Alternative Current _ Question Bank
Ranker
ALTERNATIVE CURRRENT
1)
A given LCR series circuit satisfies the condition for resonance with a given
AC source. If the angular frequency of the AC source is increased by 100%
then in order to establish resonance, without changing the value of inductance,
the capacitance must be
(1) Increased by 100%
(2) Reduced by 50%
(3) Increased by 75%
(4) Reduced by 75%
2)
The natural frequency of the circuit shown in the figure is (Assume that equal
currents i, i exist in the two branches, and the charges on the capacitors are
q
q
equal)
i+
i
3)
(2)
–
1
1
2  3LC
+ –
1 1
2  LC
(3) 1 1
 LC
(1)
C
L
C
L
L
(4) None of the above
In the LCR circuit, the voltmeter and ammeter readings are
400V 400V
(1) 200 volt, 20A
V
(2) 200 volt, 50A
C
(3) 1000 volt, 20A
50 L
A
(4) 100 volt, 20A
1000V, 50 Hz
~
4)
Figure shows a series LCR circuit connected to a variable frequency 200V
source. L = 5H, C = 80 F and R = 40. What is the source frequency which
drives the circuit at resonance?
(1) 25 Hz
(2)
25
Hz

(3) 50 Hz
(4)
50
Hz

5)
The graph of alternating emf versus time is shown in the figure. The average of
this alternating emf in the first one-half cycle will be
(1) V0 / 3
(2) V0 / 2
(3) 4V0 / 5
(4) 5V0 / 8
6)
In figure below if ZL  Zc and reading of ammeter is 1 A. Find value of source
voltage V.
(1) 80 Volt
(2) 60 Volt
(3) 100 Volt
(4) 120 Volt
Page 1
Alternative Current _ Question Bank
7)
Ranker
The graph of alternating current versus time is shown in figure. The average
value for the positive half cycle, is
i
I
(1) 0
3
5I
(3) 0
8
2I 0
(2)

I
(4) 0
2
I0
0
3T/4
T
T/4
t
-I0
8)
As shown in figure value of inductive reactance XL will be if source voltage is
100 volt
(1) 40 
(2) 30 
(3) 50 
(4) Can have any value
9)
In the series LCR circuit, the voltmeter and ammeter readings are
respectively:
(1) V=250V, I = 4 A
(2) V=150V, I = 2 A
(3) V=1000V, I = 5 A
(4) V=100V, I = 2 A
10)
When 100 volts dc is supplied across a solenoid, a current of 1.0 amperes flows
in it. When 100 volts ac is applied across the same coil, the current drops to 0.5
ampere. If the frequency of ac source is 50 Hz, then the impedance and
inductance of the solenoid are
(1) 200  and 0.55 henry
(2) 100  and 0.86 henry
(3) 200  and 1.0 henry
(4)100  and 0.93 henry
11)
In an LR-circuit, the inductive reactance is equal to the resistance R of the
circuit. An e.m.f. E  E0 cos(t) applied to the circuit. The power consumed in the
circuit is
(1)
E02
R
(2)
E02
2R
(3)
E 02
4R
(4)
E02
8R
12)
One 10 V, 60 W bulb is to be connected to 100 V line. The required induction
coil has self inductance of value ( f  50 Hz)
(1) 0.052 H
(2) 2.42 H
(3) 16.2 mH
(4) 1.62 mH
13)
In the circuit given below, what will be the reading of the voltmete
(1) 300 V
(2) 900 V
(3) 200 V
(4) 400 V
200V, 100 Hz
Page 2
Alternative Current _ Question Bank
Ranker
14)
In the circuit shown below, what will be the readings of the voltmeter and
ammeter
(1) 800 V, 2A
(2) 300 V, 2A
(3) 220 V, 2.2 A
(4) 100 V, 2A
15)
A bulb and a capacitor are connected in series to a source of alternating current.
If its frequency is increased, while keeping the voltage of the source constant,
then
(1) Bulb will give more intense light
(2) Bulb will give less intense light
(3) Bulb will give light of same intensity as before
(4) Bulb will stop radiating light
16)
An alternating e.m.f. of angular frequency  is applied across an inductance.
