Which statement correctly describes AC and DC currents?

• A. DC current reverses direction periodically
• B. AC flows in one direction
• C. DC flows in one direction; AC reverses direction periodically
• D. AC does not have frequency

Answer: (C)

Direct current flows in one direction because the source has fixed polarity; the electrons move steadily from the negative to the positive terminal, giving a constant, unidirectional flow. Alternating current, on the other hand, reverses direction periodically, so the current oscillates back and forth. The rate of that reversal is the frequency, measured in hertz. In household power, the frequency is typically 60 Hz in the US and some parts of the Americas, or 50 Hz in many other countries. The magnitude of AC can vary over time, following a waveform (usually sinusoidal), while DC tends to maintain a steady magnitude from an ideal source. The other statements misstate one or both currents: DC does not reverse direction, AC does not flow in a single direction, and AC does have a frequency.