A complete circuit — Physics, 11–13 years
Find out why a lamp lights only when electricity has a complete route through a source, wires and the lamp.
One unbroken route
A simple circuit is a complete loop: a source such as a battery, conducting wires and a device such as a lamp. The battery provides a push, and charge can move around the whole loop. Open the loop anywhere and the lamp goes out.
Why must the loop close?
Connecting a battery to one side of a lamp is not enough: charge has no complete route back to the battery. The idea of a closed circuit solves the problem of predicting when a device will work. It also makes a switch useful: the switch opens or closes the route.
Building a lamp circuit
Start with a 1.5-volt battery, two wires and a small lamp. Connect one battery terminal to one lamp contact, then connect the other lamp contact to the other terminal. If both connections are metal and firm, the loop is closed and the lamp lights; disconnect one wire and it stops.
The trap: current is not used up
It is tempting to imagine that a battery sends electricity to a lamp, where it is used up before reaching the other wire. A circuit is better pictured as a complete flow: the lamp transfers energy from the moving charges, while the battery restores the energy. This is why one break stops the whole loop.
Circuits around us
Torches, doorbells, toys and household lights all use circuits with sources, conductors and devices. A doorbell switch closes a circuit briefly, while a phone contains many tiny circuits that control its screen, sound and charging. Never experiment with household mains electricity.
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