltage v(t0) across Ceq is required by KVL to be the sum of the capacitor voltages at t0. Or according to Eq. (6.15), v(t0) = v1(t0) + v2(t0) + · · · + vN (t0) Thus, according to Eq. (6.16), The equivalent capacitance of N parallel-connected capacitors is the sum of the individual capacitances. We observe that capacitors in parallel combine in the same manner as resistors in series. v C1 (a) C2 C3 CN v (b) Ceq v v1 v2 v3 vN + ! + ! i i + ! + ! + ! + ! + ! Figure6.15 (a) Series-connected N capacitors, (b) equivalent circuit for the series capacitor. We now obtain Ceq of N capacitors connected in series by compar- ing the circuit in Fig. 6.15(a) with the equivalent circuit in Fig. 6.15(b). Note that the same current i flows (and consequently the same charge) through the capacitors. Applying KVL to the loop in Fig. 6.15(a), v = v1 + v2 + v3 + · · · + vN (6.14) But vk = 1 Ck ! t t0 i(t) dt + vk(t0). Therefore, v = 1 C1 ! t t0 i(t) dt + v1(t0) + 1 C2 ! t t0 i(t) dt + v2(t0) + · · · + 1 CN ! t t0 i(t) dt + vN (t0) = " 1 C1 + 1 C2 + · · · + 1 CN # ! t t0 i(t) dt + v1(t0) + v2(t0) + · · · + vN (t0) = 1 Ceq ! t t0 i(t) dt + v(t0) (6.15) where 1 Ceq = 1 C1 + 1 C2 + 1 C3 + · · · + 1 CN (6.16) The initial voltage v(t0) across Ceq is required by KVL to be the sum of the capacitor voltages at t0. Or according to Eq. (6.15), v(t0) = v1(t0) + v2(t0) + · · · + vN (t0) Thus, according to Eq. (6.16), v C1 (a) C2 C3 CN v (b) Ceq v v1 v2 v3 vN + ! + ! i i + ! + ! + ! + ! + ! Figure6.15 (a) Series-connected N capacitors, (b) equivalent circuit for the series capacitor. eq g the circuit in Fig. 6.15(a) with the equivalent circuit in Fig. 6.15(b). ote that the same current i flows (and consequently the same charge) ough the capacitors. Applying KVL to the loop in Fig. 6.15(a), v = v1 + v2 + v3 + · · · + vN (6.14) t vk = 1 Ck ! t t0 i(t) dt + vk(t0). Therefore, v = 1 C1 ! t t0 i(t) dt + v1(t0) + 1 C2 ! t t0 i(t) dt + v2(t0) + · · · + 1 CN ! t t0 i(t) dt + vN (t0) = " 1 C1 + 1 C2 + · · · + 1 CN # ! t t0 i(t) dt + v1(t0) + v2(t0) + · · · + vN (t0) = 1 Ceq ! t t0 i(t) dt + v(t0) (6.15) here 1 Ceq = 1 C1 + 1 C2 + 1 C3 + · · · + 1 CN (6.16) e initial voltage v(t0) across Ceq is required by KVL to be the sum of e capacitor voltages at t0. Or according to Eq. (6.15), v(t0) = v1(t0) + v2(t0) + · · · + vN (t0) us, according to Eq. (6.16), The equivalent capacitance of series-connected capacitors is the reciprocal of the v (b) Ceq v + ! i + ! Figure6.15 (a) Series-connected N capacitors, (b) equivalent circuit for the series capacitor. v = 1 C1 ! t t0 i(t) dt + v1(t0) + 1 C2 ! t t0 i(t) dt + v2(t0) + · · · + 1 CN ! t t0 i(t) dt + vN (t0) = " 1 C1 + 1 C2 + · · · + 1 CN # ! t t0 i(t) dt + v1(t0) + v2(t0) + · · · + vN (t0) = 1 Ceq ! t t0 i(t) dt + v(t0) (6.15) where 1 Ceq = 1 C1 + 1 C2 + 1 C3 + · · · + 1 CN (6.16) The initial voltage v(t0) across Ceq is required by KVL to be the sum of the capacitor voltages at t0. Or according to Eq. (6.15), v(t0) = v1(t0) + v2(t0) + · · · + vN (t0) Thus, according to Eq. (6.16), The equivalent capacitance of series-connected capacitors is the reciprocal of the sum of the reciprocals of the individual capacitances. Note that capacitors in series combine in the same manner as resistors in parallel. For N = 2 (i.e., two capacitors in series), Eq. (6.16) becomes 0 PART 1 DC Circuits or Ceq = C1C2 C1 + C2 (6.17) X A M P L E 6 . 6 Find the equivalent capacitance seen between terminals a and b of the circuit in Fig. 6.16. 5 mF 60 mF sum of the individual capacitances. We observe that capacitors in parallel combine in the same manner as resistors in series. v C1 (a) C2 C3 CN v (b) Ceq v v1 v2 v3 vN + ! + ! i i + + ! + ! + ! + ! Figure6.15 (a) Series-connected N capacitors, (b) equivalent circuit for the se capacitor. We now obtain Ceq of N capacitors connected in series by compar- ing the circuit in Fig. 6.15(a) with the equivalent circuit in Fig. 6.15(b). Note that the same current i flows (and consequently the same charge) through the capacitors. Applying KVL to the loop in Fig. 6.15(a), v = v1 + v2 + v3 + · · · + vN (6.14) But vk = 1 Ck ! t t0 i(t) dt + vk(t0). Therefore, v = 1 C1 ! t t0 i(t) dt + v1(t0) + 1 C2 ! t t0 i(t) dt + v2(t0) + · · · + 1 CN ! t t0 i(t) dt + vN (t0) = " 1 C1 + 1 C2 + · · · + 1 CN # ! t t0 i(t) dt + v1(t0) + v2(t0) + · · · + vN (t0) = 1 Ceq ! t t0 i(t) dt + v(t0) (6.15) where 1 Ceq = 1 C1 + 1 C2 + 1 C3 + · · · + 1 CN (6.16) The initial voltage v(t0) across Ceq is required by KVL to be the sum of the capacitor voltages at t0. Or according to Eq. (6.15), v(t0) = v1(t0) + v2(t0) + · · · + vN (t0) Thus, according to Eq. (6.16), The equivalent capacitance of series-connected capacitors is the reciprocal of the sum of the reciprocals of the individual capacitances. sum of the individual capacitances. We observe that capacitors in parallel combine in the same manner as resistors in series. v C1 (a) C2 C3 CN v (b) Ceq v v1 v2 v3 vN + ! + ! i i + ! + ! + ! + ! + ! Figure6.15 (a) Series-connected N capacitors, (b) equivalent circuit for the series capacitor. We now obtain Ceq of N capacitors connected in series by compar- ing the circuit in Fig. 6.15(a) with the equivalent circuit in Fig. 6.15(b). Note that the same current i flows (and consequently the same charge) through the capacitors. Applying KVL to the loop in Fig. 6.15(a), v = v1 + v2 + v3 + · · · + vN (6.14) But vk = 1 Ck ! t t0 i(t) dt + vk(t0). Therefore, v = 1 C1 ! t t0 i(t) dt + v1(t0) + 1 C2 ! t t0 i(t) dt + v2(t0) + · · · + 1 CN ! t t0 i(t) dt + vN (t0) = " 1 C1 + 1 C2 + · · · + 1 CN # ! t t0 i(t) dt + v1(t0) + v2(t0) + · · · + vN (t0) = 1 Ceq ! t t0 i(t) dt + v(t0) (6.15) where 1 Ceq = 1 C1 + 1 C2 + 1 C3 + · · · + 1 CN (6.16) The initial voltage v(t0) across Ceq is required by KVL to be the sum of the capacitor voltages at t0. Or according to Eq. (6.15), v(t0) = v1(t0) + v2(t0) + · · · + vN (t0) Thus, according to Eq. (6.16), The equivalent capacitance of series-connected capacitors is the reciprocal of the sum of the reciprocals of the individual capacitances. We observe that capacitors in parallel combine in the same manner as resistors in series. v C1 (a) C2 C3 CN v (b) Ceq v v1 v2 v3 vN + ! + ! i i + ! + ! + ! + ! + ! Figure6.15 (a) Series-connected N capacitors, (b) equivalent circuit for the series capacitor. We now obtain Ceq of N capacitors connected in series by compar- ing the circuit in Fig. 6.15(a) with the equivalent circuit in Fig. 6.15(b). Note that the same current i flows (and consequently the same charge) through the capacitors. Applying KVL to the loop in Fig. 6.15(a), v = v1 + v2 + v3 + · · · + vN (6.14) But vk = 1 Ck ! t t0 i(t) dt + vk(t0). Therefore, v = 1 C1 ! t t0 i(t) dt + v1(t0) + 1 C2 ! t t0 i(t) dt + v2(t0) + · · · + 1 CN ! t t0 i(t) dt + vN (t0) = " 1 C1 + 1 C2 + · · · + 1 CN # ! t t0 i(t) dt + v1(t0) + v2(t0) + · · · + vN (t0) = 1 Ceq ! t t0 i(t) dt + v(t0) (6.15) where 1 Ceq = 1 C1 + 1 C2 + 1 C3 + · · · + 1 CN (6.16) The initial voltage v(t0) across Ceq is required by KVL to be the sum of the capacitor voltages at t0. Or according to Eq. (6.15), v(t0) = v1(t0) + v2(t0) + · · · + vN (t0) Thus, according to Eq. (6.16), The equivalent capacitance of series-connected capacitors is the reciprocal of the sum of the reciprocals of the individual capacitances. We observe that capacitors in parallel combine in the same manner as resistors in series. v C1 (a) C2 C3 CN v (b) Ceq v v1 v2 v3 vN + ! + ! i i + ! + ! + ! + ! + ! Figure6.15 (a) Series-connected N capacitors, (b) equivalent circuit for the series capacitor. We now obtain Ceq of N capacitors connected in series by compar- ing the circuit in Fig. 6.15(a) with the equivalent circuit in Fig. 6.15(b). Note that the same current i flows (and consequently the same charge) through the capacitors. Applying KVL to the loop in Fig. 6.15(a), v = v1 + v2 + v3 + · · · + vN (6.14) But vk = 1 Ck ! t t0 i(t) dt + vk(t0). Therefore, v = 1 C1 ! t t0 i(t) dt + v1(t0) + 1 C2 ! t t0 i(t) dt + v2(t0) + · · · + 1 CN ! t t0 i(t) dt + vN (t0) = " 1 C1 + 1 C2 + · · · + 1 CN # ! t t0 i(t) dt + v1(t0) + v2(t0) + · · · + vN (t0) = 1 Ceq ! t t0 i(t) dt + v(t0) (6.15) where 1 Ceq = 1 C1 + 1 C2 + 1 C3 + · · · + 1 CN (6.16) The initial voltage v(t0) across Ceq is required by KVL to be the sum of the capacitor voltages at t0. Or according to Eq. (6.15), v(t0) = v1(t0) + v2(t0) + · · · + vN (t0) Thus, according to Eq. (6.16), The equivalent capacitance of series-connected capacitors is the reciprocal of the sum of the reciprocals of the individual capacitances.
