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Topic --- Alternating Current
Topic --- Alternating Current
21 ALTERNATING
CURRENT
21.1
Alternating
current
21.2
Root mean
square
(rms)
21.3
Resistance,
reactance
&
impedance
21.4
Power and
power
factor
Topic --- Alternating Current
(a) Define alternating current
(AC)
(b) Sketch and analyse
sinusoidal AC waveform
(c) Use sinusoidal voltage and
current equations
tII sin0 tVV sin0
21.1 ALTERNATING CURRENT
Topic --- Alternating Current
Alternating current (AC) electricity is the
type of electricity commonly used in
homes and businesses throughout the
world. While direct current (DC)
electricity flows in one direction through
a wire, AC electricity alternates its
direction in a back-and-forth motion.
The direction alternates between 50 and
60 times per second, depending on the
electrical system of the country.
AC created by an AC electric
generator, which determines
the frequency
the voltage can be readily
changed, thus making it more
suitable for long-distance
transmission than DC electricity
can employ capacitors and
inductors in electronic circuitry,
allowing for a wide range of
applications
21.1 ALTERNATING CURRENT
Topic --- Alternating Current
• is defined as an electric
current which magnitude
& direction change
periodically
• Symbol:
21.1 ALTERNATING CURRENT
Topic --- Alternating Current
Current
Voltage The output of an ac
generator is sinusoidal
and varies with time
locityangular veORfrequencyangular:
currentpeak:0I gepeak volta:0V
time:t
)2( f  In general,

21.1 ALTERNATING CURRENT
tII o sin
tVV o sin
Topic --- Alternating Current
f
T
1

21.1 ALTERNATING CURRENT
Topic --- Alternating Current
21.2 ROOT MEANC SQUARE (rms)
(a) Define root mean square
(rms) current and voltage
for AC source
(b) Use the following formula,
and
2
0
rms
I
I 
2
0
rms
V
V 
Topic --- Alternating Current
rmsI
acdc poweraveragepower 
RIRI ave
22

aveII 2

Mean or average current, Iave: the average
or mean value of current in a half-cycle
flows of current in a certain direction
  
0
2
0
ave
2II
I 
averms
II 2

The r.m.s (root mean square) current means the
square root of the average value of the current
21.2 ROOT MEAN SQUARE (rms)
Topic --- Alternating Current
• Since ac current
and
• So
and
• The root mean square (rms)
current is the effective value of
the AC
tII sin0
averms
II 2

  averms
tsinII
2
0

2
0
rms
I
I 
 
2
1
sinsin 22
 tt 
21.2 ROOT MEAN SQUARE (rms)
Topic --- Alternating Current
• The equation:
• Most household electricity is 240
V AC which means that Vrms is
240 V
2
0
rms
V
V 
21.2 ROOT MEAN SQUARE (rms)
Topic --- Alternating Current
A sinusoidal, 60.0 Hz, ac
voltage is read to be 120 V by
an ordinary voltmeter.
(a) What is the maximum value
the voltage takes on during
a cycle?
(b)b) What is the equation for
the voltage?
Solution:
(a)
(b)
2
o
rms
V
V 
)V(V rmso
2
V170
tsinVV o

tsin 120170
21.2 ROOT MEAN SQUARE (rms)
Topic --- Alternating Current
A voltage V= 60 sin 120πt is
applied across a 20 Ω resistor.
(a)What will an ac ammeter in
series with the resistor
read?
(b)Calculate the peak current
and mean power.
2
o
rms
V
V 
R
V
I rms
rms

)I(I rmso
2
2
0
I
Irms

 
W90(20)(2.12)2
2

 RIP rmsav
Solution:
(a)
(b)
21.2 ROOT MEAN SQUARE (rms)
Topic --- Alternating Current
1. An AC source
V = 500 sin t
is connected across a resistor of
250 . Calculate
(a) the rms current in the resistor,
(b) the peak current,
(c) the mean power.
2. Figure below shows a graph to
represent alternating current
passes through a resistor of 10
k. Calculate
(a) the rms current,
(b) the frequency of the AC,
(c) the mean power dissipated from
the resistor.
21.2 ROOT MEAN SQUARE (rms)
Topic --- Alternating Current
(a) Sketch and use phasor
diagram and sinusoidal
waveform to show the
phase relationship
between current and
voltage for a circuit
consisting of
(i) pure resistor
(ii)pure capacitor
(iii)pure inductor
(b) Use phasor diagram to
analyse voltage, current,
and impedance of series
circuit of:
(i) RL
(ii)RC
(iii)RLC
21.3 RESISTANCE, REACTANCE & IMPEDANCE
Topic --- Alternating Current
(c) Define and use:
(i) capacitive reactance,
(ii) inductive reactance,
(ii)impedance,
(iv)phase angle,
(d) Explain
graphically the
dependence of
R, XC, XL and Z
on f and relate it
to resonance
fC
XC
2
1

fLXL 2
 22
CL XXRZ 
R
XX CL 
tan
Topic --- Alternating Current
A diagram containing
phasor is called
phasor diagram
Phasor  a vector
that rotate
anticlockwise about
its axis with constant
angular velocity
Used to represent a
sinusoidally varying quantity
such as alternating current
(AC) & alternating voltage
Also being used to determine
the phase angle  the phase
difference between current
and voltage in AC circuit
21.3 RESISTANCE, REACTANCE & IMPEDANCE
Topic --- Alternating Current
Phase difference is
22

 





 tt
21.3 RESISTANCE, REACTANCE & IMPEDANCE
Note:
valuepositive
radian 
valuenegative
Leads
Lags behind
In antiphase
Topic --- Alternating Current
• The projection
of OP on the
y-axis is ON,
represents the
instantaneous
value
• Ao is the
peak value
of the
quantity
t0 TT
2
1 T2T
2
3
Ao
tAA o sin
y
ω
N
O
P
y
21.3 RESISTANCE, REACTANCE & IMPEDANCE
Topic --- Alternating Current
• It is defined by
OR
• It is a scalar quantity
and its unit is ohm ()
• In a DC circuit,
impedance likes the
resistance
rms
rms
I
V
Z 
2
0V
2
0I
0
0
I
V
Z 
21.3 RESISTANCE, REACTANCE & IMPEDANCE
V = IZ
 V = IR
 V = IX
 V = IXC
 V = IXL
Topic --- Alternating Current
Resistance, R: Opposition to current flow in purely resistive circuit
Reactance, X: Opposition to current flow resulting from
inductance or capacitance in ac circuit
Capacitive reactance, XC: Opposition of a capacitor to ac
Inductive reactance, XL: Opposition of an inductor to ac
Impedance, Z: Total opposition to ac (Resistance and reactance
combine to form impedance)
21.3 RESISTANCE, REACTANCE & IMPEDANCE
Topic --- Alternating Current
AC is defined as an electric
current which magnitude
& direction change
periodically
In general,

