NEET

ALTERNATING CURRENT WS 4
NEET-UG - Physics

1. The average power lost per cycle of AC is given by: [4]

a) 1

2
E0I0 sinθ b) 1

2
E0I0 ×θ

c) 1

2
E0I0 tanθ d) 1

2
E0I0 cosθ

2. In the adjoining figure a series LCR circuit is connected to a variable frequency 200 V source. L = 5 H, C = 80 [4]
μ F and R = 40 Ω . Then, the source frequency which drive the circuit at resonance is:

a) HZ b) 25 Hz
25

c) 50

π
HZ d) 50 HZ
3. A voltage of peak value 283 V and varying frequency is applied to a series LCR combination in which R = 3 Ω ; [4]
L = 25 mH and C = 400 μ F. Then, the frequency (in Hz) of the source at which maximum power is dissipated in
the above, is:

a) 50.7 b) 51.5

c) 51.1 d) 50.3
4. A direct current of 2A and an alternating current having a maximum value of 2A flow through two identical [4]
resistances. The ratio of heat produced in the two resistances will be:

a) 4 : 1 b) 1 : 1

c) 1 : 2 d) 2 : 1
5. A resistor 30 Ω , inductor of reactance 10 Ω and capacitor of reactance 10 Ω are connected in series to an AC [4]
–
voltage source e = 300 √2 sin (ωt). The current in the circuit is:
−−
a) 30
A b) 30√11 A
√11

–
c) 10 A d) 10√2 A
6. To reduce the resonant frequency in an LCR series circuit with a generator: [4]

a) the generator frequency should be reduced b) the iron core of the inductor should be
removed

c) another capacitor should be added in d) dielectric in the capacitor should

parallel to the first be removed
7. A capacitor has a resistance of 1200 mega ohm and a capacitance of 22 microfarads. It is connected to an AC [4]
supply of 80 Hz, then the emf of the AC supply required to drive a current of 10 virtual ampere is:

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 –
a) 452 volt b) 904√2 volt

c) 904 volt d) ( 904

) volt
√2

8. In a series resonant circuit, having L, C, and R as its elements, the resonant current is i. The power dissipated in [4]
circuit at resonance is:

a) i2R b) i2ωL
2

c) Zero d) i R

1
(ωL− )
ωC

9. For the given input voltage waveform Vin(t), the output voltage waveform Vo(t), across the capacitor is correctly [4]

depicted by:

a) b)

c) d)

10. A capacitor of capacitance 150.0 μ F is connected to an alternating source of emf given by E = 36 sin(120πt)V. [4]
The maximum value of current in the circuit is approximately equal to:

a) 2A b) 1
A
√2

– –
c) 2√2A d) √2 A
11. The ratio of mean value over half cycle to rms value of AC is: [4]
– –
a) 2√2 : π b) √2 :1
–
c) 2 : π d) √2 : π

12. A resistor R, an inductor L, a capacitor C and voltmeters V1, V2 and V3 are connected to an oscillator in the [4]

circuit as shown in the adjoining diagram. When the frequency of the oscillator is increased, then at the
resonance frequency, the voltmeter reading is zero in the case of:

a) voltmeter V1 b) voltmeter V3

c) voltmeter V2 d) all the three voltmeters

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13. In a pure resistivity AC circuit, which of the following sketches represents the variation of the current amplitude [4]
I0 with the frequency ω?

a) b)

c) d)

14. You have two copper cables of equal length for carrying current. One of them has a single wire of area of cross- [4]
section A, the other has ten wires of cross-section ( A

10
) each. Judge their suitability for transporting AC and DC:

a) either for DC, only multiple strands for AC b) only single strand for DC, either for AC

c) only single strand for DC and only multiple d) only single strand for AC, either for DC
strand for AC
15. In a choke coil, the reactance XL and resistance R are: [4]

a) XL = ∞ b) XL >> R

c) XL << R d) XL = R

16. An AC voltage is given by: E = E0 sin 2πt

T
[4]