  • 20. E X A M P L E 6 . 6 Find the equivalent capacitance seen between terminals a and b of the circuit in Fig. 6.16. a b Ceq 5 mF 20 mF 20 mF 6 mF 60 mF Figure6.16 For Example 6.6. Solution: The 20-µF and 5-µF capacitors are in series; their equivalent capacitance is 20 × 5 20 + 5 = 4 µF This 4-µF capacitor is in parallel with the 6-µF and 20-µF capacitors; their combined capacitance is 4 + 6 + 20 = 30 µF This 30-µF capacitor is in series with the 60-µF capacitor. Hence, the equivalent capacitance for the entire circuit is Ceq = 30 × 60 30 + 60 = 20 µF P R A C T I C E P R O B L E M 6 . 6 50 mF L E 6 . 6 Find the equivalent capacitance seen between terminals a and b of the circuit in Fig. 6.16. a b Ceq 5 mF 20 mF 20 mF 6 mF 60 mF Figure6.16 For Example 6.6. Solution: The 20-µF and 5-µF capacitors are in series; their equivalent capacitance is 20 × 5 20 + 5 = 4 µF This 4-µF capacitor is in parallel with the 6-µF and 20-µF capacitors; their combined capacitance is 4 + 6 + 20 = 30 µF This 30-µF capacitor is in series with the 60-µF capacitor. Hence, the equivalent capacitance for the entire circuit is Ceq = 30 × 60 30 + 60 = 20 µF
  • 21. CHAPTER 6 Capacitors and Inductors 211 Solution: We first find the equivalent capacitance Ceq, shown in Fig. 6.19. The two parallel capacitors in Fig. 6.18 can be combined to get 40+20 = 60 mF. This 60-mF capacitor is in series with the 20-mF and 30-mF capacitors. Thus, Ceq = 1 1 60 + 1 30 + 1 20 mF = 10 mF The total charge is q = Ceqv = 10 × 10−3 × 30 = 0.3 C This is the charge on the 20-mF and 30-mF capacitors, because they are in series with the 30-V source. (A crude way to see this is to imagine that charge acts like current, since i = dq/dt.) Therefore, v1 = q C1 = 0.3 20 × 10−3 = 15 V v2 = q C2 = 0.3 30 × 10−3 = 10 V Having determined v1 and v2, we now use KVL to determine v3 by v3 = 30 − v1 − v2 = 5 V 20 mF 40 mF 30 mF 20 mF 30 V + ! v1 v2 v3 + ! + ! + ! Figure6.18 For Example 6.7. Ceq 30 V + ! q + ! Figure6.19 Equivalent circuit for Fig. 6.18. Alternatively, since the 40-mF and 20-mF capacitors are in parallel, they have the same voltage v3 and their combined capacitance is 40 + 20 = 60 mF. This combined capacitance is in series with the 20-mF and 30-mF capacitors and consequently has the same charge on it. Hence, v3 = q 60 mF = 0.3 60 × 10−3 = 5 V E X A M P L E 6 . 7 For the circuit in Fig. 6.18, find the voltage across each capacitor. | | | | ▲ ▲ e-Text Main Menu TextbookTable of Contents Problem Solving Workbook Contents For the circuit in Fig. 6.18, find the voltage across each capacitor. | | Menu TextbookTable of Contents Problem Solving Workbook Contents
  • 22. Ceq = 30 × 60 30 + 60 = 20 µF P R A C T I C E P R O B L E M 6 . 6 Find the equivalent capacitance seen at the terminals of the circuit in Fig. 6.17. Answer: 40 µF. Ceq 120 mF 20 mF 70 F 60 mF 50 mF Figure6.17 For Practice Prob. 6.6. E X A M P L E 6 . 7 For the circuit in Fig. 6.18, find the voltage across each capacitor. | | | | ▲ ▲ e-Text Main Menu TextbookTable of Contents Problem Solving Workbook Contents This is the charge on the 20-mF and 30-mF capacitors, because they are in series with the 30-V source. (A crude way to see this is to imagine that charge acts like current, since i = dq/dt.) Therefore, v1 = q C1 = 0.3 20 × 10−3 = 15 V v2 = q C2 = 0.3 30 × 10−3 = 10 V Having determined v1 and v2, we now use KVL to determine v3 by v3 = 30 − v1 − v2 = 5 V Ceq 30 V + ! q ! Figure6.19 Equivalent circuit for Fig. 6.18. Alternatively, since the 40-mF and 20-mF capacitors are in parallel, they have the same voltage v3 and their combined capacitance is 40 + 20 = 60 mF. This combined capacitance is in series with the 20-mF and 30-mF capacitors and consequently has the same charge on it. Hence, v3 = q 60 mF = 0.3 60 × 10−3 = 5 V P R A C T I C E P R O B L E M 6 . 7 Find the voltage across each of the capacitors in Fig. 6.20. 30 mF 20 mF 60 mF 40 mF 60 V + ! v1 v3 v2 v4 + ! + ! + ! + ! Figure6.20 For Practice Prob. 6.7. Answer: v1 = 30 V, v2 = 30 V, v3 = 10 V, v4 = 20 V. 6.4 INDUCTORS An inductor is a passive element designed to store energy in its magnetic
  • 23. INDUCTORS — Inductor: is a passive element designed to store energy in its magnetic field — Inductors applications in electronic & power systems. e.g., in power supplies, transformers, radios, TVs, radars, & electric motors — When current passes through an inductor à v across the inductor is directly proportional to the time rate of change in i where L: the constant of proportionality, called inductance of inductor — The SI unit of inductance is henry (H) — 1 henry =1 volt-second per ampere. 212 PART If cur acros the c wher induc Ame that 1 Length, l Cross-sectional area, A Core material Number of turns, N Figure6.21 Typical form of an inductor. In view of Eq. (6.18), for an inductor to have voltageacrossitsterminals,itscurrentmustvary with time. Hence, v = 0 for constant current through the inductor. In PART 1 DC Circuits An inductor consists of a coil of conducting wire. If current is allowed to pass through an inductor, it is found that the voltage across the inductor is directly proportional to the time rate of change of the current. Using the passive sign convention, v = L di dt (6.18) where L is the constant of proportionality called the inductance of the inductor. The unit of inductance is the henry (H), named in honor of the American inventor Joseph Henry (1797–1878). It is clear from Eq. (6.18) that 1 henry equals 1 volt-second per ampere. onal area, A material f turns, N ductor. ave ary ent Inductance is the property whereby an inductor exhibits opposition to the change of current flowing through it, measured in henrys (H). PART 1 DC Circuits An inductor consists of a coil of conducting wire. If current is allowed to pass through an inductor, it is found that the voltage across the inductor is directly proportional to the time rate of change of the current. Using the passive sign convention, v = L di dt (6.18) where L is the constant of proportionality called the inductance of the inductor. The unit of inductance is the henry (H), named in honor of the American inventor Joseph Henry (1797–1878). It is clear from Eq. (6.18) that 1 henry equals 1 volt-second per ampere. nal area, A material turns, N uctor. ve ry nt Inductance is the property whereby an inductor exhibits opposition to the change
