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Induction and AC circuits
5 ECAT Physics questions with answers and explanations.
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The magnetic flux through a coil falls from 0.5 Wb to zero in 0.1 s. What is the average induced emf?
- 1 0.05 V
- 2 5 V
- 3 50 V
- 4 0.5 V
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An alternating current has a peak value of 10 A. What is its rms value?
- 1 5 A
- 2 7.07 A
- 3 14.1 A
- 4 10 A
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Correct answer: 7.07 A
I_rms = I_peak / sqrt(2) = 10 / 1.414, about 7.07 A.
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In a circuit containing only an ideal inductor, the current:
- 1 Leads the voltage by 90 degrees
- 2 Lags the voltage by 90 degrees
- 3 Is in phase with the voltage
- 4 Is zero
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Correct answer: Lags the voltage by 90 degrees
In an inductor the current reaches its peak a quarter cycle after the voltage, so it lags by 90 degrees.
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What is the resonant frequency of an LC circuit with L = 1 H and C = 1 microfarad? (Take pi = 3.14)
- 1 About 159 Hz
- 2 About 1000 Hz
- 3 About 50 Hz
- 4 About 6.28 Hz
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Correct answer: About 159 Hz
f = 1 / (2 pi sqrt(LC)) = 1 / (2 x 3.14 x 0.001) which is about 159 Hz.
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What is the power factor of a purely resistive AC circuit?
- 1 0
- 2 0.5
- 3 1
- 4 Infinity
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Correct answer: 1
Current and voltage are in phase, so cos(phi) = cos 0 = 1.