# Chapter 7 Alternating Current Class 12 MCQ Questions with Answers

Alternating current Class 12 MCQ is one of the best strategies to prepare for the CBSE Class 12 Board exam. If you want to complete a grasp concept or work on one’s score, there is no method except constant practice. Students can improve their speed and accuracy by doing more MCQ on Alternating Current Class 12 which will help them all through their board test.

## MCQ on Alternating Current Class 12 with Answers

Class 12 Physics MCQ with answers are given here to Alternating current. These MCQs are based on the latest CBSE board syllabus and relate to the latest Class 12 Physics syllabus. By Solving these Class 12 MCQs, you will be able to analyze all of the concepts quickly in the chapter and get ready for the Class 12 Annual exam.

Learn Class 12 Alternating current MCQ Questions with Answers PDF download according to the latest CBSE and NCERT syllabus. Students should prepare for the examination by solving CBSE MCQ on Alternating current Class 12 PDF given below.

Question 1. Which of the following combinations should be selected for better tuning of an L-C-R circuit used for communication ?
(a) R = 20 W, L = 1.5 H, C = 35 mF
(b) R = 25 W, L = 2.5 H, C = 45 mF
(c) R = 15 W, L = 3.5 H, C = 30 mF
(d) R = 25 W, L = 1.5 H, C = 45 mF

C

Question 2. A circuit when connected to an AC source of 12 V gives a current of 0.2 A. The same circuit when connected to a DC source of 12 V, gives a current of
0.4 A. The circuit is
(a) series LR
(b) series RC
(c) series LC
(d) series LCR

A

Question 3. In a circuit the coil of a choke :
(a) decreases the current
(b) increases the current
(c) has high resistance to D. C. circuit
(d) no effect with the current

A

Question 4. Turn ratio in a step up transformer is 1 : 2 if a Lechlanche cell of 1.5 V is connected across the input, then the voltage across the output will be
(a) 0.1 V
(b) 1.5 V
(c) 0.75 V
(d) zero

D

Question 5. The coefficient of mutual inductance, when magnetic flux changes by 2 × 10–2 Wb and current changes by 0.01 A is
(a) 8 henry
(b) 4 henry
(c) 3 henry
(d) 2 henry

D

Question 6. In a circuit, the current lags behind the voltage by a phase difference of p /2, the circuit will contain which of the following :
(a) only R
(b) only C
(c) R and C
(d) only L

D

Question 7. A capacitor of capacitance 2μF is connected in the tank circuit of an oscillator oscillating with a frequency of 1 kHz. If the current flowing in the circuit is 2 m A, the voltage across the capacitor will be :
(a) 0.16 V
(b) 0.32 V
(c) 79.5 V
(d) 159 V

A

Question 8. In an ideal parallel LC circuit, the capacitor is charged by connecting it to a D.C. source which is then disconnected. The current in the circuit
(a) becomes zero instantaneously
(b) grows monotonically
(c) decays monotonically
(d) oscillates instantaneously

D

Question 9. With the decrease of current in the primary coil from 2 amperes to zero value in 0.01s the emf generated in the secondary coil is 1000 volts. The mutual inductance of the two coils is
(a) 1.25 H
(b) 2.50 H
(c) 5.00 H
(d) 10.00 H

C

Question 10. In an AC circuit the potential differences across an inductance and resistance joined in series are respectively 16 V and 20 V. The total potential difference of the source is
(a) 20.0 V
(b) 25.6 V
(c) 31.9 V
(d) 53.5 V

B

Question 11. If an AC main supply is given to be 220 V. The average emf during a positive half cycle will be
(a) 198 V
(b) 220 V
(c) 240 V
(d) 220 √2V

A

Question 12. An AC source of angular frequency w 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 w/3 (but maintaining the same voltage), the current in the circuit is found to be halved. Calculate the ratio of reactance to resistance at the original frequency w
(a) √(3/5)
(b)√(2/5)
(c) √(1/5)
(d) √(4/5)

A

Question 13. A small signal voltage V(t) = V0 sinwt is applied across an ideal capacitor C
(a) Current I(t) is in phase with voltage V(t).
(b) Current I(t) leads voltage V(t) by 180°.
(c) Current I(t), lags voltage V(t) by 90°.
(d) Over a full cycle the capacitor C does not consume any energy from the voltage source.

