Physics: Alternating Current Class 12 MCQ

1. The main advantage of alternating current (AC) over direct current (DC) is that:

a) AC is safer than DC
b) AC can be easily converted to different voltages using transformers
c) AC is more economical
d) AC can be generated easily

Answer:

b) AC can be easily converted to different voltages using transformers

Explanation:

Transformers work on the principle of electromagnetic induction, which is only possible with AC.

2. The frequency of the domestic power supply in most countries is:

a) 30 Hz
b) 50 Hz
c) 60 Hz
d) 100 Hz

Answer:

b) 50 Hz

Explanation:

Most countries, especially in Europe and Asia, use a 50 Hz frequency for their domestic power supply.

3. In an AC circuit, the value which represents its maximum voltage is called:

a) Peak voltage
b) Root mean square voltage
c) Average voltage
d) Instantaneous voltage

Answer:

a) Peak voltage

Explanation:

Peak voltage represents the maximum voltage value in an AC circuit.

4. Which of the following devices works on the principle of mutual induction?

a) DC motor
b) Transformer
c) Diode
d) Resistor

Answer:

b) Transformer

Explanation:

Transformers work based on the principle of mutual induction between two or more coils.

5. The current which changes its direction periodically is:

a) Direct Current
b) Alternating Current
c) Instantaneous Current
d) Average Current

Answer:

b) Alternating Current

Explanation:

Alternating Current (AC) changes its direction and magnitude periodically.

6. In a purely resistive AC circuit, the current:

a) Leads the voltage by 90°
b) Lags behind the voltage by 90°
c) Is in phase with the voltage
d) Can be any phase with respect to the voltage

Answer:

c) Is in phase with the voltage

Explanation:

In a purely resistive circuit, voltage and current are in phase with each other.

7. In a purely capacitive AC circuit, the current:

a) Leads the voltage by 90°
b) Lags behind the voltage by 90°
c) Is in phase with the voltage
d) Is independent of the voltage

Answer:

a) Leads the voltage by 90°

Explanation:

In a purely capacitive circuit, current leads the voltage by 90°.

8. The power factor of a purely resistive AC circuit is:

a) 0
b) 0.5
c) 1
d) Undefined

Answer:

c) 1

Explanation:

Power factor is the cosine of the phase angle. For a purely resistive circuit, the phase angle is 0°, making the power factor 1.

9. The impedance of an AC circuit is the:

a) Resistance in the circuit
b) Total opposition to the flow of alternating current
c) Reactive power in the circuit
d) Rate at which work is done

Answer:

b) Total opposition to the flow of alternating current

Explanation:

Impedance combines both resistance and reactance, offering total opposition to AC.

10. A choke is used in a tube light to:

a) Decrease the current
b) Increase the current
c) Increase the voltage momentarily
d) Reduce the voltage momentarily

Answer:

c) Increase the voltage momentarily

Explanation:

The choke, being an inductor, provides a large inductive reactance that momentarily increases the voltage to start the discharge in the tube.

11. The reactance offered by a capacitor to alternating current is:

a) Directly proportional to the frequency and the capacitance
b) Inversely proportional to the frequency and the capacitance
c) Directly proportional to the frequency and inversely proportional to the capacitance
d) Inversely proportional to the frequency and directly proportional to the capacitance

Answer:

c) Directly proportional to the frequency and inversely proportional to the capacitance

Explanation:

Capacitive reactance, Xc = 1/(2πfC), where f is frequency and C is capacitance.

12. Which device converts AC into DC?

a) Transformer
b) Rectifier
c) Inverter
d) Transistor

Answer:

b) Rectifier

Explanation:

A rectifier is an electrical device that converts alternating current (AC) to direct current (DC).

13. The power consumed in a purely capacitive circuit is:

a) Maximum
b) Minimum
c) Zero
d) Equal to the power in a purely resistive circuit

Answer:

c) Zero

Explanation:

In a purely capacitive circuit, the power factor is zero, hence power consumed is zero.

14. The symbol '~' represents:

a) Direct Current
b) Alternating Current
c) High Voltage
d) Low Voltage

Answer:

b) Alternating Current

Explanation:

The '~' symbol is used to represent alternating current (AC).

15. In a series LCR circuit at resonance, the:

a) Current is minimum
b) Voltage is minimum
c) Current is maximum
d) Resistance is maximum

Answer:

c) Current is maximum

Explanation:

At resonance in a series LCR circuit, the inductive reactance equals capacitive reactance, making impedance minimum and current maximum.

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