# Physics MCQs for Class 12 with Answers Chapter 7 Alternating Current

## Alternating Current Class 12 Physics MCQs Pdf

1. The phase difference between the alternating current and emf is TE/2. Which ofthe following cannot be the constituent of the circuit?

(a) C alone

(b) L alone

(c) L and C

(d) R and L

**Answer/Explanation**

Answer: c

Explaination:

(c) When both L and C components are there then phase difference could be zero, less than or greater than \(\frac{\pi}{2}\).

2. In an LCR-series ac circuit, the voltage across each of the component L, C and R is 50 V. The voltage across the LC-combination will be

(a) 50 V

(b) 50√2 V

(c) 100 V

(d) zero

**Answer/Explanation**

Answer: d

Explaination:

(d) The voltage across L and C are out of phase. Hence the voltage across the LC combination is zero.

Physics MCQs with Answers for Class 12 Question 3. An ac circuit has a resistance of 12 ohm and an impedance of 15 ohm. The power factor of the circuit will be

(a) 0.8

(b) 0.4

(c) 0.125

(d) 1.25

**Answer/Explanation**

Answer: a

Explaination:

(a) Power factor, cos Φ = \(\frac{R}{Z}\) = \(\frac{12}{15}\) = 0.8

4. In an ac circuit the voltage applied is s = s0 sin (tit. The resulting current in the circuit is I = 70 sin (cot – nil). The power consumption in the circuit is given by

**Answer/Explanation**

Answer: c

Explaination:

(c) Here, Φ = \(\frac{\pi}{2}\),

SO p_{av} = ε_{rms} I_{rms} COS Φ = 0

Physics MCQs for Class 12 Chapter wise with Answers Pdf Question 5. In an LCR circuit, capacitance is charged from C to 2C. For resonant frequency to remain unchanged, the inductance should be changed from L to

(a) 4 L

(b) 2 L

(c) L/2

(d) L/4

**Answer/Explanation**

Answer: c

Explaination:

(c) Since, Resonance frequency,

\(v_{r}=\frac{1}{2 \pi \sqrt{L C}}\)

when C is changed 2C, L should be change to L/2.

6. The core of any transformer is laminated so as to

(a) reduce the energy loss due to eddy currents.

(b) make it light weight.

(c) make it robust and strong.

(d) increase the secondary voltage.

**Answer**

Answer: a

MCQ Questions for Class 12 Physics with Answers Pdf Question 7. In an a.c. generator, a coil with N turns, all of the same area A and total resistance R, rotates with frequency ω in a magnetic field B the maximum value of emf generated in the coil is

(a) NABR

(b) NABω

(c) NABRω

(d) NAB

**Answer/Explanation**

Answer: b

Explaination:

(b) emf induced, ε = NABGω sin ωt,

ε_{0} = NBAω

8. If coil is open, then L and R becomes

(a) infinity, zero

(b) zero, infinity

(c) infinity, infinity.

(d) zero, zero

**Answer/Explanation**

Answer: b

Explaination:

(b) No current flows through an open circuit so magnetic flux is zero and hence, L = 0.

Also as I = 0, so R = ∞.

Physics Multiple Choice Questions with Answers for Class 12 Pdf Question 9. In a series, LCR-circuit, resonant frequency depends on

**Answer/Explanation**

Answer: c

Explaination: (c) Resonance frequency, v_{r} = \(\frac{1}{\sqrt{LC}}\)

10. Which quantity is increased in a step-down transformer?

(a) Current

(b) Voltage

(c) Power

(d) Frequency

**Answer/Explanation**

Answer: a

Explaination: (a) In a step-down transformer current increases and voltage decreases.

11. If an AC voltage is applied to an ICi? circuit, which of the following is true?

(a) I and V are out of phase with each other inR.

(b) I and V are in phase in L while in C, they are out of phase.

(c) I and V are out of phase in both C and L.

(d) I and V are out of phase in L and in phase in C.

**Answer**

Answer: c

12. The peak value of ac voltage on a 220 V mains is

(a) 200√2 V

(b) 230√2 V

(c) 220√2 V

(d) 240√2 V

**Answer/Explanation**

Answer: c

Explaination:

(c) Peak value of ac voltage V0 = √2 V_{rms. = 220√2 V.}

13. Series ac circuit has inductance L, resistance R and angular frequency to, the quality factor Q is

**Answer/Explanation**

Answer: b

Explaination:

(b) Quality factor is measure of sharpness of resonance, is equal to \(\frac{\omega L}{R}\).

