# MCQ Questions Chapter 7 Alternating Current Class 12 Physics

Please refer to MCQ Questions Chapter 7 Alternating Current Class 12 Physics with answers provided below. These multiple-choice questions have been developed based on the latest NCERT book for class 12 Physics issued for the current academic year. We have provided MCQ Questions for Class 12 Physics for all chapters on our website. Students should learn the objective based questions for Chapter 7 Alternating Current in Class 12 Physics provided below to get more marks in exams.

## Chapter 7 Alternating Current MCQ Questions

Please refer to the following Chapter 7 Alternating Current MCQ Questions Class 12 Physics with solutions for all important topics in the chapter.

MCQ Questions Answers for Chapter 7 Alternating Current Class 12 Physics

Question. The r.m.s. value of potential difference V shown in the figure is

C

Question. 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. In an A.C. circuit, Irms and I0 are related as
(a) Irms = πI0
(b) Irms = 2 I0
(c) Irms = I0
(d) Irms = I0 / 2

D

Question. 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. The current I in an A.C. circuit with inductance coil varies with time according to the graph given below.

A

Question. A 40 μF 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. A transistor-oscillator using a resonant circuit with an inductor L (of negligible resistance) and a capacitor C in series produces oscillations of frequency f. If L is doubled and C is changed to 4C, the frequency will be
(a) f/2
(b) f/4
(c) 8f
(d) f /2 √2

D

Question. Resonance frequency of the given circuit is 4
(a) 50 Hz
(b) 60 Hz
(c) 100 Hz
(d) 120 Hz

A

Question. If voltage drop across a resistor, inductor, and capacitor are 120 V, 70 V, and 20 V respectively, then supply voltage is
(a) 120 V
(b) 130 V
(c) 140 V
(d) 220 V

B

Question. The square root of the product of inductance and capacitance has the dimension of
(a) length
(b) mass
(c) time
(d) no dimension

C

Question. Given the graph, low resistance is depicted by

(a) A
(b) B
(c) C
(d) D

A

Question. If resistance is 8 Ω, then impedance at 300 Hz is

(a) 5 Ω
(b) 6 Ω
(c) 8 Ω
(d) 10 Ω

D

Question. Variation of capacitive reactance with frequency is

C

Question. In RL circuit voltage and current are
(a) in same phase
(b) differ by right angle
(c) differ by complete angle
(d) none of these

B

Question. In a step-up transformer, 220 V is converted into 2200 V. The number of turns in the primary coil is 600.
What is the number of turns in the secondary coil?
(a) 60
(b) 600
(c) 6000
(d) 100

C

Question. Average value of
(a) 63.7% of maximum values sinusoidal current for a half cycle is
(b) Zero of current
(c) 70.7% of maximum value of current
(d) equal to the maximum value of current

B

Question. Given impedance of secondary coil is 220 Ω, if the voltage drops from 220 V to 22 V, current in the primary coil will be
(a) 0.1 A
(b) 1 A
(c) 10 A
(d) 0.01 A

D

Question. For AC, V = 155 sin 377t, the value of supply frequency and rms value of voltage are approximately equal to
(a) 40 Hz, 100 V
(b) 50 Hz, 120 V
(c) 60 Hz, 110 V
(d) 70 Hz, 130 V

C

Question. A small signal voltage V(t) = V0 sinϖt 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.

Question. series LCR circuit is connected to an ac voltage source. When L is removed from the circuit, the phase difference between current and voltage is π/3. If instead C is removed from the circuit, the phase difference is again p/3 between current and voltage. The power factor of the circuit is
(a) zero
(b) 0.5
(c) 1.0
(d) –1.0

C

Question. When the key K is pressed at time t = 0, then which of the following statement about the current I in the resistor AB of the given circuit is true?

(a) I oscillates between 1 mA and 2 mA
(b) At t = 0, I = 2 mA and with time it goes to 1 mA
(c) I = 1 mA at all t
(d) I = 2 mA at all t.

