AC and DC: Two Ways Current Can Flow

0:00Current is a flow of electrons around a complete circuit. Does it always flow one way? Let's graph current against time.

0:10Above the time axis, current flows one way. Below it, the other way.

0:16Direct current, or DC, flows one way only. A battery's DC is steady, so its graph is a flat line.

0:25Batteries and solar cells supply DC, and the circuits inside a phone run on it.

0:31Alternating current, or AC, keeps reversing direction, so its graph is a wave.

0:38One full back and forth is called a cycle.

0:42Power grid AC does 50 or 60 cycles a second: a frequency of 50 or 60 hertz.

0:50Power grids carry AC from power stations to homes, because its voltage is easy to change.

0:57A transformer raises or lowers voltage, the push that drives current. It only works with a changing current, like AC.

1:06Raising the voltage lets the long lines carry less current, so they lose less energy as heat.

1:13Near homes, transformers lower the voltage to a safer level.

1:18A phone charger turns AC from a wall socket into DC for the phone.

1:24A common myth is that electrons travel all the way from the power station to your home.

1:30In fact, they just jiggle back and forth in about the same place.

1:35What travels along the wires is energy, not electrons.

1:40So direct current flows one way, and alternating current keeps reversing direction.