Root mean square current
(Irms) is defined as the
effective value of a.c. which
produces the same power as
the steady d.c. when the
current passes through the
same resistor
2
0
rms
I
I 
Root mean square voltage/
p.d (Vrms): the value of the
steady direct voltage which
when applied across a resistor,
produces the same power as
the mean (average) power
produced by the alternating
voltage across the same
resistor
2
0
rms
V
V 
A diagram containing
phasor is called
phasor diagram
Phasor  a vector
that rotate
anticlockwise about
its axis with constant
angular velocity
Impedance, Z
Resistance, R
in resistor
Reactance, X
Capacitive
reactance, XC
in capacitor, C
Inductive
reactance, XL
in inductor, L
V = IZ
 V = IR
 V = IX
 V = IXC
 V = IXL
Topic --- Alternating Current
• The circuit
• The alternating current passes through
the resistor is given by
• The alternating voltage across the
resistor VR at any instant is given by
where, V = supply voltage
tII sin0
IRVR 
00 VRI  RtI sin0 and
VtVVR  sin0
21.3 RESISTANCE, REACTANCE & IMPEDANCE
Pure Capacitor In
AC Circuit
Topic --- Alternating Current
• From figure above: I = I0 sin t
and V = V0 sin t
• Thus the phase difference is
Impedance in a pure
resistor
• From the
definition of the
impedance, hence
t0
0I
0V
0I
0V
TT
2
1 T2
T
2
3
ω
VI
0 tt 
Therefore the
current I is in
phase with the
voltage V and
constant with
time
R
I
V
I
V
Z 
0
0
rms
rms
21.3 RESISTANCE, REACTANCE & IMPEDANCE
Pure Capacitor In
AC Circuit
Topic --- Alternating Current
CIRCUIT
AC source
R
I
RV
V
PHASE DIFFERENCE
tII sin0
VtVVR  sin0
0
IMPEDANCE, Z
PHASOR
DIAGRAM
VI
RZ 
In pure resistor, the current I always in phase
with the voltage V and constant with time
R
I
V
I
V
Z 
0
0
rms
rms
21.3 RESISTANCE, REACTANCE & IMPEDANCE
Pure Capacitor In
AC Circuit
Topic --- Alternating Current
• The circuit
• The alternating voltage across the
capacitor VC at any instant is equal to the
supply voltage V and is given by
• The charge accumulates at the plates of
the capacitor is
21.3 RESISTANCE, REACTANCE & IMPEDANCE
AC source
CV
V
C
I
tVVVC sin0
CCVQ 
tCVQ sin0
Pure Capacitor In
AC Circuit
Topic --- Alternating Current
• The charge and current are related by
Hence the equation of AC in the capacitor is
and
OR
dt
dQ
I 
 tCV
dt
d
I sin0
 t
dt
d
CV sin0
tCV cos0 00 ICV 
tII cos0







2
sin0

tII
21.3 RESISTANCE, REACTANCE & IMPEDANCE
Pure Capacitor In
AC Circuit
Topic --- Alternating Current
• From figure above: V = V0 sin t
and I = I0 sin (t + /2)
• Thus the phase difference is
the voltage V lags
behind the current I
by /2 radians
OR
the current I leads
the voltage V by /2
radians
21.3 RESISTANCE, REACTANCE & IMPEDANCE
t0
0I
0V
0I
0V
TT
2
1 T2
T
2
3
ω
VI
rad
2

 
22

 





 tt
Pure Capacitor In
AC Circuit
Topic --- Alternating Current
Impedance in a pure capacitor
• From the definition of the
impedance, hence
and
and
where XC:
Capacitive
(capacitative)
reactance
• Capacitive reactance is the
opposition of a capacitor to the
alternating current flows and is
defined by
• Capacitive reactance is a scalar
quantity and its unit is ohm ()
0
0
I
V
Z  00 CVI 
0
0
CV
V

CX
C
Z 

1
f 2
fC
XC
2
1

0
0
rms
rms
I
V
I
V
XC 
f0
CX
f
X C
1

21.3 RESISTANCE, REACTANCE & IMPEDANCE
Pure Capacitor In
AC Circuit
Topic --- Alternating Current
CIRCUIT
PHASE
DIFFERENCE
IMPEDANCE, Z PHASOR DIAGRAM
21.3 RESISTANCE, REACTANCE & IMPEDANCE
AC source
CV
V
C
I
tVVVC sin0







2
sin0

tII
rad
2

 
the voltage V lags behind
the current I by /2
radians.
OR
the current I leads the
voltage V by /2 radians
V
I
rad
2

 
fC
XC
2
1

f0
CX
f
X C
1

Pure Capacitor In
AC Circuit
Topic --- Alternating Current
An 8.00 μF capacitor is
connected to the terminals of
an AC generator with an rms
voltage of 150 V and a
frequency of 60.0 Hz. Find the
capacitive reactance rms
current and the peak current
in the circuit.
Solution:
21.2 ROOT MEAN SQUARE (rms)
fC
XC
2
1

0
0
rms
rms
I
V
I
V
XC 
Pure Capacitor In
AC Circuit
Topic --- Alternating Current
• The circuit
• The alternating current passes through the
inductor is given by
• When the AC passes through the inductor, the
back emf caused by the self induction is
produced and is given by
21.3 RESISTANCE, REACTANCE & IMPEDANCE
AC source
V
I
L
LV
tII sin0
dt
dI
LB  tI
dt
d
L sin0
tLI  cos0B 
At any instant,
the supply
voltage V
equals to the
back emf B in
the inductor but
the back emf
always oppose
the supply
voltage V
represents by
the negative
sign







2
sin0

 tLIVLB
Pure Inductor In
AC Circuit
Topic --- Alternating Current
• From figure above: I = I0 sin t
and V = V0 sin (t + /2)
• Thus the phase difference is
In the pure inductor,
the voltage V leads
the current I by /2
radians
OR
the current I lags
behind the voltage V
by /2 radians
21.3 RESISTANCE, REACTANCE & IMPEDANCE
22



 





 tt
t0
0I
0V
0I
0V
TT
2
1 T2
T
2
3
V
I
rad
2

 
ω
Pure Inductor In
AC Circuit
Topic --- Alternating Current
Impedance in a pure inductor
• From the definition of the
impedance, hence
where XL:
inductive reactance
• Inductive reactance is the
opposition of a inductor to the
alternating current flows and is
defined by
• Inductive reactance is a scalar
quantity and its unit is ohm ()
21.3 RESISTANCE, REACTANCE & IMPEDANCE
0
0
I
V
Z  00 LIV and
0
0
I
LI 

LXLZ  
fLXL 2
f 2and
0
0
rms
rms
I
V
I
V
XL 
f0
LX
fX L
Pure Inductor In
AC Circuit
Topic --- Alternating Current
CIRCUIT
PHASE
DIFFERENCE
IMPEDANCE, Z PHASOR DIAGRAM
21.3 RESISTANCE, REACTANCE & IMPEDANCE
the voltage V leads the
current I by /2 radians.
OR
the current I lags behind
the voltage V by /2
radians
AC source
V
I
L
LV
tII sin0
tVV cos0
OR







2
sin0

tVV
rad
2

 
V
I
rad
2

 
f0
LX
fX L
fLXL 2
Pure Inductor In
AC Circuit
Topic --- Alternating Current
A coil having an inductance of 0.5 H
is connected to a 120 V, 60 Hz
power source. If the resistance of
the coil is neglected, what is the
effective current through the coil.
A 240 V supply with a frequency of
50 Hz causes a current of 3.0 A to
flow through an pure inductor.
Calculate the inductance of the
inductor.
0
0
rms
rms
I
V
I
V
XL 
21.3 RESISTANCE, REACTANCE & IMPEDANCE
Topic --- Alternating Current
Since the voltage
on a capacitor  to
the charge on it,
the current must
lead the voltage in
time & phase to
conduct charge to
the capacitor plates
and raise the
voltage
V
I
rad
2