  • 24. — The inductance depends on the inductor dimension & construction where N: number of turns, l: length, A: cross-sectional area, μ: permeability of the core — Four factors increase the value L: 1. Increasing number of turns of coil N 2. Using core material with higher permeability μ 3. Increasing cross-sectional area A 4. Reducing the length of coil l — Commercially available inductors have different values & types — Typical practical inductors have inductance values ranging μH to tens of H — Inductors may be fixed or variable — The core may be made of iron, steel, plastic, or air — The terms coil and choke are also used for inductors ound in standard electrical engineering handbooks. For example, for the nductor (solenoid) shown in Fig. 6.21, L = N2 µA ℓ (6.19) where N is the number of turns, ℓ is the length, A is the cross-sectional area, and µ is the permeability of the core. We can see from Eq. (6.19) hat inductance can be increased by increasing the number of turns of coil, using material with higher permeability as the core, increasing the cross-sectional area, or reducing the length of the coil. Like capacitors, commercially available inductors come in different values and types. Typical practical inductors have inductance values anging from a few microhenrys (µH), as in communication systems, o tens of henrys (H) as in power systems. Inductors may be fixed or variable. The core may be made of iron, steel, plastic, or air. The terms coil and choke are also used for inductors. Common inductors are shown n Fig. 6.22. The circuit symbols for inductors are shown in Fig. 6.23, ollowing the passive sign convention. Equation (6.18) is the voltage-current relationship for an inductor. Figure 6.24 shows this relationship graphically for an inductor whose nductance is independent of current. Such an inductor is known as a inear inductor. For a nonlinear inductor, the plot of Eq. (6.18) will not be a straight line because its inductance varies with current. We will Core material Number of turns, N Figure6.21 Typical form of an inductor. In view of Eq. (6.18), for an inductor to have voltageacrossitsterminals,itscurrentmustvary with time. Hence, v = 0 for constant current through the inductor. (a) 212 PART 1 If current across the the curren where L inductor. American that 1 hen Length, l Cross-sectional area, A Core material Number of turns, N Figure6.21 Typical form of an inductor. In view of Eq. (6.18), for an inductor to have voltageacrossitsterminals,itscurrentmustvary with time. Hence, v = 0 for constant current through the inductor. Inducta The and const of differe found in s inductor ( (a) 212 PART 1 DC Circuits An inductor consists of a coil of conducting wire. If current is allowed to pass through an inductor, it is found that the voltage across the inductor is directly proportional to the time rate of change of the current. Using the passive sign convention, v = L di dt (6.18) where L is the constant of proportionality called the inductance of the inductor. The unit of inductance is the henry (H), named in honor of the American inventor Joseph Henry (1797–1878). It is clear from Eq. (6.18) that 1 henry equals 1 volt-second per ampere. Length, l Cross-sectional area, A Core material Number of turns, N Figure6.21 Typical form of an inductor. In view of Eq. (6.18), for an inductor to have voltageacrossitsterminals,itscurrentmustvary with time. Hence, v = 0 for constant current through the inductor. Inductance is the property whereby an inductor exhibits opposition to the change of current flowing through it, measured in henrys (H). The inductance of an inductor depends on its physical dimension and construction. Formulas for calculating the inductance of inductors of different shapes are derived from electromagnetic theory and can be found in standard electrical engineering handbooks. For example, for the inductor (solenoid) shown in Fig. 6.21, L = N2 µA ℓ (6.19) where N is the number of turns, ℓ is the length, A is the cross-sectional area, and µ is the permeability of the core. We can see from Eq. (6.19) that inductance can be increased by increasing the number of turns of coil, using material with higher permeability as the core, increasing the cross-sectional area, or reducing the length of the coil. Like capacitors, commercially available inductors come in different values and types. Typical practical inductors have inductance values (a) (b) 212 PART 1 D If current is across the i the current. where L is inductor. Th American in that 1 henry Length, l Cross-sectional area, A Core material Number of turns, N Figure6.21 Typical form of an inductor. In view of Eq. (6.18), for an inductor to have voltageacrossitsterminals,itscurrentmustvary with time. Hence, v = 0 for constant current through the inductor. Inductance The i and constru of different found in sta inductor (so where N is area, and µ that inducta coil, using m cross-sectio Like c values and (a) 212 PART If cu acro the whe indu Am that Length, l Cross-sectional area, A Core material Number of turns, N Figure6.21 Typical form of an inductor. In view of Eq. (6.18), for an inductor to have voltageacrossitsterminals,itscurrentmustvary with time. Hence, v = 0 for constant current through the inductor. I and of d foun indu whe area that coil cros valu rang to te vari (a) (b) 212 PART 1 DC Circuits An inductor consists o If current is allowed to pass through across the inductor is directly prop the current. Using the passive sign v = where L is the constant of propor inductor. The unit of inductance is American inventor Joseph Henry (1 that 1 henry equals 1 volt-second p Length, l Cross-sectional area, A Core material Number of turns, N Figure6.21 Typical form of an inductor. In view of Eq. (6.18), for an inductor to have voltageacrossitsterminals,itscurrentmustvary with time. Hence, v = 0 for constant current through the inductor. Inductance is the property whereby an of current flowing throug The inductance of an induct and construction. Formulas for ca of different shapes are derived fro found in standard electrical engine inductor (solenoid) shown in Fig. 6 L = where N is the number of turns, ℓ area, and µ is the permeability of that inductance can be increased coil, using material with higher pe cross-sectional area, or reducing th Like capacitors, commerciall values and types. Typical practic ranging from a few microhenrys to tens of henrys (H) as in power variable. The core may be made o coil and choke are also used for ind in Fig. 6.22. The circuit symbols following the passive sign convent (a) (b) (c) Figure6.22 Various types of inductors: Equation (6.18) is the voltag Figure 6.24 shows this relationsh inductance is independent of curr linear inductor. For a nonlinear in be a straight line because its indu Solenoidal wound inductor Chip inductor Toroidal inductor
