Which type of current alternates direction regularly?

Alternating Current (AC) is characterized by its ability to reverse its direction periodically. This means that the electric charge in the current flows back and forth, which occurs in cycles. AC voltage typically varies in a sinusoidal manner, meaning that during each cycle, the current reaches a positive peak, zero, a negative peak, and then returns to zero, creating a continuous flow of changing direction.

This property of AC is crucial in various applications, particularly in power distribution systems where alternating current is used to efficiently transmit electricity over long distances. AC's ability to easily transform between voltage levels using transformers is also a fundamental reason it is commonly used for electrical power supply instead of direct current (DC), which flows in a single direction.

In contrast, direct current (DC) flows steadily in one direction, while static current and transitory current do not represent types of current that regularly alternate direction. Static current pertains to charges at rest, and transitory current often refers to brief fluctuations or surges within a system. Thus, the defining characteristic of AC makes it the correct answer to the question about which type of current alternates direction regularly.