 
When a voltage is
applied to an
inductor, it resists
the change in
current. The current
builds up more
slowly than the
voltage, lagging it in
time and phase
V
I
rad
2

 
21.3 RESISTANCE, REACTANCE & IMPEDANCE
Topic --- Alternating CurrentPure Capacitor in AC Circuit Pure Inductor in AC Circuit
V
I
rad
2

 
the voltage V leads the current I by /2
radians
OR
the current I lags behind the voltage V
by /2 radians
tII sin0







2
sin0

tVV
rad
2

  IV
tVVVC sin0







2
sin0

tII
rad
2

  IV
V
I
rad
2

 
the voltage V lags behind the current I
by /2 radians
OR
the current I leads the voltage V by
/2 radians
tII sin0
VtVVR  sin0
0
In pure resistor, the current I always in
phase with the voltage V and constant
with time
VI
Topic --- Alternating Current
Q1
A capacitor has a rms
current of 21 mA at a
frequency of 60 Hz when
the rms voltage across it is
14 V.
(i) What is the capacitance
of the capacitor?
(ii) If the frequency is
increased, will the current
in the capacitor increase,
decrease or stay the same?
Explain.
(iii) Calculate the rms
current in the capacitor at
a frequency of 410 Hz.
Q2
A 2 F capacitor and a
1000  resistor are
placed in series with an
alternating voltage
source of 12 V and
frequency of 50 Hz.
Calculate
(i) the current flowing,
(ii) the voltage across
the capacitor,
(iii) the phase angle of
the circuit.
Q3
A rms voltage of 12.2
V with a frequency of
1.00 kHz is applied to
a 0.290 mH inductor.
(i) What is the rms
current in the circuit?
(ii) Determine the
peak current for a
frequency of 2.50
kHz.
21.3 RESISTANCE, REACTANCE & IMPEDANCE
Topic --- Alternating Current
AC source
R
I
RV
V
CV
C
IRVR  CC IXV 
22
CR VVV 
Phasor diagram:
the current I
leads the supply
voltage V by 
radians
R
C
V
V
tan
R
XC
tan

CX
Z
R
22
CXRZ 
fC
XC
2
1

22
2 1
C
RZ


A phasor diagram in terms of
R, XC and ZI

CV
RV
V
21.3 RESISTANCE, REACTANCE & IMPEDANCE
Topic --- Alternating Current
An alternating current of
angular frequency of 1.0 x 104
rad s-1 flows through a 10 k
resistor and a 0.10 F
capacitor which are connected
in series. Calculate the rms
voltage across the capacitor if
the rms voltage across the
resistor is 20 V.
Solution:
fC
XC
2
1

0
0
rms
rms
I
V
I
V
XC 
RC Series Circuit
21.3 RESISTANCE, REACTANCE & IMPEDANCE
Topic --- Alternating Current
IRVR 
Phasor diagram:
the supply voltage V leads the
current I the by  radians
22
2 1
C
RZ


A phasor diagram in terms of
R, XL and Z
AC source
R
I
RV
V
L
LV
LL IXV 

LV
V
I RV
22
LR VVV 
R
L
V
V
tan

LX
Z
R
R
XL
tan
22
LXRZ 
fLXL 2
21.3 RESISTANCE, REACTANCE & IMPEDANCE
Topic --- Alternating Current
IRVR 
LL IXV 
CC IXV 
AC source
I
V
R
RV CV
C L
LV
I

LV
RV
V
CV
 CL VV 
 22
CL XXRIV 
R
XX CL 
tan

LX
Z
CX
 CL XX 
R
 22
CL XXRZ 
21.3 RESISTANCE, REACTANCE & IMPEDANCE
Phasor diagram:
the supply voltage V
leads the current I the
by  radians
Topic --- Alternating Current
A series RLC circuit has a resistance
of 25.0 Ω, a capacitance of 50.0 μF,
and an inductance of 0.300 H. If the
circuit is driven by a 120 V, 60 Hz
source, calculate
(a) The total impedance of the circuit
(b) The rms current in the circuit
(c) The phase angle between the
voltage and the current.
Suggested Answer:
RCL Series Circuit
21.3 RESISTANCE, REACTANCE & IMPEDANCE
• 64.9 Ω ,
1.85 A,
67.3o
Topic --- Alternating Current
• is defined as the phenomenon
that occurs when the
frequency of the applied
voltage is equal to the
frequency of the RCL series
circuit
• Since resonance in series
RLC circuit occurs at
particular frequency, so it is
used for filtering and tuning
purpose as it does not allow
unwanted oscillations that
would otherwise cause signal
distortion, noise and damage
to circuit to pass through it
• Figure below shows the variation of XC,
XL, R and Z with frequency f of the RCL
series circuit
Z
fXL 
R
f
XC
1

0 f
Z
rf
•at low
frequency,
impedance Z
is large
because 1/ωC
is large
• at high
frequency,
impedance Z
is high
because ωL is
large
Topic --- Alternating Current
• From the
graph,
Zmin at fr
• This will
happen
when
 22
CL XXRZ 
02
min  RZ
RZ min
fr : resonant frequency
R
V
Z
V
I 
min
max
• The resonant
frequency, fr of the
RCL series circuit is
given by
CL XX 
C
L


1

LC
12

 
LC
f
1
2
2
r 
LC
f
2
1
r 
• At resonance in the
RCL series circuit,
the impedance is
minimum Zmin
thus the rms current
flows in the circuit is
maximum Imax and is
given by
21.3 RESISTANCE, REACTANCE & IMPEDANCE
The resistance in
the circuit is only
came from R
CL XX 
Topic --- Alternating Current
A 200  resistor, a 0.75 H inductor and a
capacitor of capacitance C are connected in
series to an alternating source 250 V, fr = 600
Hz. Calculate
(a) the inductive reactance and capacitive
reactance when resonance is occurred
(b) the capacitance C
(c) the impedance of the circuit at resonance
(d) the current flows through the circuit at
resonance
(e) Sketch the phasor diagram.
Suggested Answer:
(a)
(b)
(c)
(d)
(e)
RCL Series Circuit
21.3 RESISTANCE, REACTANCE & IMPEDANCE