The instantaneous power developed in the circuit has an angular frequenc
(1)

4
(2)

2
(3) 
(4) 2
17)
The voltage of an ac source varies with time according to the equation
V  100 sin 100t cos100t where t is in seconds and V is in volts. Then
(1) The peak voltage of the source is 100 volts
(2) The peak voltage of the source is 50 volts
(3) The peak voltage of the source is 100 / 2 volts
(4) The frequency of the source is 50 Hz
18)
The diagram shows a capacitor C and a resistor R connected in series to an ac
source. V1 and V2 are voltmeters and A is an ammeter
Consider now the following statements
I. Readings in A and V2 are always in phase
II. Reading in V1 is ahead in phase with reading in V2
III. Readings in A and V1 are always in phase which of these statements Zare/is
correct
(1) I only
(2) II only
(3) I and II only (4) II and III only
19)
In the circuit shown in figure neglecting source resistance the voltmeter and
ammeter reading will respectively, will be
(1) 0V, 3A
(2) 150V, 3A
(3) 150V, 6A
X = 25
R = 30 X = 25
(4) 0V, 8A
Page 3
Alternative Current _ Question Bank
20)
Ranker
The voltage of an ac supply varies with time (t) as V  120 sin 100  t cos 100  t. The
maximum voltage and frequency respectively are
(1) 120 volts, 100 Hz
(2) 120 volts, 100 Hz
2
(3) 60 volts, 200 Hz
(4) 60 volts, 100 Hz
21)
In the circuit shown in the figure, the ac source gives a voltage V  20 cos(2000 t).
Neglecting source resistance, the voltmeter and ammeter reading will be
(1) 0V, 0.47A
(2) 1.68V, 0.47A
A
(3) 0V, 1.4 A
(4) 5.6V, 1.4 A
22)
A telephone wire of length 200 km has a capacitance of 0.014 F per km. If it
carries an ac of frequency 5 kHz, what should be the value of an inductor
required to be connected in series so that the impedance of the circuit is
minimum
(1) 0.35 mH
(2) 35 mH
(3) 3.5 mH
(4) Zero
23)
In a certain circuit current changes with time according to i  2 t . r.m.s. value of
current between t  2 to t  4 s will be
(1) 3 A
(2) 3 3 A
(3) 2 3 A
(4) (2  2 )A
24)
Match the following
Currents
(1) x0 sin  t
r.m.s. values
(i) x0
(2)
(ii)
25)
x 0 sin  t cos  t
x0
2
x0
(3) x0 sin  t  x0 cos  t
(iii)
(1) 1. (i), 2. (ii), 3. (iii)
(3) 1. (i), 2. (iii), 3. (ii)
(2) 1. (ii), 2. (iii), 3. (i)
(4) None of these
(2 2)
The reading of ammeter in the circuit shown will be
(1) 2A
(2) 2.4 A
(3) Zero
110 V
(4) 1.7 A
Page 4
Alternative Current _ Question Bank
26)
Ranker
An ac source of angular frequency  is fed across a resistor r and a capacitor C
in series. The current registered is I. If now the frequency of source is changed
to  /3 (but maintaining the same voltage), the current in then circuit is found to
be halved. Calculate the ratio of reactance to resistance at the original
frequency 
3
5
(1)
(2)
2
5
(3)
1
5
(4)
4
5
27)
An LCR series circuit with a resistance of 100 ohm is connected to an ac source
of 200 V (r.m.s.) and angular frequency 300 rad/s. When only the capacitor is
removed, the current lags behind the voltage by 60 o . When only the inductor is
removed the current leads the voltage by 60 o . The average power dissipated is
(1) 50 W
(2) 100 W
(3) 200 W
(4) 400 W
28)
A virtual current of 4A and 50 Hz flows in an ac circuit containing a coil. The
power consumed in the coil is 240 W. If the virtual voltage across the coil is
100 V its inductance will be
(1) 1 H
(2) 1 H
(3) 1 H
(4) 1 H
3
29)
5
7
9
For a series RLC circuit R = XL = 2XC. The impedance of the circuit and phase
difference (between) V and i will be
5R
, tan 1 (2)
2
(1)
(2)
5R
1
, tan 1  
2
2
(3)
5 X C , tan 1 (2)
(4)
1 
5 R, tan 1  
2
30)
In the adjoining ac circuit the voltmeter whose reading will be zero at
resonance is
V
(1) V1
V
V
V
(2) V2
(3) V3
C
L
V
(4) V4
31)
In the adjoining figure the impedance of the circuit will be
(1) 120 ohm
(2) 50 ohm
(3) 60 ohm
(4) 90 ohm
32)
If
i  t2 0  t  T
(1)
T
2
2
then r.m.s. value of current is
