Alternating current (AC) is defined as

• A. A current that moves in one direction but varies in intensity at regular intervals.
• B. A current that changes direction and intensity at regular intervals.
• C. A constant current.
• D. A current that never flows.

Answer: (B)

Alternating current means the current reverses direction periodically while its magnitude also changes in a repeating pattern. This back-and-forth flow happens because the voltage that drives the current swings positive and negative over time, so electrons continually switch direction. In most systems, this happens at a fixed frequency (60 cycles per second in the US, 50 in many other places), producing a waveform that rises to a peak, crosses zero, then goes negative and repeats. That combination of direction reversal and changing strength is what sets AC apart from a current that always flows in one direction (even if its size varies, which would be DC with ripple), or a truly constant DC, or no current at all.