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In an A.C. circuit, in 1 s the current reduces to zero value 120 times. Hence the frequency of the A.C. current is ______ Hz.

Asked in GSEB Board July 2017 · Instantaneous values and timing

• (1) 50
• (2) 100
• (3) 60
• (4) 120

Step-by-step solution

Why the other options are wrong

• (1) 50 Hz is the mains frequency in India, but nothing in the question gives it. At 50 Hz the current would be zero 100 times a second, not 120.
• (2) No single slip gives 100 Hz. Sanity check: at 100 Hz the current would pass through zero 2×100 = 200 times each second, not 120.
• (4) Counts each zero as one complete cycle. The current passes through zero twice in every cycle (once while falling, once while rising), so 120 zeros are only 60 cycles.

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