(2)
T2
2
(3)
T2
5
(4) None of these
Page 5
Alternative Current _ Question Bank
33)
34)
35)
Ranker
Is it possible
(1) Yes
(2) No
(3) Cannot be predicted
(4) Insufficient data to reply
In a series circuit C  2F, L  1mH and R  10 , when the current in the circuit is
maximum, at that time the ratio of the energies stored in the capacitor and the
inductor will be
(1) 1 : 1
(2) 1 : 2
(3) 2 : 1
(4) 1 : 5
Which one of the following curves represents the variation of impedance (Z)
with frequency f in series LCR circuit
(1)
(2)
(3)
(4)
36)
The variation of the instantaneous current (I) and the instantaneous emf (E) in a
circuit is as shown in fig. Which of the following statements is correct
(1) The voltage lags behind the current by  / 2
E
I
(2) The voltage leads the current by  / 2
(3) The voltage and the current are in phase
(4) The voltage leads the current by 
37)
The figure shows variation of R, XL and XC with frequency f in a series L, C, R
circuit. Then for what frequency point, the circuit is inductive
(1) A
(2) B
(3) C
(4) All points
Page 6
Alternative Current _ Question Bank
38)
39)
Ranker
An alternating emf is applied across a parallel combination of a resistance R,
capacitance C and an inductance L. If IR, IL, IC are the currents through R, L and
C respectively, then the diagram which correctly represents, the phase
relationship among IR, IL, IC and source emf E, is given by
(1)
(2)
(3)
(4)
An ac source of variable frequency f is connected to an LCR series circuit.
Which one of the graphs in figure. represents the variation of current of current
I in the circuit with frequency f
(1)
(2)
(3)
(4)
40)
The r.m.s. voltage of the wave form shown is
(1) 10 V
(2) 7 V
(3) 6.37 V
(4) None of these
41)
A constant voltage at different frequencies is applied across a capacitance C as
shown in the figure. Which of the following graphs
Correctly depicts the variation of current with frequency?
(1)
(2)
Signal
Generator
V
(3)
C
(4)
Page 7
Alternative Current _ Question Bank
43)
The output current versus time curve of a rectifier is shown in the figure. The
average value of output current in this case is
(1) 0
(2) I0
(3)
2
2I0

(4)
I0
Current
42)
Ranker
The current 'i' in an inductance coil varies with time 't' according to following
graph
Which one of the following plots shows the variations of voltage in the coil
(1)
(2)
(3)
(4)
44)
When an ac source of e.m.f. e  E0 sin(100 t) is connected across a circuit, the
phase difference between the e.m.f. e and the current i in the circuit is observed
to be  / 4 , as shown in the diagram. If the circuit consists possibly only of RC
or LC in series, find the relationship between the two elements
(1) R  1k, C  10 F
i or e
(2) R  1k, C  1F
i e
(3) R  1k, L  10 H

(4) R  1k, L  1H
45)
Two sinusoidal voltages of the same frequency are shown in the diagram. What
is the frequency, and the phase relationship between the voltages.
Frequency in Hz Phase lead of N over M in radians
(1) 0.4
 / 4
(2) 2.5
 / 2
(3) 2.5
 / 2
(4) 2.5
 / 4
Page 8
Alternative Current _ Question Bank
46)
47)
Ranker
The voltage across a pure inductor is represented by the following diagram.
Which one of the following diagrams will represent the current?
(1)
(2)
(3)
(4)
In pure inductive circuit, the curves between frequency f and reciprocal of
inductive reactance 1/XL is
(1)
(2)
1
1
(3)
(4)
1
1
X
48)
The vector diagram of current and voltage for a circuit is as shown. The
components of the circuit will be
(1) LCR
(2) LR
(3) LCR or LR
(4) None of these
49)
The resonance point in X L  f and XC  f curves is
(1) P
(2) Q
(3) R
(4) S
The i -  curve for anti-resonant circuit is
50)
(1)
(2)
(3)
(4)
Page 9
Alternative Current _ Question Bank
Ranker
The graphs given below depict the dependence of two reactive impedances X1
and X2 on the frequency of the alternating e.m.f. applied individually to them.