  • 25. Circuit symbols for inductors is shown in figure i-v relationship for an inductor: — Linear inductor: inductance is independent of current i-v relationship is shown in the figure — Nonlinear inductor, inductance varies with current the plot of the equation is not a straight line à where i(t0) is the total current for −∞ < t < t0 and i(−∞) = 0 — i(−∞) = 0 is reasonable because there must be a time in the past when there was no current in the inductor Integrating gives i = 1 L ! t −∞ v(t) dt (6.20) i i i (a) v L + ! (b) v L + ! (c) v L + ! Figure6.23 Circuit symbols for inductors: (a) air-core, (b) iron-core, (c) variable iron-core. or i = 1 L ! t t0 v(t) dt + i(t0) (6.21) where i(t0) is the total current for −∞ < t < t0 and i(−∞) = 0. The idea of making i(−∞) = 0 is practical and reasonable, because there must be a time in the past when there was no current in the inductor. The inductor is designed to store energy in its magnetic field. The energy stored can be obtained from Eqs. (6.18) and (6.20). The power delivered to the inductor is p = vi = " L di dt # i (6.22) The energy stored is w = ! t −∞ p dt = ! t −∞ " L di dt # i dt = L ! t −∞ i di = 1 2 Li2 (t) − 1 2 Li2 (−∞) (6.23) Since i(−∞) = 0, w = 1 2 Li2 (6.24) Slope = L di ⁄dt 0 v Figure6.24 Voltage-current relationship of an inductor. We should note the following important properties of an inductor. 1. Note from Eq. (6.18) that the voltage across an inductor is zero when the current is constant. Thus, An inductor acts like a short circuit to dc. ypes. Typical practical inductors have inductance values m a few microhenrys (µH), as in communication systems, nrys (H) as in power systems. Inductors may be fixed or e core may be made of iron, steel, plastic, or air. The terms e are also used for inductors. Common inductors are shown The circuit symbols for inductors are shown in Fig. 6.23, e passive sign convention. on (6.18) is the voltage-current relationship for an inductor. shows this relationship graphically for an inductor whose s independent of current. Such an inductor is known as a or. For a nonlinear inductor, the plot of Eq. (6.18) will not line because its inductance varies with current. We will r inductors in this textbook unless stated otherwise. rrent-voltage relationship is obtained from Eq. (6.18) as di = 1 L v dt | e of Contents Problem Solving Workbook Contents An inductor consists of a coil of conducting wire. If current is allowed to pass through an inductor, it is found that the voltage across the inductor is directly proportional to the time rate of change of the current. Using the passive sign convention, v = L di dt (6.18) where L is the constant of proportionality called the inductance of the inductor. The unit of inductance is the henry (H), named in honor of the American inventor Joseph Henry (1797–1878). It is clear from Eq. (6.18) that 1 henry equals 1 volt-second per ampere. onal area, A material f turns, N ductor. ave ary ent Inductance is the property whereby an inductor exhibits opposition to the change of current flowing through it, measured in henrys (H). The inductance of an inductor depends on its physical dimension and construction. Formulas for calculating the inductance of inductors of different shapes are derived from electromagnetic theory and can be found in standard electrical engineering handbooks. For example, for the inductor (solenoid) shown in Fig. 6.21, CHAPTER 6 Capacitors and Inductors 213 Integrating gives i = 1 L ! t −∞ v(t) dt (6.20) i i i (a) v L + ! (b) v L + ! (c) v L + ! Figure6.23 Circuit symbols for inductors: (a) air-core, (b) iron-core, (c) variable iron-core. or i = 1 L ! t t0 v(t) dt + i(t0) (6.21) where i(t0) is the total current for −∞ < t < t0 and i(−∞) = 0. The idea of making i(−∞) = 0 is practical and reasonable, because there must be a time in the past when there was no current in the inductor. The inductor is designed to store energy in its magnetic field. The energy stored can be obtained from Eqs. (6.18) and (6.20). The power delivered to the inductor is p = vi = " L di dt # i (6.22) The energy stored is w = ! t −∞ p dt = ! t −∞ " L di dt # i dt = L ! t −∞ i di = 1 2 Li2 (t) − 1 2 Li2 (−∞) (6.23) Since i(−∞) = 0, w = 1 2 Li2 (6.24) Slope = L di ⁄dt 0 v Figure6.24 Voltage-current relationship of an inductor. CHAPTER 6 Capacitors and Inductors Integrating gives i = 1 L ! t −∞ v(t) dt (6.20) i i i (a) v L + ! (b) v L + ! ( v + ! Figure6.23 Circuit symbols for induc (a) air-core, (b) iron-core, (c) variable iron-core. or i = 1 L ! t t0 v(t) dt + i(t0) (6.21) where i(t0) is the total current for −∞ < t < t0 and i(−∞) = 0. The idea of making i(−∞) = 0 is practical and reasonable, because there must be a time in the past when there was no current in the inductor. The inductor is designed to store energy in its magnetic field. The energy stored can be obtained from Eqs. (6.18) and (6.20). The power delivered to the inductor is v CHAPTER 6 Capacitors and Inductors 213 Integrating gives i = 1 L ! t −∞ v(t) dt (6.20) i i i (a) v L + ! (b) v L + ! (c) v L + ! Figure6.23 Circuit symbols for inductors: (a) air-core, (b) iron-core, (c) variable iron-core. or i = 1 L ! t t0 v(t) dt + i(t0) (6.21) where i(t0) is the total current for −∞ < t < t0 and i(−∞) = 0. The idea of making i(−∞) = 0 is practical and reasonable, because there must be a time in the past when there was no current in the inductor. The inductor is designed to store energy in its magnetic field. The energy stored can be obtained from Eqs. (6.18) and (6.20). The power delivered to the inductor is " # v i-v relationship for linear inductor
  • 26. — The power delivered to the inductor is — The energy stored in the inductor is iron-core. must be a time in the past when there was no current in the inductor. The inductor is designed to store energy in its magnetic field. The energy stored can be obtained from Eqs. (6.18) and (6.20). The power delivered to the inductor is p = vi = " L di dt # i (6.22) The energy stored is w = ! t −∞ p dt = ! t −∞ " L di dt # i dt = L ! t −∞ i di = 1 2 Li2 (t) − 1 2 Li2 (−∞) (6.23) Since i(−∞) = 0, w = 1 2 Li2 (6.24) 0 v Figure6.24 Vol relationship of an We should note the following important properties of an inductor. 1. Note from Eq. (6.18) that the voltage across an inductor is zero when the current is constant. Thus, An inductor acts like a short circuit to dc. (a) (b) Figure6.23 Circuit symbols f (a) air-core, (b) iron-core, (c) v iron-core. where i(t0) is the total current for −∞ < t < t0 and i(−∞) = 0. The idea of making i(−∞) = 0 is practical and reasonable, because there must be a time in the past when there was no current in the inductor. The inductor is designed to store energy in its magnetic field. The energy stored can be obtained from Eqs. (6.18) and (6.20). The power delivered to the inductor is p = vi = " L di dt # i (6.22) The energy stored is w = ! t −∞ p dt = ! t −∞ " L di dt # i dt = L ! t −∞ i di = 1 2 Li2 (t) − 1 2 Li2 (−∞) (6.23) Since i(−∞) = 0, w = 1 2 Li2 (6.24) Slo 0 v Figure6.24 Voltage-curre relationship of an inductor. We should note the following important properties of an inductor. 1. Note from Eq. (6.18) that the voltage across an inductor is zero when the current is constant. Thus, An inductor acts like a short circuit to dc. (a) (b) (c) Figure6.23 Circuit symbols for inductor (a) air-core, (b) iron-core, (c) variable iron-core. t0 where i(t0) is the total current for −∞ < t < t0 and i(−∞) = 0. The idea of making i(−∞) = 0 is practical and reasonable, because there must be a time in the past when there was no current in the inductor. The inductor is designed to store energy in its magnetic field. The energy stored can be obtained from Eqs. (6.18) and (6.20). The power delivered to the inductor is p = vi = " L di dt # i (6.22) The energy stored is w = ! t −∞ p dt = ! t −∞ " L di dt # i dt = L ! t −∞ i di = 1 2 Li2 (t) − 1 2 Li2 (−∞) (6.23) Since i(−∞) = 0, w = 1 2 Li2 (6.24) Slope = L di⁄dt 0 v Figure6.24 Voltage-current relationship of an inductor. We should note the following important properties of an inductor. 1. Note from Eq. (6.18) that the voltage across an inductor is zero when the current is constant. Thus, An inductor acts like a short circuit to dc.