Physics MCQ Questions for Class 12 Pdf Question 14. A transformer is used to light a 100 W and 110 V lamp from a 220 V mains. If the main current is 0.5 A, the efficiency of the transformer is approximately

(a) 30%

(b) 50%

(c) 90%

(d) 10%

**Answer/Explanation**

Answer: c

Explaination:

(c) Since efficiency of a transformer,

15. Choose the correct statement.

(a) A capacitor can conduct a dc circuit but not an inductor.

(b) In a dc circuit the inductor can conduct but not a capacitor.

(c) In dc circuit both the inductor and capacitor cannot conduct.

(d) The inductor has infinite resistance in a dc circuit.

**Answer/Explanation**

Answer: b

Explaination:

(b) In dc circuit, an inductor offers zero resistance to dc, while a capacitor offers infinite resistance.

16. What is the value of inductance L for which the current is maximum in a series LCR- circuit with C = 10 μF and ω = 1000 s^{-1}?

(a) 100 mH

(b) 1 mH

(c) 10 mH

(d) cannot be calculated unless R is known

**Answer/Explanation**

Answer: a

Explaination:

(a) Current is maximum at resonance, when

17. A coil of self-inductance L is connected in series with a bulb B and an ac source. Brightness of the bulb decreases when

(a) frequency of the ac source is decreased.

(b) number of turns in the coil is reduced.

(c) a capacitance of reactance Xc = XL in included.

(d) an iron rod is inserted in the coil.

**Answer/Explanation**

Answer: d

Explaination:

(d) Brightness of the bulb decreases when inductive reactance increases, i.e. when iron rod is inserted in the coil, its inductance L increases, hence current decreases.

MCQs Of Physics Class 12 Question 18. The reactance of a capacitor C is X. If both the frequency and capacitance be doubled, then new reactance will be

(a) X

(b) 2X

(c) 4X

(d) \(\frac{X}{4}\)

**Answer/Explanation**

Answer:

Explaination:

when f and C both are doubled.

19. A transformer works on the principle of

(a) converter.

(b) inverter.

(c) mutual inductance.

(d) self-inductance.

**Answer/Explanation**

Answer: c

Explaination: (c) Transformer works on the principle of mutual inductance.

20. The line that draws power supply to your house from street has

(a) zero average current.

(b) 220 V average voltage.

(c) voltage and current out of phase by 90°.

(d) voltage and current are in phase.

**Answer/Explanation**

Answer: a

Explaination: (a) zero average current.

21. Alternating current cannot be measured by dc ammeter, because

(a) ac cannot pass through ac ammeter.

(b) ac charges direction.

(c) average value of current of complete cycle is zero.

(d) ac ammeter will get damaged.

**Answer/Explanation**

Answer: a

Explaination: (a) ac cannot pass through ac ammeter.

22. Average power generated in an inductor connected to an a.c. source is

(a) \(\frac{1}{2}\)LI²

(b) LI²

(c) zero

(d) none of these

**Answer/Explanation**

Answer: c

Explaination:

(c) Average power dissipated in an inductor connected to an ac source is zero,

P_{av} = V_{0}I_{0} cos = 0(Phase angle is \(\frac{\pi}{2}\))

23. The power factor varies between

(a) 2 and 2.5

(b) 3.5 to 5

(c) 0 to 1

(d) 1 to 2

**Answer/Explanation**

Answer: c

Explaination: (c) Power factor cos Φ in an ac circuit varies between 0 and 1.

24. Reciprocal of impedance is

(a) susceptance

(b) conductance

(c) admittance

(d) transconductance

**Answer/Explanation**

Answer: c

Explaination: (c) admittanc

25. Alternating voltage (V) is represented by the equation

(a) V(t) = V_{m} e^{ωt}

(b) V(t) = V_{m} sin ωt

(c) V(t) = V_{m} cot ωt

(d) V(t) = V_{m} tan ωt

**Answer**

Answer: b

26. The rms value of potential difference V shown in the figure is

**Answer**

Answer: c

27. The phase relationship between current and voltage in a pure resistive circuit is best represented by

**Answer**

Answer: b

28. In the case of an inductor

(a) voltage lags the current by \(\frac{π}{2}\)

(b) voltage leads the current by \(\frac{π}{2}\)

(c) voltage leads the current by \(\frac{π}{3}\)

(d) voltage leads the current by \(\frac{π}{4}\)

**Answer**

Answer: b

29. Which of the following graphs represents the correct variation of inductive reactance X_{L} with frequency u?

**Answer**

Answer: b

30. In a pure capacitive circuit if the frequency of ac source is doubled, then its capacitive reactance will be

(a) remains same

(b) doubled

(c) halved

(d) zero

**Answer**

Answer: c

31. Which of the following graphs represents the correct variation of capacitive reactance Xc with frequency v u?

**Answer**

Answer: c

32. In an alternating current circuit consisting of elements in series, the current increases on increasing the frequency of supply. Which of the following elements are likely to constitute the circuit?

