physics : class12 Average Power in Wattless Current : Wattless Current : Electromagnetic Induction and Alternating Currents MCQ Questions and Answers
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Average Power in Wattless Current
MCQ questions and answers.
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MCQ Questions
1. What is the average power consumed in a purely inductive AC circuit?
A.
Zero
B.
Maximum
C.
Half of the maximum power
D.
Equal to the rms current
Correct Answer:
Zero
Explanation:
In a purely inductive circuit, current lags voltage by 90°, so the power factor is cos 90° = 0. Therefore, the average power is zero.
2. In a wattless current, the phase difference between voltage and current is:
A.
0°
B.
45°
C.
90°
D.
180°
Correct Answer:
90°
Explanation:
Wattless current occurs when voltage and current differ in phase by 90°. Hence, the average power P = VI cos 90° becomes zero.
3. The average power in an AC circuit is given by P = Vᵣₘₛ Iᵣₘₛ cos φ. For a wattless current, the value of cos φ is:
A.
1
B.
0
C.
1/2
D.
−1
Correct Answer:
0
Explanation:
For wattless current, the phase angle φ is 90°. Therefore, cos φ = cos 90° = 0, making the average power zero.
4. Which type of circuit ideally carries wattless current?
A.
Purely resistive circuit
B.
Purely inductive circuit
C.
Circuit containing only a resistor
D.
DC circuit with a battery
Correct Answer:
Purely inductive circuit
Explanation:
In a purely inductive circuit, current and voltage are 90° out of phase, so the average power consumed is zero.
5. If the rms voltage is 100 V and the rms current is 2 A in a wattless AC circuit, the average power consumed is:
A.
0 W
B.
50 W
C.
100 W
D.
200 W
Correct Answer:
0 W
Explanation:
For wattless current, the power factor is zero. Thus, P = Vᵣₘₛ Iᵣₘₛ cos φ = 100 × 2 × 0 = 0 W.
6. Why is the current called wattless in a purely capacitive circuit?
A.
The current is zero
B.
The voltage is zero
C.
The average power consumed is zero
D.
The frequency is zero
Correct Answer:
The average power consumed is zero
Explanation:
In a purely capacitive circuit, current leads voltage by 90°. Therefore, the average power, proportional to cos 90°, is zero.
7. For a wattless current, the instantaneous power may be positive or negative, but its average value over one complete cycle is:
A.
Zero
B.
Positive
C.
Negative
D.
Infinite
Correct Answer:
Zero
Explanation:
In a wattless circuit, energy is alternately stored and returned during each cycle. Hence, the positive and negative powers cancel, giving zero average power.
8. A circuit has a power factor of zero. Its average power consumption is:
A.
Vᵣₘₛ Iᵣₘₛ
B.
Vᵣₘₛ/Iᵣₘₛ
C.
Zero
D.
Iᵣₘₛ²
Correct Answer:
Zero
Explanation:
Average power is P = Vᵣₘₛ Iᵣₘₛ multiplied by the power factor. When the power factor is zero, the average power is zero.
9. In a purely inductive circuit, the average power remains zero even when the current has a nonzero value because:
A.
The current is constant
B.
The current and voltage are in phase
C.
The current and voltage differ in phase by 90°
D.
The resistance is very large
Correct Answer:
The current and voltage differ in phase by 90°
Explanation:
A 90° phase difference makes the power factor cos 90° equal to zero. Therefore, nonzero current does not result in average power consumption.
10. The average power consumed by a pure inductor connected to an AC source is:
A.
VI
B.
VI/2
C.
Zero
D.
V/I
Correct Answer:
Zero
Explanation:
For a pure inductor, the phase difference between voltage and current is 90°. Thus, P = VI cos 90° = 0.
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