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A current shaped as a square wave that switches rapidly between +0.5 A and -0.5 A is passed through an AC ammeter. What reading does the ammeter show?

1. 0.25 A
2. 0.5 A
3. 0.5/sqrt(2) A
4. 0.5*sqrt(2) A

Correct answer: 0.5 A

Solution

An AC ammeter reads the root-mean-square (rms) value. For a square wave that is always +0.5 A or -0.5 A, the instantaneous magnitude is always 0.5 A, so the square of the current is always 0.25 A². The mean of the square is 0.25, and its square root is 0.5 A. Thus the rms (and the reading) is 0.5 A.

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