(a) Only resistor

(b) Resistor and inductor

(c) Resistor and capacitor

(d) Only inductor

**Answer**

Answer: c

33. In which of the following circuits the maximum power dissipation is observed?

(a) Pure capacitive circuit

(b) Pure inductive circuit

(c) Pure resistive circuit

(d) None of these

**Answer**

Answer: c

34. In series LCR circuit, the phase angle between supply voltage and current is

**Answer**

Answer: a

35. In a series LCR circuit the voltage across an inductor, capacitor and resistor are 20 V, 20 V and 40 V respectively. The phase difference between the applied voltage and the current in the circuit is

(a) 30°

(b) 45°

(c) 60°

(d) 0°

**Answer**

Answer: d

36. At resonance frequency the impedance in series LCR circuit is

(a) maximum

(b) minimum

(c) zero

(d) infinity

**Answer**

Answer: b

37. At resonant frequency the current amplitude in series LCR circuit is

(a) maximum

(b) minimum

(c) zero

(d) infinity

**Answer**

Answer: a

38. Quality factor and power factor both have the dimensions of

(a) time

(b) frequency

(c) work

(d) angle

**Answer**

Answer: d

39. The natural frequency (ca0) of oscillations in LC circuit is given by

**Answer**

Answer: c

40. A transformer works on the principle of

(a) self induction

(b) electrical inertia

(c) mutual induction

(d) magnetic effect of the electrical current

**Answer**

Answer: c

41. For an ideal-step-down transformer, the quantity which is constant for both the coils is

(a) current in the coils

(b) voltage across the coils

(c) resistance of coils

(d) power in the coils

**Answer**

Answer: d

42. If the rms current in a 50 Hz ac circuit is 5 A, the value of the current 1/300 seconds after its value becomes zero is [NCERT Exemplar] (a) 5√2A

(b) \(5 \sqrt{3 / 2}\)A

(c) 5/6 A

(d) 5√2A

**Answer**

Answer: b

43. An alternating current generator has an internal resistance R_{g} and an internal reactance X_{g}. It is used to supply power to a passive load consisting of a resistance R_{g} and a reactance X_{L} For maximum power to be delivered from the generator to the load, the value of X_{L} is equal to [NCERT Exemplar] (a) zero.

(b) X_{g}.

(c) -X_{g}.

(d) R_{g}.

**Answer**

Answer: c

44. When a voltage measuring device is connected to AC mains, the meter shows the steady input voltage of 220 V. This means [NCERT Exemplar] (a) input voltage cannot be AC voltage, but a DC voltage.

(b) maximum input voltage is 220V.

(c) the meter reads not v but < v² > and is . calibrated to read √< v² > .

(d) the pointer of the meter is stuck by some mechanical defect.

**Answer**

Answer: c

45. To reduce the resonant frequency in an LCR series circuit with a generator [NCERT Exemplar] (a) the generator frequency should be reduced.

(b) another capacitor should be added in parallel to the first.

(c) the iron core of the inductor should be removed.

(d) dielectric in the capacitor should be removed.

**Answer**

Answer: b

46. Which of the following combinations should be selected for better tuning of an LCR circuit used for communication? [NCERT Exemplar] (a) R = 20, L= 1.5 H, C = 35 μF.

(b) R = 25 , L = 2.5 H, C = 45 μF.

(c) R = 15 , L = 3.5 H, C = 30 μF.

(d) R = 25 , L = 1.5 H, C = 45 μF.

**Answer**

Answer: c

47. An inductor of reactance 1 Ω and a resistor of 2 Ω are connected in series to the terminals of a 6 V (rms) a.c. source. The power dissipated in the circuit is [NCERT Exemplar] (a) 8 W.

(b) 12 W.

(c) 14.4 W.

(d) 18 W.

**Answer**

Answer: c

48. The output of a step-down transformer is measured to be 24 V when connected to a 12 watt light bulb. The value of the peak current is [NCERT Exemplar] (a) 1/√2 A.

(b) √2A.

(c) 2 A.

(d) 2√2 A.

**Answer**

Answer: a

49. As the frequency of an ac circuit increases, the current first increases and then decreases. What combination of circuit elements is most likely to comprise the circuit? [NCERT Exemplar] (a) Inductor and capacitor.

(b) Resistor and inductor.

(c) Resistor and capacitor.

(d) Inductor only.

