physics : class12 Ninety-Degree Phase Difference : Wattless Current : Electromagnetic Induction and Alternating Currents MCQ Questions and Answers
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Ninety-Degree Phase Difference
MCQ questions and answers.
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MCQ Questions
1. In a purely inductive AC circuit, the current differs in phase from the voltage by
A.
0 degrees
B.
45 degrees
C.
90 degrees
D.
180 degrees
Correct Answer:
90 degrees
Explanation:
In a purely inductive circuit, the current lags behind the voltage by 90 degrees.
2. In a purely capacitive AC circuit, the current
A.
lags behind the voltage by 90 degrees
B.
leads the voltage by 90 degrees
C.
is in phase with the voltage
D.
is opposite in phase to the voltage
Correct Answer:
leads the voltage by 90 degrees
Explanation:
For a pure capacitor, the current leads the applied voltage by 90 degrees.
3. When the voltage and current differ in phase by 90 degrees, the average power consumed is
A.
maximum
B.
half of the maximum value
C.
zero
D.
equal to the apparent power
Correct Answer:
zero
Explanation:
The average power is P = VI cos phi. For a phase difference of 90 degrees, cos 90 degrees is zero, so the average power is zero.
4. The power factor of a circuit in which voltage and current differ by 90 degrees is
A.
0
B.
0.5
C.
1
D.
2
Correct Answer:
0
Explanation:
Power factor is cos phi. At a phase difference of 90 degrees, cos 90 degrees equals zero.
5. A current that differs in phase from the voltage by 90 degrees is called
A.
direct current
B.
wattless current
C.
short-circuit current
D.
resistive current
Correct Answer:
wattless current
Explanation:
A 90-degree phase difference makes the average power zero, so the current is called wattless current.
6. In a pure inductor, which quantity reaches its maximum value first during an AC cycle?
A.
Current
B.
Voltage
C.
Resistance
D.
Power factor
Correct Answer:
Voltage
Explanation:
In a pure inductor, voltage leads current by 90 degrees, so the voltage reaches its maximum before the current.
7. In a pure capacitor, when the current is maximum, the voltage across the capacitor is
A.
maximum
B.
minimum
C.
zero
D.
equal to the supply voltage peak at all times
Correct Answer:
zero
Explanation:
In a pure capacitive circuit, current leads voltage by 90 degrees. Therefore, current is maximum when the capacitor voltage is zero.
8. For a pure inductive or pure capacitive circuit, the phase angle between voltage and current is
A.
30 degrees
B.
60 degrees
C.
90 degrees
D.
120 degrees
Correct Answer:
90 degrees
Explanation:
A pure inductor or pure capacitor produces a 90-degree phase difference between voltage and current.
9. If the instantaneous voltage and current in an AC circuit are 90 degrees out of phase, energy is
A.
continuously consumed as heat
B.
stored and returned to the source
C.
converted entirely into light
D.
lost permanently in the circuit
Correct Answer:
stored and returned to the source
Explanation:
With a 90-degree phase difference, energy is alternately stored in the electric or magnetic field and returned to the source, resulting in zero average power.
10. Which phase difference between voltage and current corresponds to zero average power in an ideal AC circuit?
A.
0 degrees
B.
30 degrees
C.
60 degrees
D.
90 degrees
Correct Answer:
90 degrees
Explanation:
Zero average power occurs when the phase angle is 90 degrees because the power factor cos 90 degrees is zero.
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