We can then say that
(1) X1 is an inductor and X2 is a capacitor
(2) X1 is a resistor and X2 is a capacitor
(3) X1 is a capacitor and X2 is an inductor
(4) X1 is an inductor and X2 is a resistor
52)
Which of the following plots may represent the reactance of a series LC
combination
a
(1) a
(2) b
c
b
(3) c
Frequency
(4) d
Reactance
51)
d
53)
Which of the following curves correctly represents the variation of capacitive
reactance XC with frequency f
(1)
(2)
(3)
(4)
54)
A bulb and a capacitor are in series with an ac source. On increasing frequency
how will glow of the bulb change
(1) The glow decreases
(2) The glow increases
(3) The glow remain the same
(4) The bulb quenches
55)
The r.m.s. current in an ac circuit is 2 A. If the wattless current be
the power factor
(1) 1
(2) 1
(3) 1
(4) 1
3
56)
2
2
3A ,
what is
3
2.5
 F capacitor and 3000-ohm resistance are joined in series to an ac source of

200 volt and 50 sec 1 frequency. The power factor of the circuit and the power
dissipated in it will respectively
(1) 0.6, 0.06 W (2) 0.06, 0.6 W
(3) 0.6, 4.8 W
(4) 4.8, 0.6 W
57)
The self inductance of a choke coil is 10 mH. When it is connected with a 10V
dc source, then the loss of power is 20 watt. When it is connected with 10 volt
ac source loss of power is 10 watt. The frequency of ac source will be
(1) 50 Hz
(2) 60 Hz
(3) 80 Hz
(4) 100 Hz
Page 10
Alternative Current _ Question Bank
Ranker
58)
In an LCR circuit R  100 ohm. When capacitance C is removed, the current lags
behind the voltage by  / 3 . When inductance L is removed, the current leads the
voltage by  / 3 . The impedance of the circuit is
(1) 50 ohm
(2) 100 ohm
(3) 200 ohm
(4) 400 ohm
59)
A group of electric lamps having a total power rating of 1000 watt is supplied
by an ac voltage E  200 sin(310 t  60) . Then the r.m.s. value o the circuit current is
(1) 10 A
(2) 10 2 A
(3) 20 A
(4) 20 2 A
60)
Following figure shows an ac generator connected to a "block box" through a
pair of terminals. The box contains possible R, L, C or their combination,
whose elements and arrangements are not known to us. Measurements outside
the box reveals that e = 75 sin (sin  t) volt, i = 1.5 sin ( t + 45o) amp then, the
wrong statement is
(1) There must be a capacitor in the box
(2) There must be an inductor in the box
(3) There must be a resistance in the box
(4) The power factor is 0.707
61)
A resistor R, an inductor L and a capacitor C are connected in series to an
oscillator of frequency n. if the resonant frequency is nr , then the current lags
behind voltage, when
(1) n  0
(2) n  nr
(3) n  nr
(4) n  nr
62)
If power factor is 1/2 in a series RL circuit
(1)
3

Henry
(2)  Henry
(3)
R  100  .

3
ac mains is used then L is
Henry
(4) None of these
63)
What will be the self inductance of a coil, to be connected in a series with a
resistance of  3  such that the phase difference between the emf and the
current at 50 Hz frequency is 30°
(1) 0.5 Henry
(2) 0.03 Henry
(3) 0.05 Henry (4) 0.01 Henry
64)
The phase difference between the voltage and the current in an ac circuit is  / 4
. If the frequency is 50 Hz then this phase difference will be equivalent to a
time of
(1) 0.02 s
(2) 0.25 s
(3) 2.5 ms
(4) 25 ms
Page 11
Alternative Current _ Question Bank
Ranker
65)
The instantaneous values of current and emf in an ac circuit are I  1 / 2 sin 314 t
amp and E  2 sin( 314 t   / 6 )V respectively. The phase difference between E and
I will be
(1)  / 6 rad
(2)  / 3 rad
(3)  / 6 rad
(4)  / 3 rad
66)
If A and B are identical bulbs which bulbs glows brighter
(1) A
(2) B
(3) Both equally bright
(4) Cannot say
67)
The instantaneous values of current and voltage in an ac circuit are
i  100 sin 314 t amp and e  200 sin (314 t   / 3)V respectively. If the resistance is 1
then the reactance of the circuit will be
(1)  200 3 
(2) 3 
(3)  200 / 3 
(4) 100 3 
68)
What is the r.m.s. value of an alternating current which when passed through a
resistor produces heat which is thrice of that produced by a direct current of 2
amperes in the same resistor
(1) 6 amp
(2) 2 amp
(3) 3.46 amp
(4) 0.66 amp
69)
Assertion: In series LCR circuit resonance can take place.
