Alternating current, often called AC, is electrical current that rapidly switches direction back and forth, like a swing moving forward and backward. In the United States, household electricity alternates 60 times per second, far too fast to see or feel.

This might seem strange at first. Why would electricity flow backward? The answer lies in how we generate and transmit power. When power plants spin giant magnets inside coils of wire, they naturally create current that alternates direction. More importantly, alternating current travels efficiently over long distances through power lines. The electricity that lights your home might come from a power plant hundreds of miles away.

The opposite is direct current (DC), which flows steadily in one direction, like water through a hose. Batteries produce direct current. In the 1880s, Thomas Edison promoted direct current while Nikola Tesla and George Westinghouse championed alternating current. Tesla's system won because AC could travel farther with less energy loss, making it practical to build large power grids.

Today, most homes and buildings use alternating current, though many devices you plug in (like phones and laptops) convert it to direct current internally. When you see a bulky adapter on a power cord, it's doing this conversion work.