D

Question 14. A 40 mF capacitor is connected to a 200 V, 50 Hz ac supply. The r.m.s value of the current in the circuit is, nearly
(a) 1.7 A
(b) 2.05 A
(c) 2.5 A
(d) 25.1 A

C

Question 15. In a AC circuit the voltage and current are described by V 200sin (319-Π/6) volts and i 50sin (314t+Π/6) respectively. The average power dissipated in the circuit is :
(a) 2.5 watts
(b) 5.0 watts
(c) 10.0 watts
(d) 50.0 watts

A

Question 16. A coil has an inductance of 0.7 henry and is joined in series with a resistance of 220 W. When the alternating emf of 220 V at 50 Hz is applied to it then the phase through which current lags behind the applied emf and the wattless component of current in the circuit will be respectively
(a) 30°, 1 A
(b) 45°, 0.5 A
(c) 60°, 1.5 A
(d) none of these

B

Question 17. An inductor coil of inductance L is cut into two equal parts and both the parts are connected in parallel. The net inductance is :
(a) L
(b) L/2
(c) L/4
(d) 2 L.

C

Question 18. An inductance L having a resistance R is connected to an alternating source of angular frequency w. The Quality factor Q of inductance is
(a) R/ wL
(b) (wL/R)2
(c) (R /wL)1/2
(d) wL/R

D

Question 19. If we decrease the frequency of the applied A.C. with a purely capacitive load, do (1) the amplitude of Vc and (2) amplitude of Ic increase, decrease of remain the same.
(a) (1) increase (2) same
(b) (1) same (2) increase
(c) (1) same (2) decrease
(d) (1) decrease (2) same

C

Question 20. L, C, R represent physical quantities inductance, capacitance and resistance respectively. The combinations which have the dimensions of frequency are
(a) 1/RC
(b) R/L
(c) 1/ LC
(d) C/L

C

Question 21. In an A.C. circuit, the current flowing in inductance is I = 5 sin (100 t – p/2) amperes and the potential difference is V = 200 sin (100 t) volts. The power consumption is equal to
(a) 1000 watt
(b) 40 watt
(c) 20 watt
(d) zero

D

Question 22. An inductance L having a resistance R is connected to an alternating source of angular frequency w. The Quality factor Q of inductance is
(a) R/ wL
(b) (wL/R)2
(c) (R /wL)½
(d) wL/R

D

Question 23. The mutual inductance of a pair of coils, each of N turns, is M henry. If a current of I ampere in one of the coils is brought to zero in t second, the emf induced per turn in the other coil, in volt, will be
(a) MI /t
(b) NMI /t
(c) MN /It
(d) MI /Nt

A

Question 24. A step down transformer is connected to 2400 volts line and 80 amperes of current is found to flow in output load. The ratio of the turns in primary and secondary coil is 20 : 1. If transformer efficiency is 100%, then the current flowing in the primary coil will be
(a) 1600 amp
(b) 20 amp
(c) 4 amp
(d) 1.5 amp

C

Question 25. An A.C. circuit containing only capacitance, the current :
(a) lags the voltage by 90°
(b) leads the voltage by 90°
(c) remains in phase with voltage
(d) leads the voltage in 180°

B

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Question 26. The tuning circuit of a radio receiver has a resistance of 50W, an inductor of 10 mH and a variable capacitor. A 1 MHz radio wave produces a potential difference of 0.1 mV. The values of the capacitor to produce resonance is (Take Π2 = 10)
(a) 2.5 pF
(b) 5.0 pF
(c) 25 pF
(d) 50 pF

A

Question 27. A choke coil has:
(a) Low inductance and low resistance
(b) High inductance and high resistance
(c) Low inductance and high resistance
(d) High inductance and low resistance

D

Question 28. A coil has resistance 30 ohm and inductive reactance 20 ohm at 50 Hz frequency. If an ac source, of 200 volt, 100 Hz, is connected across the coil, the current in the coil will be
(a) 4.0 A
(b) 8.0 A
(c) 20√13 A
(d) 2.0 A

A

Question 29. A coil of 40 henry inductance is connected in series with a resistance of 8 ohm and the combination is joined to the terminals of a 2 volt battery. The time constant of the circuit is
(a) 5 seconds
(b) 1/5 seconds
(c) 40 seconds
(d) 20 seconds

A

Question 30. An ideal coil of 10H is connected in series with a resistance of 5W and a battery of 5V. After 2 sec the connection is made, the current flowing in ampere in the circuit is
(a) (1 – e–1)
(b) (1 – e)
(c) e
(d) e–1

A

Question 31. The primary winding of a transformer has 500 turns whereas its secondary has 5000 turns. The primary is connected to an A.C. supply of 20 V, 50 Hz. The secondary will have an output of
(a) 2 V, 50 Hz
(b) 2 V, 5 Hz
(c) 200 V, 50 Hz
(d) 200 V, 500 Hz.

C

Question 32. An inductor, a resistor and a capacitor are joined in series with an AC source. As the frequency of the source is slightly increased from a very low value, the reactance of the
(a) inductor increases
(b) resistor increases
(c) capacitor increases
(d) circuit increases

C

Question 33. Two cables of copper are of equal lengths. One of them has a single wire of area of cross-section A, while other has 10 wires of cross-sectional area A/10 each. Give their suitability for transporting A.C. and D.C.
(a) only multiple strands for A.C., either for D.C.
(b) only multiple strands for A.C., only single strand for D.C.
(c) only single strand for D.C., either for A.C.
(d) only single strand for A.C., either for D.C.

