How is alternating current (AC) different from direct current (DC)?

Alternating current (AC) is characterized by its flow, which periodically changes direction. This means that the current alternates between moving in one direction and then reversing to flow in the opposite direction. This behavior is fundamental to the operation of AC power systems, where the waveform typically follows a sine or cosine pattern over time. The ability of AC to reverse direction makes it suitable for power distribution systems since it allows for efficient energy transmission over long distances.

The other options do not accurately describe AC. The first option incorrectly suggests that AC flows in one direction; in fact, it is the characteristic of direct current (DC) to flow in a single direction. The second option misidentifies DC, stating that it periodically reverses direction, which is not true, as DC maintains a constant direction of flow. Lastly, the option indicating that DC is always switched off is misleading, as DC can be actively used in various applications, unlike what the statement implies. Thus, option C accurately encapsulates the essential feature of AC.