  • 27. Important properties of an inductor: 1. From v = L di/dt , when i is constant à v = zero 2. Inductor opposes the change in current flowing through it — An inductor opposes an abrupt change in current through it — From v = L di/dt, a discontinuous change in i through an inductor requires an infinite v, which is not physically possible — Voltage across an inductor can change abruptly w = 1 2 Li2 (6.24) We should note the following important properties of an inductor. i t (a) i (b) Figure6.25 Current through an inductor: (a) allowed, (b) not allowable; an abrupt change is not possible. 1. Note from Eq. (6.18) that the voltage across an inductor is zero when the current is constant. Thus, An inductor acts like a short circuit to dc. 2. An important property of the inductor is its opposition to the change in current flowing through it. The current through an inductor cannot change instantaneously. According to Eq. (6.18), a discontinuous change in the current through an inductor requires an infinite voltage, which is not physically possible. Thus, an inductor opposes an abrupt change in the current through it. For example, the current through an inductor may take the form shown in Fig. 6.25(a), whereas the inductor current cannot take the form shown in Fig. 6.25(b) in real-life situations due to the discontinuities. However, the voltage across an inductor can change abruptly. | | | | ▲ ▲ e-Text Main Menu TextbookTable of Contents Problem Solving Workbook Contents w = 2 Li (6.24) We should note the following important properties of an inductor. i t (a) i (b) Figure6.25 Current through an inductor: (a) allowed, (b) not allowable; an abrupt change is not possible. 1. Note from Eq. (6.18) that the voltage across an inductor is zero when the current is constant. Thus, An inductor acts like a short circuit to dc. 2. An important property of the inductor is its opposition to the change in current flowing through it. The current through an inductor cannot change instantaneously. According to Eq. (6.18), a discontinuous change in the current through an inductor requires an infinite voltage, which is not physically possible. Thus, an inductor opposes an abrupt change in the current through it. For example, the current through an inductor may take the form shown in Fig. 6.25(a), whereas the inductor current cannot take the form shown in Fig. 6.25(b) in real-life situations due to the discontinuities. However, the voltage across an inductor can change abruptly. | | | | ▲ ▲ e-Text Main Menu Textbook Table of Contents Problem Solving Workbook Contents Since i(−∞) = 0, w = 1 2 Li2 (6.24) We should note the following important properties of an inductor. i t (a) i t (b) Figure6.25 Current through an inductor: (a) allowed, (b) not allowable; an abrupt change is not possible. 1. Note from Eq. (6.18) that the voltage across an inductor is zero when the current is constant. Thus, An inductor acts like a short circuit to dc. 2. An important property of the inductor is its opposition to the change in current flowing through it. The current through an inductor cannot change instantaneously. According to Eq. (6.18), a discontinuous change in the current through an inductor requires an infinite voltage, which is not physically possible. Thus, an inductor opposes an abrupt change in the current through it. For example, the current through an inductor may take the form shown in Fig. 6.25(a), whereas the inductor current cannot take the form shown in Fig. 6.25(b) in real-life situations due to the discontinuities. However, the voltage across an inductor can change abruptly. | | | ▲ ▲ e-Text Main Menu Textbook Table of Contents Problem Solving Workbook Contents
  • 28. Important properties of an inductor: 3. Like the ideal capacitor, the ideal inductor does not dissipate energy — Inductor takes power from circuit when storing energy (switch a is closed) — Inductor delivers power to circuit when returning energy (switch b is closed) 4. A practical, nonideal inductor has a significant resistive component — This resistance is called the winding resistance Rw — Rw due to the resistance of the inductor material — Rw appears in series with the inductance — Rw makes inductor an energy storage & dissipation device — Rw is usually very small (ignorable) 5. The nonideal inductor has a winding capacitance Cw — Cw due to capacitive coupling between conducting coils — Cw is very small & can be ignored in most cases, except at high frequencies 214 PAR Since an inductor is often made of a highly con- ducting wire, it has a very small resistance. L Rw Cw Figure6.26 Circuit model for a practical inductor. E X A M P L E 6 . 8 The 214 Since an inductor is often made of a highly con- ducting wire, it has a very small resistance. The time constant tC describes the fall of the charge as well as its rise. An analogous slowing of the rise (or fall) of the current occurs if we introduce n emf ! into (or remove it from) a single-loop circuit containing a resistor R and n inductor L.When the switch S in Fig. 30-15 is closed on a, for example, the cur- ent in the resistor starts to rise. If the inductor were not present, the current would rise rapidly to a steady value !/R. Because of the inductor, however, a self- nduced emf !L appears in the circuit; from Lenz’s law, this emf opposes the rise of he current, which means that it opposes the battery emf ! in polarity. Thus, the urrent in the resistor responds to the difference between two emfs, a constant ! ue to the battery and a variable !L (! "L di/dt) due to self-induction.As long as !L is present,the current will be less than !/R. As time goes on, the rate at which the current increases becomes less rapid nd the magnitude of the self-induced emf, which is proportional to di/dt, ecomes smaller. Thus, the current in the circuit approaches !/R asymptotically. Fig. 30-15 An RL circuit.When switch S is closed on a, the current rises and ap- proaches a limiting value !/R. S a b R L – +
  • 29. very small and can be ignored in most cases, except at high frequencies. We will assume ideal inductors in this book. E X A M P L E 6 . 8 The current through a 0.1-H inductor is i(t) = 10te−5t A. Find the voltage across the inductor and the energy stored in it. Solution: Since v = L di/dt and L = 0.1 H, v = 0.1 d dt (10te−5t ) = e−5t + t(−5)e−5t = e−5t (1 − 5t) V The energy stored is w = 1 2 Li2 = 1 2 (0.1)100t2 e−10t = 5t2 e−10t J P R A C T I C E P R O B L E M 6 . 8 If the current through a 1-mH inductor is i(t) = 20 cos 100t mA, find the terminal voltage and the energy stored. Answer: −2 sin 100t mV, 0.2 cos2 100t µJ. E X A M P L E 6 . 9 Find the current through a 5-H inductor if the voltage across it is v(t) = ! 30t2 , t > 0 0, t < 0 Also find the energy stored within 0 < t < 5 s. Solution: Since i = 1 L " t t0 v(t) dt + i(t0) and L = 5 H, i = 1 5 " t 0 30t2 dt + 0 = 6 × t3 3 = 2t3 A Since v = L di/dt and L = 0.1 H, v = 0.1 d dt (10te−5t ) = e−5t + t(−5)e−5t = e−5t (1 − 5t) V The energy stored is w = 1 2 Li2 = 1 2 (0.1)100t2 e−10t = 5t2 e−10t J P R A C T I C E P R O B L E M 6 . 8 If the current through a 1-mH inductor is i(t) = 20 cos 100t mA, find the terminal voltage and the energy stored. Answer: −2 sin 100t mV, 0.2 cos2 100t µJ. E X A M P L E 6 . 9 Find the current through a 5-H inductor if the voltage across it is v(t) = ! 30t2 , t > 0 0, t < 0 Also find the energy stored within 0 < t < 5 s. Solution: Since i = 1 L " t t0 v(t) dt + i(t0) and L = 5 H, i = 1 5 " t 0 30t2 dt + 0 = 6 × t3 3 = 2t3 A | | | | ▲ ▲ e-Text Main Menu TextbookTable of Contents Problem Solving Workbook Contents
