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# Electrical circuits

A simple electric circuit made up of a power source (battery or generator) and a resistor. In current electricity, electrons are pushed along a conductor by a battery or generator. When the current is switched on, any electrons free to move in the wire all move in the same direction. They flow only if they have a complete pathway of conductors. This pathway is called a circuit. All parts of a circuit must conduct electricity and must be connected to one another. A circuit may have sections or components connected in a series or in parallel. Electrons flow from the negative terminal of a battery or generator towards the positive one, even though the electric current itself is said, by convention, to travel from positive to negative.

An electric circuit, featuring a series of batteries, a switch, an ammeter (top left, to measure the current flowing through the...Read More >>An electric circuit, featuring a series of batteries, a switch, an ammeter (top left, to measure the current flowing through the circuit) and a voltmeter (top right, to measure the voltage across a single bulb)

Series circuit (top) and parallel circuit (above). Both have a voltage source (battery) and three resistors.

## Series or parallel

Components of an electrical circuit are connected in either a series or in parallel. In a series circuit—made up of, say, four light bulbs and a single 6-volt battery—a wire links the battery to the first bulb, from there to the second, then on to the next two in turn before returning to the battery in a continuous loop. In a parallel circuit, each bulb is wired to the battery in a separate loop. When the four light bulbs are connected in series, the same current flows through all of them but the voltage is divided into four: 1.5 V for each bulb. When the light bulbs are connected in parallel, each bulb receives a quarter of the current, but the full 6 V from the battery.

The conventional symbol for an electric current is I, which comes from the French phrase "intensité de courant" (current intensity). The I symbol was used by the French scientist André Marie Ampère (1775–1836) after whom the unit of electric current, the ampere, was named.