k2.83
k2.83
C
L
X
LX 
nF93.9,
2
1
x102.83 3
 C
fC
A1.25
R
V
Z
V
I rmsrms
rms
VL
VC
VR
I
 200RZ
Topic --- Alternating Current
1. Based on the RCL series
circuit in Figure above, the
rms voltages across R, L and
C are shown.
(a)With the aid of the phasor
diagram, determine the
applied voltage and the
phase angle of the circuit.
(b) Calculate
(i) the current flows in the circuit
if the resistance of the
resistor R is 26 ,
(ii) the inductance and
capacitance if the frequency
of the AC source is 50 Hz,
(iii)the resonant frequency.
21.2 ROOT MEAN SQUARE (rms)
Topic --- Alternating Current
Q2
• A 2 F capacitor and a 1000  resistor are placed in series with an
alternating voltage source of 12 V and frequency of 50 Hz. Calculate
• (a) the current flowing,
• (b) the voltage across the capacitor,
• (c) the phase angle of the circuit.
Q3
• An AC current of angular frequency of 1.0  104 rad s1 flows
through a 10 k resistor and a 0.10 F capacitor which are
connected in series. Calculate the rms voltage across the capacitor if
the rms voltage across the resistor is 20 V.
ANS: 2.0 V
21.3 RESISTANCE, REACTANCE & IMPEDANCE
Topic --- Alternating Current
Q4A 200  resistor, a 0.75 H
inductor and a capacitor of
capacitance C are connected in
series to an alternating source
250 V, 600 Hz. Calculate
(a) the inductive reactance and
capacitive reactance when
resonance is occurred.
(b) the capacitance C.
(c) the impedance of the circuit at
resonance.
(d) the current flows through the
circuit at resonance. Sketch the
phasor diagram of the circuit.
ANS: 2.83 k, 2.83 k; 93.8 nF;
200 ; 1.25 A
Q5
A capacitor of capacitance C, a coil
of inductance L, a resistor of
resistance R and a lamp of
negligible resistance are placed in
series with alternating voltage V. Its
frequency f is varied from a low to a
high value while the magnitude of V
is kept constant.
(a) Describe and explain how the
brightness of the lamp varies.
(b) If V=0.01 V, C =0.4 F, L =0.4 H,
R = 10  and the circuit at
resonance, calculate
(i) the resonant frequency,
(ii) the maximum rms current,
(iii) the voltage across the
capacitor.
• (Advanced Level Physics,7th edition, Nelkon
& Parker, Q2, p.423)
• ANS: 400 Hz; 0.001 A; 1 V
21.3 RESISTANCE, REACTANCE & IMPEDANCE
Topic --- Alternating Current
V
V V
22
LR VVV 
22
LXRIV 
22
LXRZ 
IZV 
21.3 RESISTANCE, REACTANCE & IMPEDANCE
Topic --- Alternating Current
A circuit is made up of a 3200
pF capacitor connected in
series to a 30 H coil of
resistance 4 . Calculate
(i) impedance at frequency
30 kHz.
(ii)resonant frequency.
Solution:
C = 3200  10-12 F; L = 3010-6
H; R = 4 
(i) Given f = 30103 Hz, The
reactance of capacitor and
inductor are
PSPM 2009/ 10: Q12(c):
fC
XC
2
1

  123
10320010302
1




 1066.1 3
CX
21.3 RESISTANCE, REACTANCE & IMPEDANCE
Topic --- Alternating Current
(i) and
Therefore the impedance is given
by
(ii)Apply:
fLXL 2
  63
103010302 
 
 22
CL XXRZ 
   232
1066.166.54 
1654Z
 66.5LX
  126
10320010302
1




Hz1014.5 5
rf
LC
fr
2
1

21.3 RESISTANCE, REACTANCE & IMPEDANCE
Topic --- Alternating Current
Apply
(i) average power,
(ii)instantaneous power,
(iii)power factor,
in AC circuit consisting of R, RC, RL and RCL in
series.
21.4 POWER & POWER FACTOR
• Power factor is
a way of
measuring how
efficiently
electrical
power is being
used within a
facility's
electrical
system
cosVIP rmsrms
av
IVP 
IV
P
P
P av
a
r
cos 
Topic --- Alternating Current
• In an ac circuit , the power
is only dissipated by a
resistance, none is
dissipated by inductance or
capacitance
• From the phasor diagram
of the RCL series circuit
21.4 POWER & POWER FACTOR
RIIVP R
2
av 

ω
LV
I RV
V
CV
 CL VV 
• We get
• Then
and
cosVVR 
V
VR
cos
cosav IVP  IZV 
r
2
av cos PZIP  
where cos  is
called the
power factor of
the AC circuit,
Pr is the
average real
power and I2Z
is called the
apparent
power
Topic --- Alternating Current
• Power factor is defined as
• From
• the power factor also can be
calculated by using the equation
below
• When  = 0o (cos  =+1) ,the circuit
is completely resistive or when the
circuit is in resonance (RCL)
• When  = +90o (cos  = 0), the
circuit is completely inductive
• When  = -90o (cos  =0), the circuit
is completely capacitive
a
r
2
r
cos
P
P
ZI
P

IZ
IR
V
VR
cos
Z
R
cos
 22
CL XXRIV 
21.4 POWER & POWER FACTOR
Topic --- Alternating Current
An oscillator set for 500 Hz puts out a sinusoidal voltage of 100 V
effective. A 24.0 Ω resistor, a 10.0μF capacitor, and a 50.0 mH
inductor in series are wired across the terminals of the oscillator.
(a)What will an ammeter in the circuit read?
(b)What will a voltmeter read across each element?
(c)What is the real power dissipated in the circuit?
(d)Calculate the power supply
(e)Find the power factor
(f) What is the phase angle?
 R1
Z
V
I rms
rms

CC
LL
R
IXV
IXV
IRV



cosVIP rmsave rmsrmsplysup
VIP 
Z
R
cos  





 
Z
R
cos 1

21.4 POWER & POWER FACTOR
Topic --- Alternating Current
A 100 F capacitor, a 4.0 H inductor and a 35  resistor are
connected in series with an alternating source given by the
equation. V = 520 sin 100t. Calculate:
(a)the frequency of the source,
(b)the capacitive reactance and inductive reactance,
(c)the impedance of the circuit,
(d)the peak current in the circuit,
(e)the phase angle,
(f) the power factor of the circuit.
21.4 POWER & POWER FACTOR
Topic --- Alternating Current
Suggested Answer:
By comparing
V = 520 sin 100t
to the, V = V0 sin t
Thus
V0 = 520V,  = 100 rad s-1
(a) The frequency of AC source is
given by
(b) The capacitive reactance is
and the inductive reactance is
f 2
Hz9.15f
f2100 
fC
XC
2
1

100CX
  6
101009.152
1




CX
fLXL 2
 400LX
  0.49.152
21.4 POWER & POWER FACTOR
Topic --- Alternating Current
(c) The impedance of the circuit is
(d) The peak current in the circuit is
(e) The phase angle between the
current and the supply voltage is
OR
f. The power factor of the circuit is
given by
 22
CL XXRZ 
   22
10040035 
 302Z
ZIV 00 
 302520 0I
A72.10 I
R
XX CL 
tan





 
 
35
100400
tan 1
rad45.1





 
 
