physics//electromagnetism//electric circuit

An electric circuit is a closed path along which a source pushes electric charge through components that use, store or control it, and it is the unit of everything electrical, from a phone charger to the grid. Four quantities describe it: the voltage that pushes, the current that flows, the resistance that opposes the flow, and the power that results.

An electric circuit is a closed path along which a source pushes electric charge through components that use, store or control it, and it is the unit of everything electrical, from a phone charger to the grid. Four quantities describe it: the voltage that pushes, the current that flows, the resistance that opposes the flow, and the power that results.

The three are tied by Ohm's law, V=IRV = I RV=IR: one volt across one ohm drives one ampere. Power, the rate at which energy is delivered, is voltage times current, P=VIP = V IP=VI, measured in watts (joules per second). A few everyday numbers make the units concrete: a phone charges at 5 to 9 V and one to a few amperes, about 10 to 30 W; a kettle on a 230 V socket draws about 10 A, about 2.3 kW; one joule is roughly the energy needed to lift a 100 g apple by one metre.

Voltage pushes, current flows, resistance resists, and power is push times flow.

With V=IRV = IRV=IR and P=VIP = VIP=VI most questions about a circuit (will it overheat, how long will the battery last, what fuse does it need) are a line of arithmetic.

The path must be closed. Current flows only around a loop; an open switch, a cut wire or an air gap stops it everywhere at once, and a short circuit is a loop with almost no resistance, where the current is limited only by the source.

Energy goes somewhere. In a resistor it becomes heat (Joule heating), in a motor motion, in an LED light, in a capacitor or an inductor it is stored and given back.

A voltage source keeps the push constant while charge flows; without one, a circuit is a charged capacitor that runs down as it is used.