A

Question 34. In an A.C. circuit, Irms and I0 are related as
(a) Irms = ΠI0
(b) Irms = √2 I0
(c) Irms = I0/Π
(d) Irms = I0 /√ 2

C

Question 35. 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) a capacitance of reactance XC = XL is included in the same circuit.
(b) an iron rod is inserted in the coil.
(c) frequency of the AC source is decreased.
(d) number of turns in the coil is reduced.

B

Question 36. A transformer having efficiency of 90% is working on 200 V and 3 kW power supply. If the current in the secondary coil is 6 A, the voltage across the
secondary coil and the current in the primary coil respectively are
(a) 300 V, 15 A
(b) 450 V, 15 A
(c) 450 V, 13.5 A
(d) 600 V, 15 A

B

Question 37. In an ac circuit an alternating voltage 200 2 sin100 t volts is connected to a capacitor of capacity 1 mF. The r.m.s. value of the current in the circuit is
(a) 10 mA
(b) 100 mA
(c) 200 mA
(d) 20 mA

D

Question 38. The primary of a transformer when connected to a dc battery of 10 volt draws a current of 1 mA. The number of turns of the primary and secondary windings are 50 and 100 respectively. The voltage in the secondary and the current drawn by the circuit in the secondary are respectively
(a) 20 V and 2.0 mA
(b) 10 V and 0.5 mA
(c) Zero volt and therefore no current
(d) 20 V and 0.5 mA

C

Question. 39 An alternating voltage V = V0 sin ϖt is applied across a circuit. As a result, a current I = I0 sin (ϖt – π/2) flows in it. The power consumed per cycle is
(a) zero
(b) 0.5 V0I0
(c) 0.707 V0I0
(d) 1.414 V0I0

A

Question 40. A capacitor of capacity C has reactance X. If capacitance and frequency become double then reactance will be
(a) 4X
(b) X/2
(c) X/4
(d) 2X

C

Question 41. A transformer is used to light a 140 W, 24 V bulb from a 240 V a.c. mains. The current in the main cable is 0.7 A. The efficiency of the transformer is
(a) 63.8 %
(b) 83.3 %
(c) 16.7 %
(d) 36.2 %

B

Question 42. The frequency of A.C. mains in India is
(a) 30 c/s
(b) 50 c/s
(c) 60 c/s
(d) 120 c/s

B

Question 43. The impedance in a circuit containing a resistance of 1 Ω and an inductance of 0.1 H in series, for AC of 50 Hz, is
(a) 100 √10Ω
(b) 10 √10Ω
(c) 100Ω
(d) 10Ω

B

Question 44. The primary winding of a transformer has 100 turns and its secondary winding has 200 turns. The primary is connected to an A.C. supply of 120 V and the current flowing in it is 10 A. The voltage and the current in the secondary are
(a) 240 V, 5 A
(b) 240 V, 10 A
(c) 60 V, 20 A
(d) 120 V, 20 A

A

Question 45. A 12 W resistor and a 0.21 henry inductor are connected in series to an a.c. source operating at 20 volt, 50 cycle. The phase angle between the current and source voltage is
(a) 30º
(b) 40º
(c) 80º
(d) 90º

C

Question 46. A step down transformer is connected to 2400 volts line and 80 amperes of current is found to flow in output load. The ratio of the turns in primary and secondary coil is 20 : 1. If transformer efficiency is 100%, then the current flowing in the primary coil will be
(a) 1600 amp
(b) 20 amp
(c) 4 amp
(d) 1.5 amp

C

Question 47. The primary winding of transformer has 500 turns whereas its secondary has 5000 turns. The primary is connected to an A.C. supply of 20 V, 50 Hz. The secondary will have an output of
(a) 2 V, 5 Hz
(b) 200 V, 500 Hz
(c) 2V, 50 Hz
(d) 200 V, 50 Hz

D

Question 48. In the given circuit, the current drawn from the source is
(a) 20 A
(b) 10 A
(c) 5 A
(d) 5 √2 A

D

Question 49. In an A.C. circuit, a resistance of R ohm is connected in series with an inductance L. If phase angle between voltage and current be 45°, the value of inductive reactance will be
(a) R/4
(b) R/2
(c) R
(d) cannot be found with given data

C

Question 50. An alternating voltage E (in volts) = 200 √2 sin 100 t is connected to one micro farad capacitor through an a.c. ammeter. The reading of the ammeter shall be
(a) 100 mA
(b) 20 mA
(c) 40 mA
(d) 80 mA

B

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