Alternating current
Current that periodically reverses direction.
An ideal alternating waveform crosses the zero line as it reverses sign.
Alternating current changes direction periodically. Most transmission and distribution networks use AC, allowing transformers to change voltage and connecting many resources to a shared electrical frequency.
A repeating change in direction
In an ideal sinusoidal AC waveform, the value rises and falls smoothly, passing through zero and reversing sign each half-cycle. Real waveforms can also contain distortion. Frequency describes how often the pattern repeats.
AC networks became widespread partly because transformers make it convenient to change voltage levels. Direct current also has important grid uses, including batteries, solar panels, and high-voltage transmission links.
Why it matters
Recognizing whether a resource supplies AC or DC helps explain which equipment is needed to connect it to the wider network.