Physics · Electromagnetic induction and AC · chapter 7 of 14

Class 12 Physics Chapter 7: Alternating Current Assertion-reason questions with answers

2 assertion-reason questions on Alternating Current, each with its answer and the reason, written from the NCERT chapter. For the CBSE board, JEE Main and NEET, which are set from the same NCERT chapter. Open a question's solution to check your working; then take the free test to find your next chapter. Each question gives an Assertion (A) and a Reason (R); choose how they stand to each other.

Question 1
Assertion (A): The average power supplied by an ac source to an ideal inductor over a complete cycle is zero. Reason (R): In an ideal inductor the current lags the voltage by π/2, so the instantaneous power p = −(im vm/2) sin 2ωt is positive for half the time and negative for the other half.
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Answer: (a) Both A and R are true, and R is the correct explanation of A.

Both are true. Because of the π/2 lag the product vi is a sin 2ωt term, whose average over a cycle is zero: R is exactly why A holds.

Question 2
Assertion (A): The peak value of a 220 V household supply is about 311 V. Reason (R): The average value of a sinusoidal alternating current over a complete cycle is zero.
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Answer: (b) Both A and R are true, but R is not the correct explanation of A.

A is true: 220 V is the rms value, and vm = √2 × 220 ≈ 311 V. R is true: the current is positive for half the cycle and negative for the other half. But a zero average says nothing about how the peak relates to the rms value, so R does not explain A.

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