R
XX CL1
tan

3.83
cosfactorpower 

383cos .
117.0factorpower 
21.4 POWER & POWER FACTOR
Topic --- Alternating Current
Q1
A 22.5 mH inductor, a
105  resistor and a
32.3 F capacitor are
connected in series to
the alternating source
240 V, 50 Hz.
(a) Sketch the phasor
diagram for the circuit
(b) Calculate the power
factor of the circuit
(c) Determine the
average power
consumed by the
circuit.
ANS: 0.755, 313 W
Q2
A coil having inductance 0.14
H and resistance of 12  is
connected to an alternating
source 110 V, 25 Hz.
Calculate
(a) the rms current flows in
the coil
(b) the phase angle between
the current and supply
voltage
(c) the power factor of the
circuit
(d) the average power loss in
the coil.
ANS: 4.4 A, 61.3o , 0.48, 0.23
kW
Q3
A series RCL circuit
contains a 5.10 μF
capacitor and a generator
whose voltage is 11.0 V.
At a resonant frequency
of 1.30 kHz the power
dissipated in the circuit is
25.0 W. Calculate
(a) the inductance
(b) the resistance
(c) the power factor when
the generator frequency
is 2.31 kHz.
ANS: 2.94 x 10-3 H , 4.84
Ω , 0.163
21.4 POWER & POWER FACTOR
Topic --- Alternating Current
Q4
• An RLC circuit has a resistance of 105 , an inductance of 85.0 mH and a
capacitance of 13.2 F.
• What is the power factor of the circuit if it is connected to a 125 Hz AC
generator?
• Will the power factor increase, decrease or stay the same if the resistance is
increased? Explain.
• (Physics, 3rd edition, James S. Walker, Q47, p.834)
• ANS: 0.962; U think
Q5
• A 1.15 k resistor and a 505 mH inductor are connected in series to a 14.2 V,1250 Hz AC
generator.
• What is the rms current in the circuit?
• What is the capacitance’s value must be inserted in series with the resistor and inductor to
reduce the rms current to half of the value in part (a)?
• (Physics, 3rd edition, James S. Walker, Q69, p.835)
• ANS: 3.44 mA, 10.5 nF
21.4 POWER & POWER FACTOR
Topic --- Alternating Current
G E O M E T R I C A L
O P T I C S

