Both a battery and a wall socket push electrons through wires, but they push in different ways. A battery gives a steady push in one direction forever — that's direct current, or DC, and it runs your phone and your torch. A wall socket doesn't push one way; it sloshes the electrons back and forth, pulling them one direction and then shoving them back, over and over, fifty times every second. That's alternating current, or AC, and it's what comes out of every plug in your house. The funny thing about AC is the electrons hardly go anywhere — they just jiggle on the spot. Flip the switch in the simulator between AC and DC and watch the electrons drift steadily one way, then start jiggling back and forth.
Most people think AC electrons race all the way down the wire to power your home. In fact they barely move, oscillating a fraction of a millimetre on the spot, yet the energy still flows.
What's actually happening
Picture the electrons in a wire as a crowd. Direct current is the crowd all walking the same way down a corridor, slowly and steadily, never turning around. That is what a battery does: it provides a fixed push, a constant voltage with a plus end and a minus end that never swap. Every battery-powered thing you own runs on DC, and so does almost all electronics inside, because chips want a steady, predictable supply.
Alternating current is the same crowd, but now the corridor tips back and forth and everyone sways with it — forward, back, forward, back. The wall socket reverses its push 50 times a second (60 in the Americas), so the electrons never get anywhere; they just oscillate around one spot, a tiny shuffle a fraction of a millimetre wide. It feels like it should be useless, but the energy still flows, because the pushing and pulling does work whichever way the electrons happen to be moving at that instant.
So why did the world wire its homes with the sloshing kind? Because of the transformer. A transformer can only change voltage when the magnetism in its core is changing, and that needs a current that keeps reversing — AC obliges, DC does not. That single fact let AC be cheaply stepped up to enormous voltages for efficient travel across the country and back down for safe use at home. In the 1890s this became a genuine fight, the 'war of the currents', between Edison's DC and the AC backed by Tesla and Westinghouse. AC won the grid because it could be transformed; DC simply could not keep up over distance. The irony is that long-distance power lines today sometimes convert back to DC for the very longest hauls, now that modern electronics can do the voltage-changing job that once only a transformer could.
AC reverses dozens of times a second while DC pushes one steady way, and grids chose AC because only a changing current can be transformed.
- 1Wave a small mains-powered LED light (or look at a streetlight) while moving your eyes or phone camera quickly past it in a dark room.
- 2Instead of a smooth streak you may see a dashed line (bright, dark, bright, dark) because the AC current is swinging through zero 100 or 120 times a second.
- 3Now do the same with a battery-powered LED torch: a clean, continuous streak, because DC never dips. You have just told AC and DC apart by eye.
Common questions
The pushing and pulling does work whichever way the electrons happen to be moving at that instant. They net almost nowhere, oscillating a fraction of a millimetre, yet the energy still travels down the wire.
In the 1890s Edison backed DC while Tesla and Westinghouse backed AC. AC won the grid because transformers could step it up for efficient long-distance travel, a job DC simply could not do at the time.
Not always. For the very longest links, such as undersea cables, engineers now convert AC to high-voltage DC, because over huge distances DC loses less and modern electronics can change its voltage.