What is the difference between alternating current (AC) and direct current (DC) in terms of current direction and frequency?

• A. Both are unidirectional but differ in voltage level.
• B. DC is unidirectional with constant magnitude; AC periodically reverses direction with a defined frequency.
• C. DC alternates direction with varying frequency; AC is always constant.
• D. DC has frequency; AC has zero frequency.

Answer: (B)

The essential idea is how current changes over time. Direct current flows in a single direction and its magnitude stays essentially constant, so there’s no repeating cycle—its frequency is effectively zero. Alternating current reverses direction in a regular pattern, and the rate of those reversals is described by a defined frequency (for example, 50 Hz or 60 Hz in household power). In practice, AC varies its magnitude as the waveform cycles, while DC remains steady. So the correct description is that DC is unidirectional with constant magnitude, while AC reverses direction periodically with a defined frequency. The other statements either claim AC doesn’t reverse direction or that DC has a frequency, which doesn’t match how they behave.