Reason: Resonance takes place if inductance and capacitive reactances are
equal and opposite.
(1) Both Assertion and Reason are true and Reason is the correct explanation of
Assertion
(2) Both Assertion and Reason are true but Reason is not the correct
explanation of Assertion
(3) Assertion is true, Reason is false
(4) Assertion is false, Reason is true
70)
Assertion: The alternating current lags behind the e.m.f. by a phase angle of
 / 2 , when ac flows through an inductor.
Reason: The inductive reactance increases as the frequency of ac source
decreases.
(1) Both Assertion and Reason are true and Reason is the correct explanation
of Assertion
(2) Both Assertion and Reason are true but Reason is not the correct
explanation of Assertion
(3) Assertion is true, Reason is false
(4) Assertion is false, Reason is true
Page 12
Alternative Current _ Question Bank
71)
72)
Ranker
Assertion: Capacitor serves as a block for dc and offers an easy path to ac.
Reason: Capacitive reactance is inversely proportional to frequency.
(1) Both Assertion and Reason are true and Reason is the correct explanation
of Assertion
(2) Both Assertion and Reason are true but Reason is not the correct
explanation of Assertion
(3) Assertion is true, Reason is false
(4) Assertion is false, Reason is true
Assertion: When capacitive reactance is smaller than the inductive reactance in
LCR current, e.m.f. leads the current .
Reason: The phase angle is the angle between the alternating e.m.f. and
alternating current of the circuit.
(1) Both Assertion and Reason are true and Reason is the correct explanation
of Assertion
(2) Both Assertion and Reason are true but Reason is not the correct
explanation of Assertion
(3) Assertion is true, Reason is false
(4) Assertion is false, Reason is true
73)
Assertion: Chock coil is preferred over a resistor to adjust current in an ac
circuit.
Reason: Power factor for inductance is zero.
(1) Both Assertion and Reason are true and Reason is the correct explanation
of Assertion
(2) Both Assertion and Reason are true but Reason is not the correct
explanation of Assertion
(3) Assertion is true, Reason is false
(4) Assertion is false, Reason is true
74)
Assertion: If the frequency of alternating current in an ac circuit consisting of
an inductance coil is increased then current gets decreased.
Reason: The current is inversely proportional to frequency of alternating
current.
(1) Both Assertion and Reason are true and Reason is the correct explanation
of Assertion
(2) Both Assertion and Reason are true but Reason is not the correct
explanation of Assertion
(3) Assertion is true, Reason is false
(4) Assertion is false, Reason is true
Page 13
Alternative Current _ Question Bank
Ranker
75)
Assertion: An alternating current does not show any magnetic effect.
Reason: Alternating current varies with time.
(1) Both Assertion and Reason are true and Reason is the correct explanation
of Assertion
(2) Both Assertion and Reason are true but Reason is not the correct
explanation of Assertion
(3) Assertion is true, Reason is false
(4) Assertion is false, Reason is true
76)
Assertion: The dc and ac both can be measured by a hot wire instrument.
Reason: The hot wire instrument is based on the principle of magnetic effect of
current.
(1) Both Assertion and Reason are true and Reason is the correct explanation
of Assertion
(2) Both Assertion and Reason are true but Reason is not the correct
explanation of Assertion
(3) Assertion is true, Reason is false
(4) Assertion is false, Reason is true
77)
Assertion: ac is more dangerous than dc
Reason: Frequency of ac is dangerous for human body.