  • 30. CHAPTER 6 Capacitors and Inductors 215 The power p = vi = 60t5 , and the energy stored is then w = ! p dt = ! 5 0 60t5 dt = 60 t6 6 " " " " 5 0 = 156.25 kJ Alternatively, we can obtain the energy stored using Eq. (6.13), by writing w " "5 0 = 1 2 Li2 (5) − 1 2 Li(0) = 1 2 (5)(2 × 53 )2 − 0 = 156.25 kJ as obtained before. P R A C T I C E P R O B L E M 6 . 9 The terminal voltage of a 2-H inductor is v = 10(1−t) V. Find the current flowing through it at t = 4 s and the energy stored in it within 0 < t < 4 s. Assume i(0) = 2 A. Answer: −18 A, 320 J. E X A M P L E 6 . 1 0 Consider the circuit in Fig. 6.27(a). Under dc conditions, find: (a) i, vC, and iL, (b) the energy stored in the capacitor and inductor. 12 V 1 F + ! 4 " 5 " 1 " 2 H i iL vC + ! (a) (a) Solution: (a) Under dc conditions, we replace the capacitor with an open circuit and the inductor with a short circuit, as in Fig. 6.27(b). It is evident from Fig. 6.27(b) that i = iL = 12 = 2 A
  • 31. Answer: −18 A, 320 J. E X A M P L E 6 . 1 0 Consider the circuit in Fig. 6.27(a). Under dc conditions, find: (a) i, vC, and iL, (b) the energy stored in the capacitor and inductor. 12 V 1 F + ! 4 " 5 " 1 " 2 H i iL vC + ! vC + ! (a) (a) 12 V + ! 4 " 5 " 1 " i iL (b) Figure6.27 For Example 6.10. Solution: (a) Under dc conditions, we replace the capacitor with an open circuit and the inductor with a short circuit, as in Fig. 6.27(b). It is evident from Fig. 6.27(b) that i = iL = 12 1 + 5 = 2 A The voltage vC is the same as the voltage across the 5-! resistor. Hence, vC = 5i = 10 V (b) The energy in the capacitor is wC = 1 2 Cv2 C = 1 2 (1)(102 ) = 50 J and that in the inductor is wL = 1 2 Li2 L = 1 2 (2)(22 ) = 4 J P R A C T I C E P R O B L E M 6 . 1 0 Determine vC, iL, and the energy stored in the capacitor and inductor in the circuit of Fig. 6.28 under dc conditions. 4 A 2 F 3 " 1 " 0.25 H iL vC + ! Figure6.28 For Practice Prob. 6.10. Answer: 3 V, 3 A, 9 J, 1.125 J. @ !" = 5 1 + 5 12 = 10)
  • 32. w = 1 2 Li2 = 1 2 (0.1)100t2 e−10t = 5t2 e−10t J P R A C T I C E P R O B L E M 6 . 8 If the current through a 1-mH inductor is i(t) = 20 cos 100t mA, find the terminal voltage and the energy stored. Answer: −2 sin 100t mV, 0.2 cos2 100t µJ. E X A M P L E 6 . 9 Find the current through a 5-H inductor if the voltage across it is v(t) = ! 30t2 , t > 0 0, t < 0 Also find the energy stored within 0 < t < 5 s. Solution: Since i = 1 L " t t0 v(t) dt + i(t0) and L = 5 H, i = 1 5 " t 0 30t2 dt + 0 = 6 × t3 3 = 2t3 A | | | | ▲ ▲ e-Text Main Menu TextbookTable of Contents Problem Solving Workbook Contents The power p = vi = 60t5 , and the energy stored is then w = ! p dt = ! 5 0 60t5 dt = 60 t6 6 " " " " 5 0 = 156.25 kJ Alternatively, we can obtain the energy stored using Eq. (6.13), by writing w " "5 0 = 1 2 Li2 (5) − 1 2 Li(0) = 1 2 (5)(2 × 53 )2 − 0 = 156.25 kJ as obtained before. P R A C T I C E P R O B L E M 6 . 9 The terminal voltage of a 2-H inductor is v = 10(1−t) V. Find the current flowing through it at t = 4 s and the energy stored in it within 0 < t < 4 s. Assume i(0) = 2 A. Answer: −18 A, 320 J. E X A M P L E 6 . 1 0 Consider the circuit in Fig. 6.27(a). Under dc conditions, find: (a) i, vC, and iL, (b) the energy stored in the capacitor and inductor. 12 V 1 F + ! 4 " 5 " 1 " 2 H i iL vC + ! (a) (a) 5 " 1 " i Solution: (a) Under dc conditions, we replace the capacitor with an open circuit and the inductor with a short circuit, as in Fig. 6.27(b). It is evident from Fig. 6.27(b) that i = iL = 12 1 + 5 = 2 A The voltage vC is the same as the voltage across the 5-! resistor. Hence, Consider the circuit in Fig. 6.27(a). Under dc conditions, find: (a) i, vC, and iL, (b) the energy stored in the capacitor and inductor. 12 V 1 F + ! 4 " 5 " 1 " 2 H i iL vC + ! vC + ! (a) (a) 12 V + ! 4 " 5 " 1 " i iL (b) Figure6.27 For Example 6.10. Solution: (a) Under dc conditions, we replace the capacitor with an open circuit and the inductor with a short circuit, as in Fig. 6.27(b). It is evident from Fig. 6.27(b) that i = iL = 12 1 + 5 = 2 A The voltage vC is the same as the voltage across the 5-! resistor. Hence, vC = 5i = 10 V (b) The energy in the capacitor is wC = 1 2 Cv2 C = 1 2 (1)(102 ) = 50 J and that in the inductor is wL = 1 2 Li2 L = 1 2 (2)(22 ) = 4 J P R A C T I C E P R O B L E M 6 . 1 0 Determine vC, iL, and the energy stored in the capacitor and inductor in the circuit of Fig. 6.28 under dc conditions. 4 A 2 F 3 " 1 " 0.25 H iL vC + ! Figure6.28 For Practice Prob. 6.10. Answer: 3 V, 3 A, 9 J, 1.125 J. @
  • 33. SERIES INDUCTORS The equivalent inductance Leq of N inductors in series: — Same i flows through the inductors — Applying KVL to the loop (Inductors in series are combined in exactly the same way as resistors in series) 6.5 S Now tha necessar need to k or parall Co 6.29(a), have the L1 (a) L2 L3 LN (b) Leq i i v + ! v + ! + ! v1 + ! v2 + ! v3 + ! vN . . . Figure6.29 (a) A series connection of N inductors, (b) equivalent circuit for the series inductors. Substitu where Thus, Inductor series. PART 1 DC Circuits 6.5 SERIES AND PARALLEL INDUCTORS Now that the inductor has been added to our list of passive elements, it is necessary to extend the powerful tool of series-parallel combination. We need to know how to find the equivalent inductance of a series-connected or parallel-connected set of inductors found in practical circuits. Consider a series connection of N inductors, as shown in Fig. 6.29(a), with the equivalent circuit shown in Fig. 6.29(b). The inductors have the same current through them. Applying KVL to the loop, v = v1 + v2 + v3 + · · · + vN (6.25) LN Leq + ! vN . . ection of N for the Substituting vk = Lk di/dt results in v = L1 di dt + L2 di dt + L3 di dt + · · · + LN di dt = (L1 + L2 + L3 + · · · + LN ) di dt = ! N " k=1 Lk # di dt = Leq di dt (6.26) where Leq = L1 + L2 + L3 + · · · + LN (6.27) Thus, The equivalent inductance of series-connected inductors is the sum of the individual inductances. Inductors in series are combined in exactly the same way as resistors in series. to know how to find the equivalent inductance of a series-connected rallel-connected set of inductors found in practical circuits. Consider a series connection of N inductors, as shown in Fig. a), with the equivalent circuit shown in Fig. 6.29(b). The inductors the same current through them. Applying KVL to the loop, v = v1 + v2 + v3 + · · · + vN (6.25) tituting vk = Lk di/dt results in v = L1 di dt + L2 di dt + L3 di dt + · · · + LN di dt = (L1 + L2 + L3 + · · · + LN ) di dt = ! N " k=1 Lk # di dt = Leq di dt (6.26) e Leq = L1 + L2 + L3 + · · · + LN (6.27) , The equivalent inductance of series-connected inductors is the sum of the individual inductances. need to know how to find the equivalent inductance of a series-connected or parallel-connected set of inductors found in practical circuits. Consider a series connection of N inductors, as shown in Fig. 6.29(a), with the equivalent circuit shown in Fig. 6.29(b). The inductors have the same current through them. Applying KVL to the loop, v = v1 + v2 + v3 + · · · + vN (6.25) LN + ! vN n of N he Substituting vk = Lk di/dt results in v = L1 di dt + L2 di dt + L3 di dt + · · · + LN di dt = (L1 + L2 + L3 + · · · + LN ) di dt = ! N " k=1 Lk # di dt = Leq di dt (6.26) where Leq = L1 + L2 + L3 + · · · + LN (6.27) Thus, The equivalent inductance of series-connected inductors is the sum of the individual inductances. Inductors in series are combined in exactly the same way as resistors in series.