**Answer**

Answer: a

50. In an alternating current circuit consisting of elements in series, the current increases on increasing the frequency of supply. Which of the following elements are likely to constitute the circuit?

(a) Only resistor.

(b) Resistor and an inductor.

(c) Resistor and a capacitor.

(d) Only an inductor.

**Answer**

Answer: c

51. Electrical energy is transmitted over large distances at high alternating voltages. Which of the following statements is not correct?

(a) For a given power level, there is a lower current.

(b) Lower current implies less power loss.

(c) Transmission lines can be made thinner.

(d) It is easy to reduce the voltage at the receiving end using step-down transformers.

**Answer**

Answer: c

52. For an LCR circuit, the power transferred from the driving source to the driven oscillator is P = I²Z cos Φ. Which of the following is incorrect?

(a) Here, the power factor cos Φ ≥ 0, P ≥ 0.

(b) The driving force can give no energy to the oscillator (P = 0) in some cases.

(c) The driving force cannot syphon out (P < 0) the energy out of oscillator.

(d) The driving force can take away energy out of the oscillator.

**Answer**

Answer: d

53. When an AC voltage of 220 V is applied to the capacitor C

(a) the maximum voltage between plates is 220 V.

(b) the current is in phase with the applied voltage.

(c) the charge on the plates is not in phase with the applied voltage.

(d) power delivered to the capacitor is zero.

**Answer**

Answer: d

54. Average value of ac over a complete cycle _________ .

**Answer/Explanation**

Answer:

Explaination: zero

55. Inductive reactance increases with _________ in frequency of ac.

**Answer/Explanation**

Answer:

Explaination: increase

56. In ac generator _________ is converted into electrical energy of alternating form.

**Answer/Explanation**

Answer:

Explaination: mechanical energy

57. Electric current that varies in magnitude dontinuously and reverses its direction periodically is known as _________ .

**Answer/Explanation**

Answer:

Explaination: alternating current.

58. Current which flows with a constant magnitude in same fixed direction is known as _________ .

**Answer/Explanation**

Answer:

Explaination: direct current.

59. Average or mean value of ac over a half cycle is _________ .

**Answer/Explanation**

Answer:

Explaination: 0.637 I_{0}

60. Rms or virtual value of alternating current is _________ .

**Answer/Explanation**

Answer:

Explaination: 0.707 I_{0}

61. Current in the inductive a.c. circuit _________ the voltage.

**Answer/Explanation**

Answer:

Explaination: lag behind

62. Capacitive reactance increases with _________ in frequency of ac.

**Answer/Explanation**

Answer:

Explaination: increase

63. Capacitive reactance Xc increases with _________ in frequency of ac and current in the capacitive ac circuit _________ the ac voltage.

**Answer/Explanation**

Answer:

Explaination: decrease, leading ahead

64. Can ever the rms value be equal to the peak value of an ac?

**Answer/Explanation**

Answer:

Explaination: Yes, when the ac is a square wave.

65. Which is more dangerous 220 ac or 220 dc and why?

**Answer/Explanation**

Answer:

Explaination:

∴ ac is more dangerous than dc because its peak value is very high and also ac is in nature.

66. The peak value of emf in ac is E_{0}. Write its

(i) rms, and (ii) average value over a complete cycle. [Foreign 2011]

**Answer/Explanation**

Answer:

Explaination: (i) E_{rms} = \(\frac{E_{0}}{\sqrt{2}}\) (ii) Zero.

67. The current flowing through a pure inductor of inductance 4mH is i= 12 cos 300 t ampere. What is (i) rms, and (ii) average value of the current for a complete cycle? [Foreign 2011]

**Answer/Explanation**

Answer:

Explaination: (i) I_{rms} = \(\frac{12}{\sqrt{2}}\) (ii) I_{av} = zero

68. Calculate the rms value of the alternating current shown in the figure. [HOTS]

**Answer/Explanation**

Answer:

Explaination:

The rms value of the alternating current shown in the figure is 2 A.

69. When an alternating current is passed through a moving coil galvanometer, it shows no deflection. Why?

**Answer/Explanation**

Answer:

Explaination: A moving coil galvanometer measures an average value of current, which is zero for ac. Hence, no deflection is shown by galvanometer

70. Which of the following curves may represent the reactance of a series LC combination? [DoE]

**Answer/Explanation**

Answer:

Explaination:

(b) AS X_{C}-X_{L} = \(\frac{1}{2 \pi v C}\) – 2πvL

71. In a series LCR circuit, the voltages across an inductor, a capacitor and a resistor are 30 V, 30V and 60V respectively. What is the phase difference between the applied voltage and the current in the circuit?