B

Question. The time constant of C-R circuit is
(a) 1/CR
(b) C/R
(c) CR
(d) R/C

C

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

C

Question. 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) 2.0 A
(b) 4.0 A
(c) 8.0 A
(d) (20/√13) A

B

Question. In the given circuit the reading of voltmeter V1 and V2 are 300 volts each. The reading of the voltmeter V3 and ammeter A are respectively

(a) 150 V, 2.2 A
(b) 220 V, 2.2 A
(c) 220 V, 2.0 A
(d) 100 V, 2.0 A

B

Question. What is the value of inductance L for which the current is maximum in a series LCR circuit with C = 10 μF and w = 1000 s–1?
(a) 1 mH
(b) cannot be calculated unless R is known
(c) 10 mH
(d) 100 mH

D

Question. In an electrical circuit R, L, C and ac voltage source are all connected in series. When L is removed from the circuit, the phase difference between the voltage and the current in the circuit is π/3. If instead, C is removed from the circuit, the phase difference is again π/3. The power factor of the circuit is
(a) 1/2
(b) 1/√2
(c) 1
(d) √3/2

C

Question. The instantaneous values of alternating current and voltages in a circuit are given as

(a) 1/2
(b) 1/√2
(c) 1
(d) √3/2

D

Question. In a circuit L, C and R are connected in series with an alternating voltage source of frequency f. The current leads the voltage by 45°. The value of C is

D

Question. The value of quality factor is

A, B

Question. An series L-C-R circuit is connected to a source of A.C. current. At resonance, the phase difference between the applied voltage and the current in the circuit, is
(a) π
(b) zero
(c) π/4
(d) π/2

B

Question. An inductor 20 mH, a capacitor 100 mF and a resistor 50 W are connected in series across a source of emf, V = 10 sin 314t. The power loss in the circuit is
(a) 0.79 W
(b) 0.43 W
(c) 2.74 W
(d) 1.13 W

A

Question. The potential differences across the resistance, capacitance and inductance are 80 V, 40 V and 100 V respectively in an L-C-R circuit. The power factor of this circuit is
(a) 0.4
(b) 0.5
(c) 0.8
(d) 1.0

C

Question. An inductor 20 mH, a capacitor 50 μF and a resistor 40 Ω are connected in series across a source of emf V = 10 sin 340t. The power loss in A.C. circuit is
(a) 0.76 W
(b) 0.89 W
(c) 0.51 W
(d) 0.67 W

C

Question. A 220 volt input is supplied to a transformer. The output circuit draws a current of 2.0 ampere at 440 volts. If the efficiency of the transformer is 80%, the current drawn by the primary windings of the transformer is
(a) 3.6 ampere
(b) 2.8 ampere
(c) 2.5 ampere
(d) 5.0 ampere

D

Question. Power dissipated in an LCR series circuit connected to an A.C. source of emf e is

D

Question. In an a.c. circuit the e.m.f. (e) and the current (i) at any instant are given respectively by e = E0sinwt , i = I0sin(ϖt – Φ) The average power in the circuit over one cycle of a.c. is

A

Question. A coil of inductive reactance 31 Ω has a resistance of 8 W. It is placed in series with a condenser of capacitative reactance 25 Ω. The combination is connected to an a.c. source of 110 V. The power factor of the circuit is
(a) 0.33
(b) 0.56
(c) 0.64
(d) 0.80

D

Question. For a series LCR circuit, the power loss at resonance is

C

Question. A condenser of capacity C is charged to a potential difference of V1. The plates of the condenser are then connected to an ideal inductor of inductance L. The current through the inductor when the potential difference across the condenser reduces to V2 is

D

Question. 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. 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. 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. The primary and secondary coils of a transformer have 50 and 1500 turns respectively. If the magnetic flux f linked with the primary coil is given by Φ = Φ0 + 4t, where f is in webers, t is time in seconds and f0 is a constant, the output voltage across the secondary coil is
(a) 120 volts
(b) 220 volts
(c) 30 volts
(d) 90 volts

A

Question. In an a.c. circuit with phase voltage V and current I, the power dissipated is
(a) V.I
(b) depends on phase angle between V and I

B

Question. In an A.C. circuit, the current flowing is I = 5 sin (100t – π/2) ampere and the potential difference is V = 200 sin (100t) volts. The power
consumption is equal to
(a) 20 W
(b) 0 W
(c) 1000 W
(d) 40 W

B

Question. 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 amp, the efficiency of the transformer is approximately
(a) 50%
(b) 90%
(c) 10%
(d) 30%

B

Question. The core of a transformer is laminated because
(a) ratio of voltage in primary and secondary may be increased
(b) energy losses due to eddy currents may be minimised
(c) the weight of the transformer may be reduced
(d) rusting of the core may be prevented.

