physics//electromagnetism//electric circuit//electric current//alternating current

Alternating current is an electric current that reverses direction many times a second, usually as a smooth sine wave, and it is what the power grid delivers to every socket: 50 times a second in Europe and most of the world, 60 in the Americas. Batteries give direct current; nearly every device converts the socket's alternating current back to direct current inside its charger.

Alternating current is an electric current that reverses direction many times a second, usually as a smooth sine wave, and it is what the power grid delivers to every socket: 50 times a second in Europe and most of the world, 60 in the Americas. Batteries give direct current; nearly every device converts the socket's alternating current back to direct current inside its charger.

The grid uses it because of the transformer. A transformer changes voltage by induction, and induction needs a changing current; with alternating current the change is built in, so voltage can be raised for transmission and lowered for homes with a simple, nearly lossless device. That matters because the heat lost in a line grows with the square of the current: sending the same power at a thousand times the voltage takes a thousandth of the current and loses a millionth of the heat.

High to travel, low to arrive.

Alternating current won the grid because transformers let it travel at hundreds of kilovolts and enter the house at 230 V, a trade direct current could not make cheaply until modern power electronics.

The 230 V of a European socket is an effective value. The voltage actually swings between about +325 V and -325 V; 230 V is the steady voltage that would heat a resistor equally.

Low-frequency alternating current is the most dangerous to people. Around 50 to 60 Hz it is best at disturbing the heart's rhythm and at locking the muscles of a hand around a conductor (the let-go threshold is about 10 mA), developed in electric shock.

Very long links and undersea cables now use high-voltage direct current, converted at each end by power electronics, because alternating current in long cables wastes energy charging the cable's own capacitance.