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Ch 21 Alternating Current

  • 2. Topic --- Alternating Current 21 ALTERNATING CURRENT 21.1 Alternating current 21.2 Root mean square (rms) 21.3 Resistance, reactance & impedance 21.4 Power and power factor
  • 3. Topic --- Alternating Current (a) Define alternating current (AC) (b) Sketch and analyse sinusoidal AC waveform (c) Use sinusoidal voltage and current equations tII sin0 tVV sin0 21.1 ALTERNATING CURRENT
  • 4. Topic --- Alternating Current Alternating current (AC) electricity is the type of electricity commonly used in homes and businesses throughout the world. While direct current (DC) electricity flows in one direction through a wire, AC electricity alternates its direction in a back-and-forth motion. The direction alternates between 50 and 60 times per second, depending on the electrical system of the country. AC created by an AC electric generator, which determines the frequency the voltage can be readily changed, thus making it more suitable for long-distance transmission than DC electricity can employ capacitors and inductors in electronic circuitry, allowing for a wide range of applications 21.1 ALTERNATING CURRENT
  • 5. Topic --- Alternating Current • is defined as an electric current which magnitude & direction change periodically • Symbol: 21.1 ALTERNATING CURRENT
  • 6. Topic --- Alternating Current Current Voltage The output of an ac generator is sinusoidal and varies with time locityangular veORfrequencyangular: currentpeak:0I gepeak volta:0V time:t )2( f  In general,  21.1 ALTERNATING CURRENT tII o sin tVV o sin
  • 7. Topic --- Alternating Current f T 1  21.1 ALTERNATING CURRENT
  • 8. Topic --- Alternating Current 21.2 ROOT MEANC SQUARE (rms) (a) Define root mean square (rms) current and voltage for AC source (b) Use the following formula, and 2 0 rms I I  2 0 rms V V 
  • 9. Topic --- Alternating Current rmsI acdc poweraveragepower  RIRI ave 22  aveII 2  Mean or average current, Iave: the average or mean value of current in a half-cycle flows of current in a certain direction    0 2 0 ave 2II I  averms II 2  The r.m.s (root mean square) current means the square root of the average value of the current 21.2 ROOT MEAN SQUARE (rms)
  • 10. Topic --- Alternating Current • Since ac current and • So and • The root mean square (rms) current is the effective value of the AC tII sin0 averms II 2    averms tsinII 2 0  2 0 rms I I    2 1 sinsin 22  tt  21.2 ROOT MEAN SQUARE (rms)
  • 11. Topic --- Alternating Current • The equation: • Most household electricity is 240 V AC which means that Vrms is 240 V 2 0 rms V V  21.2 ROOT MEAN SQUARE (rms)
  • 12. Topic --- Alternating Current A sinusoidal, 60.0 Hz, ac voltage is read to be 120 V by an ordinary voltmeter. (a) What is the maximum value the voltage takes on during a cycle? (b)b) What is the equation for the voltage? Solution: (a) (b) 2 o rms V V  )V(V rmso 2 V170 tsinVV o  tsin 120170 21.2 ROOT MEAN SQUARE (rms)
  • 13. Topic --- Alternating Current A voltage V= 60 sin 120πt is applied across a 20 Ω resistor. (a)What will an ac ammeter in series with the resistor read? (b)Calculate the peak current and mean power. 2 o rms V V  R V I rms rms  )I(I rmso 2 2 0 I Irms    W90(20)(2.12)2 2   RIP rmsav Solution: (a) (b) 21.2 ROOT MEAN SQUARE (rms)
  • 14. Topic --- Alternating Current 1. An AC source V = 500 sin t is connected across a resistor of 250 . Calculate (a) the rms current in the resistor, (b) the peak current, (c) the mean power. 2. Figure below shows a graph to represent alternating current passes through a resistor of 10 k. Calculate (a) the rms current, (b) the frequency of the AC, (c) the mean power dissipated from the resistor. 21.2 ROOT MEAN SQUARE (rms)
  • 15. Topic --- Alternating Current (a) Sketch and use phasor diagram and sinusoidal waveform to show the phase relationship between current and voltage for a circuit consisting of (i) pure resistor (ii)pure capacitor (iii)pure inductor (b) Use phasor diagram to analyse voltage, current, and impedance of series circuit of: (i) RL (ii)RC (iii)RLC 21.3 RESISTANCE, REACTANCE & IMPEDANCE
  • 16. Topic --- Alternating Current (c) Define and use: (i) capacitive reactance, (ii) inductive reactance, (ii)impedance, (iv)phase angle, (d) Explain graphically the dependence of R, XC, XL and Z on f and relate it to resonance fC XC 2 1  fLXL 2  22 CL XXRZ  R XX CL  tan
  • 17. Topic --- Alternating Current A diagram containing phasor is called phasor diagram Phasor  a vector that rotate anticlockwise about its axis with constant angular velocity Used to represent a sinusoidally varying quantity such as alternating current (AC) & alternating voltage Also being used to determine the phase angle  the phase difference between current and voltage in AC circuit 21.3 RESISTANCE, REACTANCE & IMPEDANCE
  • 18. Topic --- Alternating Current Phase difference is 22          tt 21.3 RESISTANCE, REACTANCE & IMPEDANCE Note: valuepositive radian  valuenegative Leads Lags behind In antiphase
  • 19. Topic --- Alternating Current • The projection of OP on the y-axis is ON, represents the instantaneous value • Ao is the peak value of the quantity t0 TT 2 1 T2T 2 3 Ao tAA o sin y ω N O P y 21.3 RESISTANCE, REACTANCE & IMPEDANCE
  • 20. Topic --- Alternating Current • It is defined by OR • It is a scalar quantity and its unit is ohm () • In a DC circuit, impedance likes the resistance rms rms I V Z  2 0V 2 0I 0 0 I V Z  21.3 RESISTANCE, REACTANCE & IMPEDANCE V = IZ  V = IR  V = IX  V = IXC  V = IXL
  • 21. Topic --- Alternating Current Resistance, R: Opposition to current flow in purely resistive circuit Reactance, X: Opposition to current flow resulting from inductance or capacitance in ac circuit Capacitive reactance, XC: Opposition of a capacitor to ac Inductive reactance, XL: Opposition of an inductor to ac Impedance, Z: Total opposition to ac (Resistance and reactance combine to form impedance) 21.3 RESISTANCE, REACTANCE & IMPEDANCE
  • 22. Topic --- Alternating Current AC is defined as an electric current which magnitude & direction change periodically In general,  Root mean square current (Irms) is defined as the effective value of a.c. which produces the same power as the steady d.c. when the current passes through the same resistor 2 0 rms I I  Root mean square voltage/ p.d (Vrms): the value of the steady direct voltage which when applied across a resistor, produces the same power as the mean (average) power produced by the alternating voltage across the same resistor 2 0 rms V V  A diagram containing phasor is called phasor diagram Phasor  a vector that rotate anticlockwise about its axis with constant angular velocity Impedance, Z Resistance, R in resistor Reactance, X Capacitive reactance, XC in capacitor, C Inductive reactance, XL in inductor, L V = IZ  V = IR  V = IX  V = IXC  V = IXL
  • 23. Topic --- Alternating Current • The circuit • The alternating current passes through the resistor is given by • The alternating voltage across the resistor VR at any instant is given by where, V = supply voltage tII sin0 IRVR  00 VRI  RtI sin0 and VtVVR  sin0 21.3 RESISTANCE, REACTANCE & IMPEDANCE Pure Capacitor In AC Circuit
  • 24. Topic --- Alternating Current • From figure above: I = I0 sin t and V = V0 sin t • Thus the phase difference is Impedance in a pure resistor • From the definition of the impedance, hence t0 0I 0V 0I 0V TT 2 1 T2 T 2 3 ω VI 0 tt  Therefore the current I is in phase with the voltage V and constant with time R I V I V Z  0 0 rms rms 21.3 RESISTANCE, REACTANCE & IMPEDANCE Pure Capacitor In AC Circuit
  • 25. Topic --- Alternating Current CIRCUIT AC source R I RV V PHASE DIFFERENCE tII sin0 VtVVR  sin0 0 IMPEDANCE, Z PHASOR DIAGRAM VI RZ  In pure resistor, the current I always in phase with the voltage V and constant with time R I V I V Z  0 0 rms rms 21.3 RESISTANCE, REACTANCE & IMPEDANCE Pure Capacitor In AC Circuit
  • 26. Topic --- Alternating Current • The circuit • The alternating voltage across the capacitor VC at any instant is equal to the supply voltage V and is given by • The charge accumulates at the plates of the capacitor is 21.3 RESISTANCE, REACTANCE & IMPEDANCE AC source CV V C I tVVVC sin0 CCVQ  tCVQ sin0 Pure Capacitor In AC Circuit
  • 27. Topic --- Alternating Current • The charge and current are related by Hence the equation of AC in the capacitor is and OR dt dQ I   tCV dt d I sin0  t dt d CV sin0 tCV cos0 00 ICV  tII cos0        2 sin0  tII 21.3 RESISTANCE, REACTANCE & IMPEDANCE Pure Capacitor In AC Circuit