**Answer/Explanation**

Answer:

Explaination: Zero. As V_{L} = V_{C}, circuit is resistive in nature.

72. Can a capacitor of suitable capacitance replace a inductor coil in an AC circuit?

**Answer/Explanation**

Answer:

Explaination: Yes, because average power consumed in both is least while controlling an AC.

73. At an airport, a person is made to walk through the doorway of a metal detector, for security reasons. Is she/he is carrying anything made of metal, the metal detector emits a sound. On what principle does this detector work? [NCERT Exempler]

**Answer/Explanation**

Answer:

Explaination: The metal detector works on the principle of resonances in ac circuits.

74. A capacitor blocks dc and allows ac. Why?

**Answer/Explanation**

Answer:

Explaination:

The capacitive reactance is X_{C} =\(\frac{1}{2 \pi v C}\).

For dc, v = 0 ⇒ X_{C} = ∞

i.e. a capacitor offers infinite resistance to dc and hence blocks it.

For ac, v ≠ 0, ⇒ X_{C} ≠ ∞ , but has some finite value.

Therefore, an ac can pass through the capacitor.

75. A bulb and a capacitor are connected in series to an ac source of variable frequency. How will the brightness of the bulb change on increasing the frequency of the ac source?

**Answer/Explanation**

Answer:

Explaination:

The brightness will increase (∵ X_{C} ∝ \(\frac{1}{v}\)), and heat produced H ∝ I², where

76. Is there are any device by which direct current can be controlled without any loss of energy?

**Answer/Explanation**

Answer:

Explaination:

By the definition, value of the alternating voltage is equal to 110V.

77. A 110 V dc heater is used on an ac source, such that the heat produced is the same. What would be the value of the alternating voltage?

**Answer/Explanation**

Answer:

Explaination: cos Φ = 1.

78. The power factor of an ac circuit is 0.5. What is the phase difference between voltage and current in this circuit? [Foreign 2015]

**Answer/Explanation**

Answer:

Explaination:

∵ Power factor, cos Φ=0.5

∴ Φ = 60° or π/3

So, the phase difference between voltage and current is \(\frac{\pi}{3}\).

79. Why does a low power factor imply large power loss in transmission for circuits used for transporting electric power? [NCERT Example]

**Answer/Explanation**

Answer:

Explaination:

We know that P = TV cos Φ, where cos Φ is the power factor. To supply a given power, at a given voltage, if cos Φ is small, we have to increase the current accordingly. But this will lead to large power loss (I²R) in transmission.

80. The instantaneous current and voltage of an ac circuitaregivenby I = 10sin314tA and V=50sin (314 t + \(\frac{\pi}{2}\)) V. What is the power of dissipation m the circuit?

**Answer/Explanation**

Answer:

Explaination:

Phase difference between the current and voltage is π/2. So, the power dissipation P_{av} = P_{rms} cos Φ is zero.

81. Why is the use of ac voltage preferred over dc voltage? [AI2014]

**Answer/Explanation**

Answer:

Explaination:

An ac voltage can be stepped up or down using a transformer, but not the dc voltage.

82. Does a step down transformer violate the principle of conservation of energy?

**Answer/Explanation**

Answer:

Explaination:

No. In a transformer, if a voltage is increased, the current is decreased in the same ratio and the product VI (power) remains the same.

83. Can we measure 110 V, 50 Hz a.c. using moving coil galvanometers? How can we measure it.

**Answer/Explanation**

Answer:

Explaination:

No.

Reason: As average value of a.c. voltage over one complete cycle is zero.

We can measure the rms value of a.c. voltage using hot wire meters.

84. What is an acceptor circuit and where it is used?

**Answer/Explanation**

Answer:

Explaination: Series LCR circuit is called acceptor circuit radio or TV sets.

85. Why do we prefer carbon brushes than copper in an ac generator? [DoE]

**Answer/Explanation**

Answer:

Explaination: Corrosion free and small expansion on heating maintains proper contact.

86. In a series LCR circuit, X_{L}, X_{c} and R are the inductive reactance, capacitive reactance and resistance respectively at a certain frequency f If the frequency of ac is doubled, what will be the values of reactances and resistance of the circuit? [DoE]

**Answer/Explanation**

Answer:

Explaination:

Resistance R remains unchanged; X_{L} will be doubled and X_{C} will be halved.

87. In a series L-R circuit, X_{L} = R and power factor of the circuit is P_{1} When a capacitor with capacitance C such that X_{L} = X_{c} is put in series, the power factor becomes P_{2} Find P_{1}/P_{2}

**Answer/Explanation**

Answer:

Explaination: 1 : √2