Which of the following describes alternating current?

Alternating current (AC) is defined as an electric current that periodically reverses direction. This means it alternates between both positive and negative values over time, creating a waveform that typically follows a sine wave pattern. Understanding AC is crucial, especially in residential and commercial power systems, where it is the standard form of electricity supply.

The correct choice captures this characteristic accurately by stating that current alternates between positive and negative directions. This is essential because it highlights the nature of AC—it's not confined to merely oscillating around zero but rather shifts back and forth across a range of values, which includes both positive and negative phases.

The other options do not fully encompass the behavior of alternating current. While some aspects of them may touch on parts of AC's behavior (such as transitioning through zero), they fail to mention the duality of positive and negative alternation that is fundamental to its definition. This distinction is key for understanding how AC behaves in circuits, how it can be transformed, and how it is utilized in electrical systems.