Then, the mean value of voltage calculated over any time interval of T

2
seconds:

a) is always zero b) is never zero

2e0
c) is ( π
) always d) may be zero
17. For a series L-C-R circuit, the power loss at resonance is: [4]
2

a) I2ωC b) V

1
ωL−
ωC

c) I2R d) V

ωC

18. In an AC circuit having pure capacitor the current: [4]

a) lags voltage by π b) lags voltage by

π

c) leads voltage by π d) leads voltage by

π

19. In a circuit, L, C, and R are connected in series with an alternating voltage source of frequency f. The current [4]

leads the voltage by 45o. The value of C is:

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 a) 1
b) 1

2πf(2πfL−R) πf(2πfL−R)

c) 1

2πf(2πfL+R)
d) 1

πf(2πfL+R)

20. In a series L-C-R circuit R = 200Ω and the voltage and the frequency of the main supply is 220 V and 50 Hz [4]

respectively. On taking out the capacitance from the circuit the current lags behind the voltage by 30o. On taking

out the inductor from the circuit the current leads the voltage by 30o. The power dissipated in the L-C-R circuit
is:

a) 305 W b) zero

c) 242 W d) 210 W
21. In a pure inductive AC circuit: [4]

a) voltage lags behind the current by π b) current lags behind the voltage by
π

c) current leads the voltage by π

2
d) current and voltage are in phase
22. The rms value of an alternating current which produces thrice the heat in a resistor as produced by direct current [4]
of 2 amp, when flowing through the same resistance:

a) 2 A b) 4 A
– –
c) 3√2 A d) 2√3 A
23. A 0.7 henry inductor is connected across a 120 V, 60 Hertz AC source. The current in the inductor will be vary [4]
nearly:

a) 3.55 amp b) 4.55 amp

c) 0.455 amp d) 0.355 amp

–
24. An AC voltage source E = 200 √2 sin 100t is connected across a circuit containing an AC ammeter and [4]
capacitor of capacity 1μ F. The reading of ammeter is:

a) 20 mA b) 40 mA

c) 80 mA d) 10 mA
25. An inductance L having a resistance is connected to an alternating source of angular frequency ω. The quality [4]
factor (Q) of the inductance is:

a) (ωL/R)2 b) (R/ωL)

c) (ωL/R) d) (R/ωL)1/2

26. An alternating current generator has an internal resistance Rg and an internal reactance Xg. It is used to supply [4]

power to a passive load consisting of a resistance Rg and a reactance XL. For maximum power to be delivered

from the generator to the load, the value of XL is equal to:

a) -Xg b) Xg

c) Rg d) zero

27. A 50 Hz AC current of rest value of 1 A flows through the primary coil of a transformer. If the mutual [4]
inductance between the primary and secondary coils is 1.5 H. The rest value of voltage induced in the secondary
is:

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 a) 225 V b) 300 V

c) 75 V d) 250 V
28. If value of R is changed then: [4]

a) voltage across C remains same b) voltage across L-C combination remains

same

c) voltage across L-C combination changes d) voltage across L remains same

29. The peak value of an alternating emf E given by: [4]
E = E0 cos ωt
is 10 volt and frequency is 50 Hz. At time t = ( 1

600
) sec, the instantaneous value of emf is:

a) 1 volt b) 5 volt
–
c) 5√3 volt d) 10 volt
30. A choke coil has: [4]

a) low inductance and low resistance b) high inductance and high resistance

c) high inductance and low resistance d) low inductance and high resistance
31. An inductor 20 mH, a capacitor 50 μ F and a resistor 40Ω are connected in series across a source of emf V = 10 [4]
sin 340 t. The power loss in A.C. circuit is:

a) 0.76 W b) 0.89 W

c) 0.67 W d) 0.51 W
32. The quality factor of LCR circuit having resistance (R) and inductance (L) at resonance frequency (ω) is given [4]
by:
1

a) R

ωL
b) (
ωL
) 2

c) ωL

R
d) (
ωL

R
)
2

33. For an RLC circuit driven with voltage of amplitude vm and frequency ω = 0
1
the current exhibits resonance. [4]
√LC

The quality factor, Q is given by:

ω0 R
a) b)
CR

I ω0

c) R
d) ω0 L

(ω0 C) R

34. The voltage of an AC source varies with time according to the equation: [4]
V = 100 sin 100πt cos 100πt
where t is in second and V is in volt. Then:

a) the peak voltage of the source is ( ) volt b) the peak voltage of the source is 100 volt
100

√2

c) the peak voltage of the source is 50 volt d) the frequency of the source is 50 Hz
35. The rms value of an AC of 50 Hz is 10 amp. The time taken by an alternating current in reaching from zero to [4]
maximum value and the peak value will be:

a) 2 × 10-2 sec and 14.14 amp b) 5 × 10-3 sec and 7.07 amp

c) 5 × 10-3 sec and 14.14 amp d) 1 × 10-2 sec and 7.07 amp

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36. In a transformer loss due to eddy currents is minimised by: [4]
i. laminating the core
ii. laminating the core of steel
iii. increasing the number of turns of the primary coil
iv. Lubricating the core

a) iv and i b) ii and iii

c) only i d) iii and iv

37. A thick resistor of the uniform cross-section is connected to an AC source. If the current densities near the axis [4]
and the circumference of the resistor cross-section be denoted by J1 and J2, then:

a) J1 >/< J2 b) J1 < J2

c) J1 = J2 d) J1 > J2

38. If the frequency of the source emf in an AC circuit is n, the power varies with a frequency: [4]

a) zero b) n

c) d) 2n
n

39. A transformer rated at 10 kW is used to connect a 5 kV transmission line to a 240 V circuit. The ratio of turns in [4]
the windings of the transformer is:

a) 104 b) 40

c) 20.8 d) 5
40. An ideal choke takes a current of 8 ampere when connected to an AC supply of 100 volt and 50 Hz. A pure [4]
resistor under the same conditions takes a current of 10 ampere. If the two are connected to an AC supply of 150
volt and 40 Hz, then the total current through a parallel combination of the resistor and the inductor, when
connected to an AC supply of 150 volt and 40 Hz, is:

a) 30 amp b) 15 amp
–
c) 15√2 amp d) zero amp
41. In a transformer, the core is made of soft iron to reduce: [4]
i. hysteresis losses
ii. eddy current losses
iii. force opposing electric current
iv. none of the above

a) iii and iv b) iv and i

c) only i d) ii and iii

42. For the given figures, choose the correct options: [4]

a) At resonance, current in (b) is less than that b) The rms current in figure (a) is always equal

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 in (a) to that in figure (b)

c) The rms current in circuit (b) can never be d) The rms current in circuit (b) can be larger
larger than that in (a) than that in (a)
43. The reactance of a coil, when used in the domestic AC power supply (220 volt, 50 cycles per second) is 50 ohm. [4]
The inductance of the coil is nearly:

a) 0.22 henry b) 2.2 henry

c) 1.6 henry d) 0.16 henry

44. The current in resistance R at resonance is: [4]

a) infinite b) maximum but finite

c) minimum but finite d) zero

45. If wattless current flows in the AC circuit, then the circuit is [4]

a) Purely Resistive circuit b) Purely Inductive circuit

c) RC series circuit only d) LCR series circuit

46. Which of the following statements is incorrect: [4]
i. In LCR series AC circuit, as the frequency of the source increases, the impedance of the circuit first
decreases and then increases.
ii. If the net reactance of an LCR series AC circuit is same as its resistance, then the current lags behind the
voltage by 45°.
iii. At resonance, the impedance of an AC circuit becomes purely resistive.
iv. Below resonance, voltage leads the current while above it, current leads the voltage.

a) If the net reactance of an LCR series AC b) In LCR series AC circuit, as the frequency
circuit is same as its resistance, then the of the source increases, the impedance of
current lags behind the voltage by 45°. the circuit first decreases and then increases.

