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Alternating current

5 Class 12 Physics questions with answers and explanations.

Printable worksheet with answer key

All questions with answers
  1. The rms value of an AC with peak value 10 A is:

    • 1 14.1 A
    • 2 7.07 A
    • 3 10 A
    • 4 5 A
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    Correct answer: 7.07 A

    I rms = I0 / square root of 2 = 7.07 A.

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  2. In a purely resistive AC circuit, the current and voltage are:

    • 1 180 degrees apart
    • 2 90 degrees apart
    • 3 Unrelated
    • 4 In phase
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    Correct answer: In phase

    For a resistor the current is in phase with the voltage.

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  3. Capacitive reactance is given by:

    • 1 2 pi f L
    • 2 2 pi f C
    • 3 1 / (2 pi f L)
    • 4 1 / (2 pi f C)
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    Correct answer: 1 / (2 pi f C)

    X_C = 1 / (omega C) = 1 / (2 pi f C).

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  4. Inductive reactance:

    • 1 Increases with frequency
    • 2 Decreases with frequency
    • 3 Is zero always
    • 4 Does not change
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    Correct answer: Increases with frequency

    X_L = 2 pi f L, so it rises with frequency.

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  5. At resonance in a series LCR circuit, the impedance is:

    • 1 Infinite
    • 2 Minimum and equal to R
    • 3 Maximum
    • 4 Zero
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    Correct answer: Minimum and equal to R

    X_L = X_C cancel, so impedance equals R.

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