Which of the following is a fundamental characteristic of AC voltage?

The fundamental characteristic of AC (Alternating Current) voltage is that it exhibits time-varying polarity. In an AC voltage supply, the voltage alternates in polarity, meaning it changes direction periodically over time. This characteristic is what differentiates AC from DC (Direct Current), where the voltage has a constant direction.

The fluctuations in voltage that define AC are typically sinusoidal, resulting in the voltage alternating from positive to negative over a cycle. This time-varying behavior is crucial for the operation of many electrical devices and for the transmission of power over long distances, as it allows for efficient generation and transformation of voltage levels.

Understanding this concept is essential in electrical work, as the time-varying nature leads to various parameters such as frequency and phase that are critical in AC circuit analysis and design. The other characteristics mentioned do not accurately describe the nature of AC voltage, helping to clarify the uniqueness of the time-varying polarity in alternating current systems.