What is unique about the character of alternating current (AC)?

The uniqueness of alternating current (AC) lies in its ability to change polarity, which happens 60 times a second in standard U.S. electrical systems, corresponding to a frequency of 60 Hz. This characteristic enables AC to efficiently transmit electricity over long distances, making it the most widely used form of electricity for residential and commercial power distribution.

The alternating nature of AC is essential for the function of various electrical devices and systems, as it allows for the use of transformers to step voltage levels up or down easily. This capability is fundamental to the design and functionality of electrical grids, as well as household appliances that operate on AC power.

In contrast, the other choices do not accurately describe the nature of AC current. For instance, a current that flows in one direction characterizes direct current (DC), while constant voltage is not a trait specific to AC; AC voltage can vary in amplitude over time. Additionally, AC is used in a wide range of applications beyond just residential circuits, including commercial and industrial settings.