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

Electric current is the rate at which electric charge flows past a point, and it is the quantity that heats wires, trips fuses, turns motors and, in a body, stops hearts. It is measured in amperes: one ampere is one coulomb of charge per second. A current that always flows the same way is **direct current** (DC), what batteries give and what nearly all electronics use inside.

Electric current is the rate at which electric charge flows past a point, and it is the quantity that heats wires, trips fuses, turns motors and, in a body, stops hearts. It is measured in amperes: one ampere is one coulomb of charge per second. A current that always flows the same way is direct current (DC), what batteries give and what nearly all electronics use inside.

The charges that move are the free carriers: electrons in a metal, ions in a salty solution like blood or the fluid around neurons, electrons and holes in a semiconductor. In a copper wire the electrons drift remarkably slowly, a fraction of a millimetre per second, even though the signal that sets them moving travels along the wire at a large fraction of the speed of light; the wire is already full of electrons, and pushing at one end moves them all at once, as water in a full pipe. How much flows for a given voltage is set by the electrical resistance of the path: double the resistance and the current halves.

Typical numbers: a phone charger delivers about one to three amperes, a kettle about 10 A, a car starter motor a few hundred; the nerve and muscle effects of current through the body start at milliamperes.

Current is what heats. The heat in a conductor grows with the square of the current (Joule heating), which is why wires are rated in amperes and why power travels long distances at high voltage and low current.

Every current makes a magnetic field around it, the link that ties circuits to motors, transformers and the magnetic recording of brain activity.

A current that reverses direction many times a second is alternating current.