  • 28. Topic --- Alternating Current • From figure above: V = V0 sin t and I = I0 sin (t + /2) • Thus the phase difference is the voltage V lags behind the current I by /2 radians OR the current I leads the voltage V by /2 radians 21.3 RESISTANCE, REACTANCE & IMPEDANCE t0 0I 0V 0I 0V TT 2 1 T2 T 2 3 ω VI rad 2    22          tt Pure Capacitor In AC Circuit
  • 29. Topic --- Alternating Current Impedance in a pure capacitor • From the definition of the impedance, hence and and where XC: Capacitive (capacitative) reactance • Capacitive reactance is the opposition of a capacitor to the alternating current flows and is defined by • Capacitive reactance is a scalar quantity and its unit is ohm () 0 0 I V Z  00 CVI  0 0 CV V  CX C Z   1 f 2 fC XC 2 1  0 0 rms rms I V I V XC  f0 CX f X C 1  21.3 RESISTANCE, REACTANCE & IMPEDANCE Pure Capacitor In AC Circuit
  • 30. Topic --- Alternating Current CIRCUIT PHASE DIFFERENCE IMPEDANCE, Z PHASOR DIAGRAM 21.3 RESISTANCE, REACTANCE & IMPEDANCE AC source CV V C I tVVVC sin0        2 sin0  tII rad 2    the voltage V lags behind the current I by /2 radians. OR the current I leads the voltage V by /2 radians V I rad 2    fC XC 2 1  f0 CX f X C 1  Pure Capacitor In AC Circuit
  • 31. Topic --- Alternating Current An 8.00 μF capacitor is connected to the terminals of an AC generator with an rms voltage of 150 V and a frequency of 60.0 Hz. Find the capacitive reactance rms current and the peak current in the circuit. Solution: 21.2 ROOT MEAN SQUARE (rms) fC XC 2 1  0 0 rms rms I V I V XC  Pure Capacitor In AC Circuit
  • 32. Topic --- Alternating Current • The circuit • The alternating current passes through the inductor is given by • When the AC passes through the inductor, the back emf caused by the self induction is produced and is given by 21.3 RESISTANCE, REACTANCE & IMPEDANCE AC source V I L LV tII sin0 dt dI LB  tI dt d L sin0 tLI  cos0B  At any instant, the supply voltage V equals to the back emf B in the inductor but the back emf always oppose the supply voltage V represents by the negative sign        2 sin0   tLIVLB Pure Inductor In AC Circuit
  • 33. Topic --- Alternating Current • From figure above: I = I0 sin t and V = V0 sin (t + /2) • Thus the phase difference is In the pure inductor, the voltage V leads the current I by /2 radians OR the current I lags behind the voltage V by /2 radians 21.3 RESISTANCE, REACTANCE & IMPEDANCE 22            tt t0 0I 0V 0I 0V TT 2 1 T2 T 2 3 V I rad 2    ω Pure Inductor In AC Circuit
  • 34. Topic --- Alternating Current Impedance in a pure inductor • From the definition of the impedance, hence where XL: inductive reactance • Inductive reactance is the opposition of a inductor to the alternating current flows and is defined by • Inductive reactance is a scalar quantity and its unit is ohm () 21.3 RESISTANCE, REACTANCE & IMPEDANCE 0 0 I V Z  00 LIV and 0 0 I LI   LXLZ   fLXL 2 f 2and 0 0 rms rms I V I V XL  f0 LX fX L Pure Inductor In AC Circuit
  • 35. Topic --- Alternating Current CIRCUIT PHASE DIFFERENCE IMPEDANCE, Z PHASOR DIAGRAM 21.3 RESISTANCE, REACTANCE & IMPEDANCE the voltage V leads the current I by /2 radians. OR the current I lags behind the voltage V by /2 radians AC source V I L LV tII sin0 tVV cos0 OR        2 sin0  tVV rad 2    V I rad 2    f0 LX fX L fLXL 2 Pure Inductor In AC Circuit
  • 36. Topic --- Alternating Current A coil having an inductance of 0.5 H is connected to a 120 V, 60 Hz power source. If the resistance of the coil is neglected, what is the effective current through the coil. A 240 V supply with a frequency of 50 Hz causes a current of 3.0 A to flow through an pure inductor. Calculate the inductance of the inductor. 0 0 rms rms I V I V XL  21.3 RESISTANCE, REACTANCE & IMPEDANCE
  • 37. Topic --- Alternating Current Since the voltage on a capacitor  to the charge on it, the current must lead the voltage in time & phase to conduct charge to the capacitor plates and raise the voltage V I rad 2    When a voltage is applied to an inductor, it resists the change in current. The current builds up more slowly than the voltage, lagging it in time and phase V I rad 2    21.3 RESISTANCE, REACTANCE & IMPEDANCE
  • 38. Topic --- Alternating CurrentPure Capacitor in AC Circuit Pure Inductor in AC Circuit V I rad 2    the voltage V leads the current I by /2 radians OR the current I lags behind the voltage V by /2 radians tII sin0        2 sin0  tVV rad 2    IV tVVVC sin0        2 sin0  tII rad 2    IV V I rad 2    the voltage V lags behind the current I by /2 radians OR the current I leads the voltage V by /2 radians tII sin0 VtVVR  sin0 0 In pure resistor, the current I always in phase with the voltage V and constant with time VI
  • 39. Topic --- Alternating Current Q1 A capacitor has a rms current of 21 mA at a frequency of 60 Hz when the rms voltage across it is 14 V. (i) What is the capacitance of the capacitor? (ii) If the frequency is increased, will the current in the capacitor increase, decrease or stay the same? Explain. (iii) Calculate the rms current in the capacitor at a frequency of 410 Hz. Q2 A 2 F capacitor and a 1000  resistor are placed in series with an alternating voltage source of 12 V and frequency of 50 Hz. Calculate (i) the current flowing, (ii) the voltage across the capacitor, (iii) the phase angle of the circuit. Q3 A rms voltage of 12.2 V with a frequency of 1.00 kHz is applied to a 0.290 mH inductor. (i) What is the rms current in the circuit? (ii) Determine the peak current for a frequency of 2.50 kHz. 21.3 RESISTANCE, REACTANCE & IMPEDANCE
  • 40. Topic --- Alternating Current AC source R I RV V CV C IRVR  CC IXV  22 CR VVV  Phasor diagram: the current I leads the supply voltage V by  radians R C V V tan R XC tan  CX Z R 22 CXRZ  fC XC 2 1  22 2 1 C RZ   A phasor diagram in terms of R, XC and ZI  CV RV V 21.3 RESISTANCE, REACTANCE & IMPEDANCE
  • 41. Topic --- Alternating Current An alternating current of angular frequency of 1.0 x 104 rad s-1 flows through a 10 k resistor and a 0.10 F capacitor which are connected in series. Calculate the rms voltage across the capacitor if the rms voltage across the resistor is 20 V. Solution: fC XC 2 1  0 0 rms rms I V I V XC  RC Series Circuit 21.3 RESISTANCE, REACTANCE & IMPEDANCE
  • 42. Topic --- Alternating Current IRVR  Phasor diagram: the supply voltage V leads the current I the by  radians 22 2 1 C RZ   A phasor diagram in terms of R, XL and Z AC source R I RV V L LV LL IXV   LV V I RV 22 LR VVV  R L V V tan  LX Z R R XL tan 22 LXRZ  fLXL 2 21.3 RESISTANCE, REACTANCE & IMPEDANCE
  • 43. Topic --- Alternating Current IRVR  LL IXV  CC IXV  AC source I V R RV CV C L LV I  LV RV V CV  CL VV   22 CL XXRIV  R XX CL  tan  LX Z CX  CL XX  R  22 CL XXRZ  21.3 RESISTANCE, REACTANCE & IMPEDANCE Phasor diagram: the supply voltage V leads the current I the by  radians
  • 44. Topic --- Alternating Current A series RLC circuit has a resistance of 25.0 Ω, a capacitance of 50.0 μF, and an inductance of 0.300 H. If the circuit is driven by a 120 V, 60 Hz source, calculate (a) The total impedance of the circuit (b) The rms current in the circuit (c) The phase angle between the voltage and the current. Suggested Answer: RCL Series Circuit 21.3 RESISTANCE, REACTANCE & IMPEDANCE • 64.9 Ω , 1.85 A, 67.3o
  • 45. Topic --- Alternating Current • is defined as the phenomenon that occurs when the frequency of the applied voltage is equal to the frequency of the RCL series circuit • Since resonance in series RLC circuit occurs at particular frequency, so it is used for filtering and tuning purpose as it does not allow unwanted oscillations that would otherwise cause signal distortion, noise and damage to circuit to pass through it • Figure below shows the variation of XC, XL, R and Z with frequency f of the RCL series circuit Z fXL  R f XC 1  0 f Z rf •at low frequency, impedance Z is large because 1/ωC is large • at high frequency, impedance Z is high because ωL is large
  • 46. Topic --- Alternating Current • From the graph, Zmin at fr • This will happen when  22 CL XXRZ  02 min  RZ RZ min fr : resonant frequency R V Z V I  min max • The resonant frequency, fr of the RCL series circuit is given by CL XX  C L   1  LC 12    LC f 1 2 2 r  LC f 2 1 r  • At resonance in the RCL series circuit, the impedance is minimum Zmin thus the rms current flows in the circuit is maximum Imax and is given by 21.3 RESISTANCE, REACTANCE & IMPEDANCE The resistance in the circuit is only came from R CL XX 
