What is the primary characteristic of an alternating current (AC) signal?

An alternating current (AC) signal is characterized primarily by its ability to change direction periodically. This means that instead of flowing in one consistent direction as with direct current (DC), AC reverses its flow, usually in a sinusoidal pattern. As a result, the voltage and current levels oscillate above and below a zero reference point over time.

This periodic change allows for a more efficient transmission of power over long distances, as it can easily be transformed to different voltage levels using transformers. The changing direction is crucial in various applications, including power generation and distribution, where AC is favored for its ease of generation and manipulation.

The other characteristics mentioned do not apply to AC. For instance, AC does not necessarily maintain a constant voltage level, nor is it unidirectional, nor is it limited to being always positive, as it spans both positive and negative values as it oscillates.