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
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
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
LB 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 2and
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 s1 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 = 3010-6
H; R = 4
(i) Given f = 30103 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
Hz9.15f
f2100
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