c) At resonance, the impedance of an AC d) Below resonance, voltage leads the current

circuit becomes purely resistive. while above it, current leads the voltage.
47. What is the value of inductance L for which the current is maximum in a series LCR circuit with C = 10μ F and [4]

ω = 1000 s-1?

a) cannot be calculated unless R is known b) 10 mH

c) 1 mH d) 100 mH
48. An alternating emf of angular frequency ω is applied across an inductance. The instantaneous power developed [4]
in the circuit has an angular frequency:

a) b)
ω
ω
2

c) ω

4
d) 2ω

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49. The ratio of the secondary to the primary turns in a transformer is 3 : 2 and the output power is P. Neglecting all [4]
power losses, the input power must be:

a) P b)
3P

c) P

2
d) 2P

50. The graphs given below depict the dependence of two reactive impedances X1 and X2 on the frequency of the [4]
alternating emf applied individually to them. We can then say that:

a) X1 is an inductor and X2 is a resistor b) X1 is an inductor and X2 is a capacitor

c) X1 is a resistor and X2 is a capacitor d) X1 is a capacitor and X2 is an inductor

51. Find the capacitive reactance of 10 μ F capacitor, when it is part of a circuit, whose frequency is 100 Hz. [4]
[Hint: Take π = 3.14]

a) 612.1 Ω b) 412.1 Ω

c) 812.1 Ω d) 159.2 Ω
52. An L-C circuit contains a 20 mH inductor and a 50 μ F capacitor with an initial charge of 10mC. The resistance [4]
of the circuit is negligible. Let the instant the circuit is closed at t = 0. At what time is the energy stored
completely magnetic?

a) t = 1.57 ms b) t = 3.14 ms

c) t = 6.28 ms d) t = 0
53. An alternating voltage of 220 V, 50 Hz frequency is applied across a capacitor of capacitance 2μ F. The [4]
impedance of the circuit is:

a) b)
5000 π

π 5000

c) 500 π d)
1000

54. The overall efficiency of a transformer is 90%. The transformer is rated for an output of 9000 watt. The primary [4]
voltage is 1000 volt. The ratio of turns in the primary to the secondary coil is 5 : 1. The iron losses at full load
are 700 watt. The primary voltage is 1000 volt. The primary coil has a resistance of 1 ohm. Then the current in
the primary coil is:

a) 9 amp b) 1 amp

c) 4.5 amp d) 10 amp

55. An alternating emf E = 440 sin 100 πt is applied to a circuit containing an inductance of
√2
H. If an a.c. [4]
π

ammeter is connected in the circuit, its reading will be:

a) 1.55 A b) 4.4 A

c) 2.2 A d) 3.11 A

8 / 11
56. A circuit element X when connected to an a.c. supply of peak voltage 100 V gives a peak current of 5 A which is [4]
in phase with the voltage. A second element Y when connected to the same a.c. supply also gives the same value
of peak current which lags behind the voltage by . If X and Y are connected in series to the same supply, what
π

will be the rms value of the current in ampere?

a) b)
5 10

2 √2

–
c) 5√2 d)
5

√2

57. A step-up transformer operates on a 230 V line and supplies a current of 2 A. The ratio of primary and secondary [4]
windings is 1 : 25. The primary current is

a) 12.5 A b) 50 A

c) 25 A d) 8.8 A
58. A 10 μ F capacitor is connected across a 200 V, 50 Hz AC supply. The peak current through the circuit is: [4]

a) 0.6 A b) A
π 0.6

2 √2

–
c) 0.6√2 A d) 0.6 A
59. A series resonant LCR circuit has a quality factor (Q-factor) 0.4 and a bandwidth of 1.3 kHz. If R = 2kΩ , C = [4]
0.1 μ F, then the value of inductance is:

a) 5 H b) 0.64 H

c) 0.1 H d) 2 H
60. If value of R is changed then: [4]