  • 47. Topic --- Alternating Current A 200  resistor, a 0.75 H inductor and a capacitor of capacitance C are connected in series to an alternating source 250 V, fr = 600 Hz. Calculate (a) the inductive reactance and capacitive reactance when resonance is occurred (b) the capacitance C (c) the impedance of the circuit at resonance (d) the current flows through the circuit at resonance (e) Sketch the phasor diagram. Suggested Answer: (a) (b) (c) (d) (e) RCL Series Circuit 21.3 RESISTANCE, REACTANCE & IMPEDANCE   k2.83 k2.83 C L X LX  nF93.9, 2 1 x102.83 3  C fC A1.25 R V Z V I rmsrms rms VL VC VR I  200RZ
  • 48. Topic --- Alternating Current 1. Based on the RCL series circuit in Figure above, the rms voltages across R, L and C are shown. (a)With the aid of the phasor diagram, determine the applied voltage and the phase angle of the circuit. (b) Calculate (i) the current flows in the circuit if the resistance of the resistor R is 26 , (ii) the inductance and capacitance if the frequency of the AC source is 50 Hz, (iii)the resonant frequency. 21.2 ROOT MEAN SQUARE (rms)
  • 49. Topic --- Alternating Current Q2 • A 2 F capacitor and a 1000  resistor are placed in series with an alternating voltage source of 12 V and frequency of 50 Hz. Calculate • (a) the current flowing, • (b) the voltage across the capacitor, • (c) the phase angle of the circuit. Q3 • An AC current of angular frequency of 1.0  104 rad s1 flows through a 10 k resistor and a 0.10 F capacitor which are connected in series. Calculate the rms voltage across the capacitor if the rms voltage across the resistor is 20 V. ANS: 2.0 V 21.3 RESISTANCE, REACTANCE & IMPEDANCE
  • 50. Topic --- Alternating Current Q4A 200  resistor, a 0.75 H inductor and a capacitor of capacitance C are connected in series to an alternating source 250 V, 600 Hz. Calculate (a) the inductive reactance and capacitive reactance when resonance is occurred. (b) the capacitance C. (c) the impedance of the circuit at resonance. (d) the current flows through the circuit at resonance. Sketch the phasor diagram of the circuit. ANS: 2.83 k, 2.83 k; 93.8 nF; 200 ; 1.25 A Q5 A capacitor of capacitance C, a coil of inductance L, a resistor of resistance R and a lamp of negligible resistance are placed in series with alternating voltage V. Its frequency f is varied from a low to a high value while the magnitude of V is kept constant. (a) Describe and explain how the brightness of the lamp varies. (b) If V=0.01 V, C =0.4 F, L =0.4 H, R = 10  and the circuit at resonance, calculate (i) the resonant frequency, (ii) the maximum rms current, (iii) the voltage across the capacitor. • (Advanced Level Physics,7th edition, Nelkon & Parker, Q2, p.423) • ANS: 400 Hz; 0.001 A; 1 V 21.3 RESISTANCE, REACTANCE & IMPEDANCE
  • 51. Topic --- Alternating Current V V V 22 LR VVV  22 LXRIV  22 LXRZ  IZV  21.3 RESISTANCE, REACTANCE & IMPEDANCE
  • 52. Topic --- Alternating Current A circuit is made up of a 3200 pF capacitor connected in series to a 30 H coil of resistance 4 . Calculate (i) impedance at frequency 30 kHz. (ii)resonant frequency. Solution: C = 3200  10-12 F; L = 3010-6 H; R = 4  (i) Given f = 30103 Hz, The reactance of capacitor and inductor are PSPM 2009/ 10: Q12(c): fC XC 2 1    123 10320010302 1      1066.1 3 CX 21.3 RESISTANCE, REACTANCE & IMPEDANCE
  • 53. Topic --- Alternating Current (i) and Therefore the impedance is given by (ii)Apply: fLXL 2   63 103010302     22 CL XXRZ     232 1066.166.54  1654Z  66.5LX   126 10320010302 1     Hz1014.5 5 rf LC fr 2 1  21.3 RESISTANCE, REACTANCE & IMPEDANCE
  • 54. Topic --- Alternating Current Apply (i) average power, (ii)instantaneous power, (iii)power factor, in AC circuit consisting of R, RC, RL and RCL in series. 21.4 POWER & POWER FACTOR • Power factor is a way of measuring how efficiently electrical power is being used within a facility's electrical system cosVIP rmsrms av IVP  IV P P P av a r cos 
  • 55. Topic --- Alternating Current • In an ac circuit , the power is only dissipated by a resistance, none is dissipated by inductance or capacitance • From the phasor diagram of the RCL series circuit 21.4 POWER & POWER FACTOR RIIVP R 2 av   ω LV I RV V CV  CL VV  • We get • Then and cosVVR  V VR cos cosav IVP  IZV  r 2 av cos PZIP   where cos  is called the power factor of the AC circuit, Pr is the average real power and I2Z is called the apparent power
  • 56. Topic --- Alternating Current • Power factor is defined as • From • the power factor also can be calculated by using the equation below • When  = 0o (cos  =+1) ,the circuit is completely resistive or when the circuit is in resonance (RCL) • When  = +90o (cos  = 0), the circuit is completely inductive • When  = -90o (cos  =0), the circuit is completely capacitive a r 2 r cos P P ZI P  IZ IR V VR cos Z R cos  22 CL XXRIV  21.4 POWER & POWER FACTOR
  • 57. Topic --- Alternating Current An oscillator set for 500 Hz puts out a sinusoidal voltage of 100 V effective. A 24.0 Ω resistor, a 10.0μF capacitor, and a 50.0 mH inductor in series are wired across the terminals of the oscillator. (a)What will an ammeter in the circuit read? (b)What will a voltmeter read across each element? (c)What is the real power dissipated in the circuit? (d)Calculate the power supply (e)Find the power factor (f) What is the phase angle?  R1 Z V I rms rms  CC LL R IXV IXV IRV    cosVIP rmsave rmsrmsplysup VIP  Z R cos          Z R cos 1  21.4 POWER & POWER FACTOR
  • 58. Topic --- Alternating Current A 100 F capacitor, a 4.0 H inductor and a 35  resistor are connected in series with an alternating source given by the equation. V = 520 sin 100t. Calculate: (a)the frequency of the source, (b)the capacitive reactance and inductive reactance, (c)the impedance of the circuit, (d)the peak current in the circuit, (e)the phase angle, (f) the power factor of the circuit. 21.4 POWER & POWER FACTOR
  • 59. Topic --- Alternating Current Suggested Answer: By comparing V = 520 sin 100t to the, V = V0 sin t Thus V0 = 520V,  = 100 rad s-1 (a) The frequency of AC source is given by (b) The capacitive reactance is and the inductive reactance is f 2 Hz9.15f f2100  fC XC 2 1  100CX   6 101009.152 1     CX fLXL 2  400LX   0.49.152 21.4 POWER & POWER FACTOR
  • 60. Topic --- Alternating Current (c) The impedance of the circuit is (d) The peak current in the circuit is (e) The phase angle between the current and the supply voltage is OR f. The power factor of the circuit is given by  22 CL XXRZ     22 10040035   302Z ZIV 00   302520 0I A72.10 I R XX CL  tan          35 100400 tan 1 rad45.1          R XX CL1 tan  3.83 cosfactorpower   383cos . 117.0factorpower  21.4 POWER & POWER FACTOR
  • 61. Topic --- Alternating Current Q1 A 22.5 mH inductor, a 105  resistor and a 32.3 F capacitor are connected in series to the alternating source 240 V, 50 Hz. (a) Sketch the phasor diagram for the circuit (b) Calculate the power factor of the circuit (c) Determine the average power consumed by the circuit. ANS: 0.755, 313 W Q2 A coil having inductance 0.14 H and resistance of 12  is connected to an alternating source 110 V, 25 Hz. Calculate (a) the rms current flows in the coil (b) the phase angle between the current and supply voltage (c) the power factor of the circuit (d) the average power loss in the coil. ANS: 4.4 A, 61.3o , 0.48, 0.23 kW Q3 A series RCL circuit contains a 5.10 μF capacitor and a generator whose voltage is 11.0 V. At a resonant frequency of 1.30 kHz the power dissipated in the circuit is 25.0 W. Calculate (a) the inductance (b) the resistance (c) the power factor when the generator frequency is 2.31 kHz. ANS: 2.94 x 10-3 H , 4.84 Ω , 0.163 21.4 POWER & POWER FACTOR
  • 62. Topic --- Alternating Current Q4 • An RLC circuit has a resistance of 105 , an inductance of 85.0 mH and a capacitance of 13.2 F. • What is the power factor of the circuit if it is connected to a 125 Hz AC generator? • Will the power factor increase, decrease or stay the same if the resistance is increased? Explain. • (Physics, 3rd edition, James S. Walker, Q47, p.834) • ANS: 0.962; U think Q5 • A 1.15 k resistor and a 505 mH inductor are connected in series to a 14.2 V,1250 Hz AC generator. • What is the rms current in the circuit? • What is the capacitance’s value must be inserted in series with the resistor and inductor to reduce the rms current to half of the value in part (a)? • (Physics, 3rd edition, James S. Walker, Q69, p.835) • ANS: 3.44 mA, 10.5 nF 21.4 POWER & POWER FACTOR
  • 63. Topic --- Alternating Current G E O M E T R I C A L O P T I C S