B

Question. A series R-C circuit is connected to an alternating voltage source. Consider two situations :
(i) When capacitor is air filled.
(ii) When capacitor is mica filled.
Current through resistor is i and voltage across capacitor is V then
(a) ia > ib
(b) Va = Vb
(c) Va < Vb
(d) Va > V

D

Question. 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. An ac voltage is applied to a resistance R and an inductor L in series. If R and the inductive reactance are both equal to 3 Ω, the phase difference between the applied voltage and the current in the circuit is
(a) π/6
(b) π/4
(c) π/2
(d) zero

B

Question. A step-up transformer operates on a 230 V line and supplies a load of 2 ampere. The ratio of the primary and secondary windings is 1 : 25. The current in the primary is
(a) 15 A
(b) 50 A
(c) 25 A
(d) 12.5 A

B

Question. 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. Figure shows a circuit that contains three identical resistors with resistance R = 9.0 Ω each, two identical inductors with inductance L = 2.0 mH each, and an ideal battery with emf e = 18 V. The current i through the battery just after the switch closed is

(a) 0.2 A
(b) 2 A
(c) 0 ampere
(d) 2 mA

B

Question. 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. In the circuit given in figure, 1 and 2 are ammeters. Just after key K is pressed to complete the circuit, the reading will be

(a) zero in 1, maximum in 2
(b) maximum in both 1 and 2
(c) zero in both 1 and 2
(d) maximum in 1, zero in 2.

D

Assertion and Reasoning Based Questions :

(a) If both assertion and reason are true and reason is the correct explanation of assertion.
(b) If both assertion and reason are true but reason is not the correct explanation of assertion.
(c) If assertion is true but reason is false.
(d) If both assertion and reason are false.

Question. Assertion. In series L-C-R circuit, the resonance occurs at one frequency only.
Reason. At resonance, the inductive reactance is equal and opposite to the capacitive reactance.

A

Question. Assertion. At resonance, L-C-R series circuit has a minimum current.
Reason. At resonance, in L-C-R series circuit, the current and emf are not in phase with each other.

D

Question. Assertion. Transformers are used only in alternating current source, not in direct current.
Reason. Only AC can be stepped up or down by means of transformers.

A

Question. Assertion. At resonance, the inductive reactance is equal to the capacitive reactance.
Reason. In series L-C-R circuit, the inductive reactance is equal and opposite to the capacitive reactance.

C

Question. Assertion. The quantity L/R possesses the dimension of time
Reason. In order to reduce the rate of increase of current through a solenoid, we should increase the time constant.

C

Question. Assertion. It is advantageous to transmit electric power at high voltage.
Reason. High voltage implies high current.

C

Assertion-Reason Questions

In the following questions, a statement of Assertion (A) is followed by a statement of Reason (R). Choose the correct answer out of the following choices.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false and R is also false.

Question. Assertion (A): Capacitor serves as a block for DC and offers an easy path to AC.
Reason (R): Capacitive reactance is inversely proportional to frequency.

A

Question. Assertion (A): In series LCR resonance circuit, the impedance is equal to the ohmic resistance.
Reason (R): At resonance, the inductive reactance exceeds the capacitive reactance.

C

Question. Assertion (A): An alternating current of frequency 50 Hz becomes zero, 100 times in one second.
Reason (R): Alternating current changes direction and becomes zero twice in a cycle.

A

Question. Assertion (A): A capacitor of suitable capacitance can be used in an AC circuit in place of the choke coil.
Reason (R): A capacitor blocks DC and allows AC only.

B

Question. Assertion (A): 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 lags behind potential difference across the inductance by an angle π/2.
Reason (R): In LR circuit voltage leads the current by phase angle which depends on the value of inductance and resistance both.

B

Question. Assertion (A): An alternating current does not show any magnetic effect.
Reason (R): Alternating current does not vary with time.

D

Question. Assertion (A): The quantity L/R possesses the dimension of time.
Reason (R): In order to reduce the rate of increase of current through a solenoid, we should increase the time constant.

B

Question. Assertion (A): In series LCR-circuit, the resonance occurs at one frequency only.
Reason (R): At resonance, the inductive reactance is equal and opposite to the capacitive reactance.

A

Question. Assertion (A): When capacitive reactance is smaller than the inductive reactance in LCR circuit, emf leads the current.
Reason (R): The phase angle is the angle between the alternating emf and alternating current of the circuit.