a) voltage across L-C combination changes b) voltage across C remains same

c) voltage across L remains same d) voltage across L-C combination remains the
same
61. The alternating current through an inductor is given by: I = I0 sin 2πnt then the magnetic field energy in the [4]
inductor changes from a minimum to a maximum value in a time equal to:

a) 1

2n
b) 1

8n

c) 1

4n
d) 1

62. A coil of resistance 10 ohm and inductance 5 henry is connected to a 100 volt battery. Then, the energy stored in [4]
the coil is:

a) 125 erg b) 250 joule

c) 125 joule d) 250 erg

63. An oscillating circuit contains an inductor of inductance 10-5 H and two capacitors each of capacitance 5 × 10-6 [4]
farad connected in parallel. Then the resonance frequency of the circuit is:
5

a) 10

2π
b) √2×10 5

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 5 5

c) 2×10

π
d) 10

64. In an ac circuit, an inductor, a capacitor and a resistor are connected in series with XL = R = Xc. Impedance of [4]
this circuit is:
–
a) R b) R√2

c) Zero d) 2R2

65. An LCR circuit contains resistance of 110 Ω and a supply of 220 V at 300 rad/s angular frequency. If only [4]
capacitance is removed from the circuit, current lags behind the voltage by 45o. If on the other hand, only
inductor is removed the current leads by 45o with the applied voltage. The rms current flowing in the circuit will
be

a) 2.5 A b) 2 A

c) 1.5 A d) 1 A
[4]
2√3
66. When a DC voltage of 200 V is applied to a coil of self inductance( π
) H, a current of 1 A flows through it.
But by replacing DC source with AC source of 200 V, the current in the coil is reduced to 0.5 A. Then, the
frequency of AC supply is:

a) 50 Hz b) 75 Hz

c) 100 Hz d) 60 Hz
–
67. If the impedance is √3 times of resistance, find the phase difference: [4]

a) data is incomplete b) zero

c) 60o d) 30o

68. Find the current passing through the battery immediately after the key (K) is closed. It is given that initially, all [4]
the capacitors are uncharged. (Given that: R = 6Ω . and C = 4μ F)

a) 3 A b) 5 A

c) 1 A d) 2 A
69. A current I flows in an AC circuit. If potential of E = E0 sin ωt has been applied, then the [4]
π
= I0 sin(ωt − )
2

power consumption P in the circuit will be:

E0 I0
a) P =
2
b) P =
EI

√2

E0 I0
c) P = zero d) P =
√2

70. The primary and secondary coils of a transformer have 50 and 1500 turns respectively. If the magnetic flux ϕ [4]
linked with the primary coil is given by: ϕ = ϕ + 41, where ϕ is in webers, t is time in seconds and ϕ is a
0 0

constant, the output voltage across the secondary coil is:

10 / 11
 a) 90 volt b) 120 volt

c) 30 volt d) 220 volt

71. The potential difference across the resistance, capacitance and inductance are 80V, 40V and 100 V respectively [4]
in an L-C-R circuit. The power factor of the circuit is:

a) 0.4 b) 0.8

c) 1.0 d) 0.5
72. A capacitor is a perfect insulator for: [4]

a) none of these b) alternating current

c) direct as well as alternating current d) direct current

73. The phase difference between the current and voltage of LCR circuit in series combination at resonance is: [4]

a) 0o b) −π

c) π d)
π

74. A coil of 10 Ω and 10 mH is connected in parallel to a capacitor of 0.1 μ F. The impedance of the circuit at [4]
resonance is:

a) 103 Ω b) 102 Ω

c) 106 Ω d) 104 Ω

75. A voltage of peak value 283 V and varying frequency is applied to a series L-C-R combination in which R = 3 [4]
Ω ; L = 25 mH and C = 400 μ F. Then, the frequency (in Hz) of the source at which maximum power is
dissipated in the above, is:

a) 51.5 b) 50.7

c) 51.1 d) 50.3

11 / 11