What Is the Difference Between AC and DC?

DC (direct current) and AC (alternating current) are the two basic ways electricity flows. In the shortest terms: in DC the current always flows in the same direction; in AC the current changes direction at regular intervals.

What Is DC (Direct Current)?

In DC the voltage and current direction are constant. Batteries, USB and adapter outputs are DC. Almost all electronic circuits (phones, microcontrollers, sensors) run on DC because they want a stable, predictable supply.

What Is AC (Alternating Current)?

In AC the voltage periodically rises, falls, reverses and returns — usually as a sine wave. The mains outlet is AC (for example 230 V at 50 Hz in much of the world, or 120 V at 60 Hz in North America). So it completes 50 full cycles per second. That 230 V is not the peak value of the wave but its effective (RMS) value.

Why Is the Grid AC and Electronics DC?

AC is used for electricity distribution because it can be carried over long distances with low losses and easily stepped up or down with a transformer. Electronic circuits, on the other hand, want a stable voltage, so they need DC. That's exactly why device adapters convert AC to DC.

How Is AC Converted to DC?

Inside an adapter, a transformer first lowers the voltage, then rectifier diodes turn the AC one-way, and capacitors smooth the remaining ripple. The result is the clean DC the device wants.

The quantity that sets how many cycles per second AC completes is frequency; to go deeper, see our "What Are Frequency and Period?" article.