(1) Both Assertion and Reason are true and Reason is the correct explanation
of Assertion
(2) Both Assertion and Reason are true but Reason is not the correct
explanation of Assertion
(3) Assertion is true, Reason is false
(4) Assertion is false, Reason is true
78)
Assertion: Average value of ac over a complete cycle is always zero.
Reason: Average value of ac is always defined over half cycle.
(1) Both Assertion and Reason are true and Reason is the correct explanation
of Assertion
(2) Both Assertion and Reason are true but Reason is not the correct
explanation of Assertion
(3) Assertion is true, Reason is false
(4) Assertion is false, Reason is true
Page 14
Alternative Current _ Question Bank
Ranker
79)
Assertion: The division are equally marked on the scale of ac ammeter.
Reason: Heat produced is directly proportional to the current.
(1) Both Assertion and Reason are true and Reason is the correct explanation
of Assertion
(2) Both Assertion and Reason are true but Reason is not the correct
explanation of Assertion
(3) Assertion is true, Reason is false
(4) Assertion is false, Reason is true
80)
Assertion: When ac circuit contain resistor only, its power is minimum.
Reason: Power of a circuit is independent of phase angle.
(1) Both Assertion and Reason are true and Reason is the correct explanation
of Assertion
(2) Both Assertion and Reason are true but Reason is not the correct
explanation of Assertion
(3) Assertion is true, Reason is false
(4) Assertion is false, Reason is true
81)
Assertion: An electric lamp connected in series with a variable capacitor and ac
source, its brightness increases with increase in capacitance.
Reason: Capacitive reactance decrease with increase in capacitance of
capacitor.
(1) Both Assertion and Reason are true and Reason is the correct explanation
of Assertion
(2) Both Assertion and Reason are true but Reason is not the correct
explanation of Assertion
(3) Assertion is true, Reason is false
(4) Assertion is false, Reason is true
82)
Assertion: An inductance and a resistance are connected in series with an ac
circuit. In this circuit the current and the potential difference across the
resistance lag behind potential difference across the inductance by an angle /2.
Reason: In LR circuit voltage leads the current by phase angle which depends
on the value of inductance and resistance both.
(1) Both Assertion and Reason are true and Reason is the correct explanation
of Assertion
(2) Both Assertion and Reason are true but Reason is not the correct
explanation of Assertion
(3) Assertion is true, Reason is false
(4) Assertion is false, Reason is true
Page 15
Alternative Current _ Question Bank
Ranker
83)
Assertion: A capacitor of suitable capacitance can be used in an ac circuit in
place of the choke coil.
Reason: A capacitor blocks dc and allows ac only.
(1) Both Assertion and Reason are true and Reason is the correct explanation
of Assertion
(2) Both Assertion and Reason are true but Reason is not the correct
explanation of Assertion
(3) Assertion is true, Reason is false
(4) Assertion is false, Reason is true
84)
The components shown in the figure are ideal. The angular frequency ω of the
input voltage to the circuit is continuously varied keeping V0 constant. The
potential drops VC, VL,VR across C,L,R and those VL,C and VR,L across the
pairs L,C and L,R are measured. ω0 is the angular frequency at which the
current in the circuit is maximum.
C
L
R
~
V=V0 Cos(ωt)
Column I
(1)
ω= ω0
(2)
ω >ω0
(3)
ω <ω0
(4)
Power factor < 1, then the
possibility is
(1) A-q,r,s; B-p,r,s; C-rs; D-prs
(3) A-q,r,s; B-p,r,s; C-sr; D-srp
Column II
(P) VL >VC
(Q) VL,C = 0
(R) VL, C < VL
(S) VR,L > VR
(2) A-q,s,r; B-p,r,s; C-rs; D-prs
(4) A-q,r,s; B-s,r,p; C-rs; D-prs
85)
Column I
Column II
In series LCR circuit at resonance
A.
(P) Maximum
impedance will be
In parallel LCR circuit at
B.
(Q) Minimum
resonance impedance will be
At resonance current in series LCR
C.
(R) Second
circuit will be
D.
Unit of L/R will be
(S) Hours
(1) A-q; B-s; C-r; D-r,s
(2) A-q; B-p; C-q; D-r,s
(3) A-p; B-q; C-q; D-r,s
(4) A-q; B-p; C-s; D-r,s
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