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The peak value of an alternating current is 5 A and its frequency is 60 Hz. Find its rms value and the time taken to reach the peak value of the current starting from zero.

Asked in GUJCET 2011 · RMS and mean values

• (1) 3.536 A; 4.167 ms
• (2) 3.536 A; 15 ms
• (3) 6.07 A; 10 ms
• (4) 2.536 A; 4.167 ms

Step-by-step solution

Why the other options are wrong

• (2) The r.m.s. value is right but the time is not. From zero to the peak is a quarter cycle, T/4=4.167 ms; no single slip gives 15 ms, which is nearly a whole cycle (T=16.67 ms).
• (3) An r.m.s. value of 6.07 A is larger than the 5 A peak, which is impossible. No single slip gives it, or 10 ms: the peak comes at T/4=4.167 ms.
• (4) The time is right, but no single slip gives 2.536 A. Sanity check: 5/(√2)=3.536 A.

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