  • 34. PARALLEL INDUCTORS The equivalent inductance Leq of N inductors in parallel: — Inductors have same v across them — Applying KCL For N = 2 (Inductors in parallel are combined in the same way as resistors in parallel) Induct series. (a) (b) Leq i v + ! v + ! L1 L2 L3 LN i i1 i2 i3 iN Figure6.30 (a) A parallel connection of N inductors, (b) equivalent circuit for the parallel inductors. in Fig. have th But ik i | | | ▲ ▲ e-Text Main Menu Textboo CHAPTER 6 Capacitors and Inductors 217 where 1 Leq = 1 L1 + 1 L2 + 1 L3 + · · · + 1 LN (6.30) he initial current i(t0) through Leq at t = t0 is expected by KCL to be he sum of the inductor currents at t0. Thus, according to Eq. (6.29), i(t0) = i1(t0) + i2(t0) + · · · + iN (t0) According to Eq. (6.30), The equivalent inductance of series-connected inductors is the sum of the individual inductances. Inductors in series are combined in exactly the same way as resistors in series. LN iN n of N e parallel We now consider a parallel connection of N inductors, as shown in Fig. 6.30(a), with the equivalent circuit in Fig. 6.30(b). The inductors have the same voltage across them. Using KCL, i = i1 + i2 + i3 + · · · + iN (6.28) But ik = 1 Lk $ t t0 v dt + ik(t0); hence, i = 1 L1 $ t t0 v dt + i1(t0) + 1 L2 $ t t0 v dt + i2(t0) + · · · + 1 LN $ t t0 v dt + iN (t0) = % 1 L1 + 1 L2 + · · · + 1 LN & $ t t0 v dt + i1(t0) + i2(t0) + · · · + iN (t0) = ! N " k=1 1 Lk # $ t t0 v dt + N " k=1 ik(t0) = 1 Leq $ t t0 v dt + i(t0) (6.29) | | Menu TextbookTable of Contents Problem Solving Workbook Contents Inductors in series are combined in exactly the same way as resistors in series. (a) (b) Leq i v + ! v + ! L1 L2 L3 LN i i1 i2 i3 iN Figure6.30 (a) A parallel connection of N inductors, (b) equivalent circuit for the parallel inductors. We now consider a parallel connection of N inductors, as shown in Fig. 6.30(a), with the equivalent circuit in Fig. 6.30(b). The inductors have the same voltage across them. Using KCL, i = i1 + i2 + i3 + · · · + iN (6.28) But ik = 1 Lk $ t t0 v dt + ik(t0); hence, i = 1 L1 $ t t0 v dt + i1(t0) + 1 L2 $ t t0 v dt + i2(t0) + · · · + 1 LN $ t t0 v dt + iN (t0) = % 1 L1 + 1 L2 + · · · + 1 LN & $ t t0 v dt + i1(t0) + i2(t0) + · · · + iN (t0) = ! N " k=1 1 Lk # $ t t0 v dt + N " k=1 ik(t0) = 1 Leq $ t t0 v dt + i(t0) (6.29) | | | | ▲ ▲ e-Text Main Menu TextbookTable of Contents Problem Solving Workbook Contents sum of the individual inductances. Inductors in series are combined in exactly the same way as resistors in series. LN iN on of N he parallel We now consider a parallel connection of N inductors, as shown in Fig. 6.30(a), with the equivalent circuit in Fig. 6.30(b). The inductors have the same voltage across them. Using KCL, i = i1 + i2 + i3 + · · · + iN (6.28) But ik = 1 Lk $ t t0 v dt + ik(t0); hence, i = 1 L1 $ t t0 v dt + i1(t0) + 1 L2 $ t t0 v dt + i2(t0) + · · · + 1 LN $ t t0 v dt + iN (t0) = % 1 L1 + 1 L2 + · · · + 1 LN & $ t t0 v dt + i1(t0) + i2(t0) + · · · + iN (t0) = ! N " k=1 1 Lk # $ t t0 v dt + N " k=1 ik(t0) = 1 Leq $ t t0 v dt + i(t0) (6.29) | | Menu TextbookTable of Contents Problem Solving Workbook Contents Leq L3 LN iN connection of N cuit for the parallel We now consider a parallel connection of N inductors, as shown in Fig. 6.30(a), with the equivalent circuit in Fig. 6.30(b). The inductors have the same voltage across them. Using KCL, i = i1 + i2 + i3 + · · · + iN (6.28) But ik = 1 Lk $ t t0 v dt + ik(t0); hence, i = 1 L1 $ t t0 v dt + i1(t0) + 1 L2 $ t t0 v dt + i2(t0) + · · · + 1 LN $ t t0 v dt + iN (t0) = % 1 L1 + 1 L2 + · · · + 1 LN & $ t t0 v dt + i1(t0) + i2(t0) + · · · + iN (t0) = ! N " k=1 1 Lk # $ t t0 v dt + N " k=1 ik(t0) = 1 Leq $ t t0 v dt + i(t0) (6.29) | | Main Menu TextbookTable of Contents Problem Solving Workbook Contents series. LN N of N parallel We now consider a parallel connection of N inductors, as shown in Fig. 6.30(a), with the equivalent circuit in Fig. 6.30(b). The inductors have the same voltage across them. Using KCL, i = i1 + i2 + i3 + · · · + iN (6.28) But ik = 1 Lk $ t t0 v dt + ik(t0); hence, i = 1 L1 $ t t0 v dt + i1(t0) + 1 L2 $ t t0 v dt + i2(t0) + · · · + 1 LN $ t t0 v dt + iN (t0) = % 1 L1 + 1 L2 + · · · + 1 LN & $ t t0 v dt + i1(t0) + i2(t0) + · · · + iN (t0) = ! N " k=1 1 Lk # $ t t0 v dt + N " k=1 ik(t0) = 1 Leq $ t t0 v dt + i(t0) (6.29) | | enu TextbookTable of Contents Problem Solving Workbook Contents CHAPTER 6 Capacitors and Inductors 217 here 1 Leq = 1 L1 + 1 L2 + 1 L3 + · · · + 1 LN (6.30) e initial current i(t0) through Leq at t = t0 is expected by KCL to be e sum of the inductor currents at t0. Thus, according to Eq. (6.29), i(t0) = i1(t0) + i2(t0) + · · · + iN (t0) cording to Eq. (6.30), The equivalent inductance of parallel inductors is the reciprocal of the sum of the reciprocals of the individual inductances. ote that the inductors in parallel are combined in the same way as resis- s in parallel. CHAPTER 6 Capacitors and Inductors 217 where 1 Leq = 1 L1 + 1 L2 + 1 L3 + · · · + 1 LN (6.30) The initial current i(t0) through Leq at t = t0 is expected by KCL to be the sum of the inductor currents at t0. Thus, according to Eq. (6.29), i(t0) = i1(t0) + i2(t0) + · · · + iN (t0) According to Eq. (6.30), The equivalent inductance of parallel inductors is the reciprocal of the sum of the reciprocals of the individual inductances. Note that the inductors in parallel are combined in the same way as resis- CHAPTER 6 Capacitors and Inductors where 1 Leq = 1 L1 + 1 L2 + 1 L3 + · · · + 1 LN (6.30) The initial current i(t0) through Leq at t = t0 is expected by KCL to be the sum of the inductor currents at t0. Thus, according to Eq. (6.29), i(t0) = i1(t0) + i2(t0) + · · · + iN (t0) According to Eq. (6.30), The equivalent inductance of parallel inductors is the reciprocal of the sum of the reciprocals of the individual inductances. Note that the inductors in parallel are combined in the same way as resis- tors in parallel. For two inductors in parallel (N = 2), Eq. (6.30) becomes 1 Leq = 1 L1 + 1 L2 or Leq = L1L2 L1 + L2 (6.31) It is appropriate at this point to summarize the most important character- istics of the three basic circuit elements we have studied. The summary is given in Table 6.1. TABLE 6.1 Important characteristics of the basic elements.†
