In a 60 Hz circuit, how many times per second does the current cross zero?

In a 60 Hz circuit, how many times per second does the current cross zero?

• A. 60
• B. 120
• C. 180
• D. 240

Answer: (B)

In an AC circuit, the current follows a sine wave. A sine wave crosses zero twice in each full cycle—once while going from positive to negative, and once while going from negative to positive. At 60 cycles per second, that means 60 cycles × 2 zero-crossings per cycle = 120 zero crossings per second. The time between zero crossings is half the period (the period is 1/60 s, so half-period is about 8.33 ms), which also shows why you get zeros every half-cycle. So the current crosses zero 120 times each second.

In an AC circuit, the current follows a sine wave. A sine wave crosses zero twice in each full cycle—once while going from positive to negative, and once while going from negative to positive. At 60 cycles per second, that means 60 cycles × 2 zero-crossings per cycle = 120 zero crossings per second. The time between zero crossings is half the period (the period is 1/60 s, so half-period is about 8.33 ms), which also shows why you get zeros every half-cycle. So the current crosses zero 120 times each second.