What characteristic defines AC voltage?

AC voltage is defined by its ability to alternate polarity and direction. This means that the voltage can both rise and fall, transitioning from positive to negative. As a result, the flow of electric current reverses direction periodically, which is a fundamental characteristic of alternating current (AC). This alternating nature is critical for the effective transmission of electricity over long distances, as it allows for the use of transformers to change voltage levels.

The fixed frequency often associated with AC voltage systems—such as 60 Hz in North America or 50 Hz in many other parts of the world—relates to how quickly these changes occur, but the defining characteristic is the alternation of polarity and direction itself. Thus, the distinction between alternating current and direct current (DC), where the flow of electrons remains in one direction, underscores why option B accurately defines the nature of AC voltage.