CBSE Class 12 Physics Alternating Current MCQs

Question: Alternating current cannot be measured by D.C. ammeter, because

(a) A. C. is virtual
(b) A. C. changes its direction
(c) A. C. cannot pass through D.C. ammeter
(d) average value of A. C for complete cycle is zero

Answer: D

Question: A.C. power is transmitted from a power house at a high voltage as

(a) the rate of transmission is faster at high voltages
(b) it is more economical due to less power loss
(c) power cannot be transmitted at low voltages
(d) a precaution against theft of transmission lines

Answer: B

Question: If the frequency of an A.C. is made 4 times of its initial value, the inductive reactance will
(a) be 4 times
(b) be 2 times
(c) be half     
(d) remain the same

Answer: A

Question: When an ac voltage of 220 V is applied to the capacitor C, then
(a) the maximum voltage between plates is 220 V.
(b) the current is in phase with the applied voltage.
(c) the charge on the plate is not in phase with the applied votage.
(d) power delivered to the capacitor per cycle is zero

Answer: D

Question: An A.C. source is connected to a resistive circuit. Which of the following is true?
(a) Current leads ahead of voltage in phase
(b) Current lags behind voltage in phase
(c) Current and voltage are in same phase
(d) Any of the above may be true depending upon the value of resistance.

Answer: C

Question: With increase in frequency of an A.C. supply, the inductive reactance
(a) decreases
(b) increases directly with frequency
(c) increases as square of frequency
(d) decreases inversely with frequency

Answer: B

Question: The capacitive reactance in an A.C. circuit is
(a) effective resistance due to capacity
(b) effective wattage
(c) effective voltage
(d) None of these

Answer: A

Question: 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: C

Question: In an L.C.R. series a.c. circuit, the current
(a) is always in phase with the voltage
(b) always lags the generator voltage
(c) always leads the generator voltage
(d) None of these

Answer: D

Question: If an LCR series circuit is connected to an ac source, then at resonance the voltage across
(a) R is zero
(b) R equals the applied voltage
(c) C is zero
(d) L equals the applied voltage

Answer: B

Question: The power factor in a circuit connected to an A.C. The value of power factor is 
(a) unity when the circuit contains an ideal inductance only
(b) unity when the circuit contains an ideal resistance only
(c) zero when the circuit contains an ideal resistance only
(d) unity when the circuit contains an ideal capacitance only

Answer: B

Question: In series combination of R, L and C with an A.C. source at resonance, if R = 20 ohm, then impedence Z of the combination is 
(a) 20 ohm
(b) zero
(c) 10 ohm
(d) 400 ohm

Answer: A

Question: In an LCR series a.c. circuit, the voltage across each of the components, L, C and R is 50V. The voltage across the LC combination will be 
(a) 100 V
(b) 150 V
(c) 50 V 
(d) 0 V

Answer: D

Question: Choose the correct option
Assertion : When the frequency of the AC source in an LCR circuit equals the resonant frequency, the reactance of the circuit is zero, and so there is no current through the inductor or the capacitor.
Reason : The net current in the inductor and capacitor is zero. 
(a) Assertion is correct, reason is correct; reason is a correct explanation for assertion.
(b) Assertion is correct, reason is correct; reason is not a correct explanation for assertion
(c) Assertion is correct, reason is incorrect
(d) Assertion is incorrect, reason is correct

Answer: D

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) 20 seconds
(b) 5 seconds
(c) 1/5 seconds
(d) 40 seconds

Answer: B

Question: A current of 4A flows in a coil when connected to a 12V dc source. If the same coil is connected to a 12V, 50 rad/s a.c. source, a current of 2.4A flows in the circuit. Determine the inductance of the coil.

(a) 0.08 H
(b) 0.04 H
(c) 0.02 H
(d) 1 H

Answer: A

Question: An inductive circuit contains resistance of 10 ohms and an inductance of 2 henry. If an A.C. voltage of 120 Volts and frequency 60 Hz is applied to this circuit, the current would be nearly 

(a) 0.32 A
(b) 0.16 A
(c) 0.48 A
(d) 0.80 A

Answer: B

Question: A transformer is employed to

(a) convert A.C. into D.C.
(b) convert D.C. into A.C.
(c) obtain a suitable A.C. voltage
(d) obtain a suitable D.C. voltage

Answer: C

Question: The transformer voltage induced in the secondary coil of a transformer is mainly due to

(a) a varying electric field
(b) a varying magnetic field
(c) the vibrations of the primary coil
(d) the iron core of the transformer

Answer: B

Question:  The loss of energy in the form of heat in the iron core of a transformer is

(a) iron loss           
(b) copper loss
(c) mechanical loss
(d) None of these

Answer: A