  • 35. E X A M P L E 6 . 1 1 Find the equivalent inductance of the circuit shown in Fig. 6.31. | | | | ▲ ▲ e-Text Main Menu Textbook Table of Contents Problem Solving Workbook Contents 218 PART 1 DC Circuits 4 H 20 H 8 H 10 H 12 H 7 H Leq Figure6.31 For Example 6.11. Solution: The 10-H, 12-H, and 20-H inductors are in series; thus, combining them gives a 42-H inductance. This 42-H inductor is in parallel with the 7-H inductor so that they are combined, to give 7 × 42 7 + 42 = 6 H This 6-H inductor is in series with the 4-H and 8-H inductors. Hence, Leq = 4 + 6 + 8 = 18 H P R A C T I C E P R O B L E M 6 . 1 1 Calculate the equivalent inductance for the inductive ladder network in Fig. 6.32. 20 mH 100 mH 40 mH 30 mH 20 mH 40 mH 50 mH Leq Figure6.32 For Practice Prob. 6.11. Leq = 4 + 6 + 8 = 18 H P R A C T I C E P R O B L E M 6 . 1 1 Calculate the equivalent inductance for the inductive ladder network in Fig. 6.32. 20 mH 100 mH 40 mH 30 mH 20 mH 40 mH 50 mH Leq Figure6.32 For Practice Prob. 6.11. Answer: 25 mH. E X A M P L E 6 . 1 2 For the circuit in Fig. 6.33, i(t) = 4(2 − e−10t ) mA. If i2(0) = −1 mA, find: (a) i1(0); (b) v(t), v1(t), and v2(t); (c) i1(t) and i2(t). 4 H 12 H 4 H v + ! v2 v1 + + ! ! i i1 i2 Figure6.33 For Example 6.12. Solution: (a) From i(t) = 4(2 − e−10t ) mA, i(0) = 4(2 − 1) = 4 mA. Since i = i1 + i2, i1(0) = i(0) − i2(0) = 4 − (−1) = 5 mA (b) The equivalent inductance is Leq = 2 + 4 ∥ 12 = 2 + 3 = 5 H Thus, 2 H
  • 36. (b) The equivalent inductance is Leq = 2 + 4 ∥ 12 = 2 + 3 = 5 H Thus, v(t) = Leq di dt = 5(4)(−1)(−10)e−10t mV = 200e−10t mV and v1(t) = 2 di dt = 2(−4)(−10)e−10t mV = 80e−10t mV Since v = v1 + v2, v2(t) = v(t) − v1(t) = 120e−10t mV | | ain Menu TextbookTable of Contents Problem Solving Workbook Contents CHAPTER 6 Capacitors and Inductors (c) The current i1 is obtained as i1(t) = 1 4 ! t 0 v2 dt + i1(0) = 120 4 ! t 0 e−10t dt + 5 mA = −3e−10t " "t 0 + 5 mA = −3e−10t + 3 + 5 = 8 − 3e−10t mA Similarly, i2(t) = 1 12 ! t 0 v2 dt + i2(0) = 120 12 ! t 0 e−10t dt − 1 mA = −e−10t " "t 0 − 1 mA = −e−10t + 1 − 1 = −e−10t mA Note that i1(t) + i2(t) = i(t). P R A C T I C E P R O B L E M 6 . 1 2 For the circuit in Fig. 6.33, i(t) = 4(2 − e−10t ) mA. If i2(0) = −1 mA, find: (a) i1(0); (b) v(t), v1(t), and v2(t); (c) i1(t) and i2(t). 2 H 2 Solution: (a) From i(t) = 4(2 − e−10t ) mA, i(0) = 4(2 − 1) = 4 mA. Since i = i1 + i2, i1(0) = i(0) − i2(0) = 4 − (−1) = 5 mA (b) The equivalent inductance is Leq = 2 + 4 ∥ 12 = 2 + 3 = 5 H Thus, v(t) = Leq di dt = 5(4)(−1)(−10)e−10t mV = 200e−10t mV and v1(t) = 2 di dt = 2(−4)(−10)e−10t mV = 80e−10t mV Since v = v1 + v2, v2(t) = v(t) − v1(t) = 120e−10t mV | | nu TextbookTable of Contents Problem Solving Workbook Contents E X A M P L E 6 . 1 2 For find 4 H 12 H 4 H v + ! v2 v1 + + ! ! i i1 i2 Figure6.33 For Example 6.12. Solu (a) F i1 + (b) T Thu and Sinc | | | ▲ ▲ e-Text Main Menu Textbo 2 H 212 PART 1 DC Circuits An inductor consists of a coil of conducting wire. If current is allowed to pass through an inductor, it is found that the across the inductor is directly proportional to the time rate of ch the current. Using the passive sign convention, v = L di dt where L is the constant of proportionality called the inductance inductor. The unit of inductance is the henry (H), named in hono American inventor Joseph Henry (1797–1878). It is clear from Eq that 1 henry equals 1 volt-second per ampere. Length, l Cross-sectional area, A Core material Number of turns, N Figure6.21 Typical form of an inductor. In view of Eq. (6.18), for an inductor to have voltageacrossitsterminals,itscurrentmustvary with time. Hence, v = 0 for constant current through the inductor. Inductance is the property whereby an inductor exhibits opposition to the ch of current flowing through it, measured in henrys (H). The inductance of an inductor depends on its physical dim and construction. Formulas for calculating the inductance of in of different shapes are derived from electromagnetic theory and found in standard electrical engineering handbooks. For example inductor (solenoid) shown in Fig. 6.21, L = N2 µA ℓ where N is the number of turns, ℓ is the length, A is the cross-se area, and µ is the permeability of the core. We can see from Eq that inductance can be increased by increasing the number of t coil, using material with higher permeability as the core, increas cross-sectional area, or reducing the length of the coil. Like capacitors, commercially available inductors come in d values and types. Typical practical inductors have inductance (a) CHAPTER 6 Capacitors and Integrating gives i = 1 L ! t −∞ v(t) dt or i = 1 L ! t t0 v(t) dt + i(t0) where i(t0) is the total current for −∞ < t < t0 and i(−∞) = idea of making i(−∞) = 0 is practical and reasonable, becaus must be a time in the past when there was no current in the induc The inductor is designed to store energy in its magnetic fie energy stored can be obtained from Eqs. (6.18) and (6.20). The delivered to the inductor is p = vi = " L di dt # i The energy stored is w = ! t −∞ p dt = ! t −∞ " L di dt # i dt = L ! t −∞ i di = 1 2 Li2 (t) − 1 2 Li2 (−∞) Since i(−∞) = 0, w = 1 2 Li2 We should note the following important properties of an in 1. Note from Eq. (6.18) that the voltage across an inductor
  • 37. Leq = 4 + 6 + 8 = 18 H P R A C T I C E P R O B L E M 6 . 1 1 Calculate the equivalent inductance for the inductive ladder network in Fig. 6.32. 20 mH 100 mH 40 mH 30 mH 20 mH 40 mH 50 mH Leq Figure6.32 For Practice Prob. 6.11. Answer: 25 mH. E X A M P L E 6 . 1 2 For the circuit in Fig. 6.33, i(t) = 4(2 − e−10t ) mA. If i2(0) = −1 mA, find: (a) i1(0); (b) v(t), v1(t), and v2(t); (c) i1(t) and i2(t). 4 H 12 H 4 H v + ! v2 v1 + + ! ! i i1 i2 Figure6.33 For Example 6.12. Solution: (a) From i(t) = 4(2 − e−10t ) mA, i(0) = 4(2 − 1) = 4 mA. Since i = i1 + i2, i1(0) = i(0) − i2(0) = 4 − (−1) = 5 mA (b) The equivalent inductance is Leq = 2 + 4 ∥ 12 = 2 + 3 = 5 H Thus,