Current, voltage and resistance: how a circuit works

A bulb lights up only when there is a complete path, called a circuit, from one end of the battery, through the bulb and back to the other end. Break the loop anywhere, with a switch or a loose clip, and the light goes out.

Three big ideas

The scientist Georg Ohm found a simple rule linking them: voltage equals current times resistance (V = I × R). Double the resistance with the same battery and the current halves.

Series and parallel

In a series circuit everything is in one single loop. The current has only one path, so if one bulb breaks, all of them go out. Adding more bulbs adds more resistance and they all get dimmer, because they share the battery's voltage.

In a parallel circuit each bulb has its own branch. Every branch gets the full voltage, so each bulb stays bright, and if one breaks the others keep working. The currents in the branches add up to the total current leaving the battery. That is why the lights in your house are wired in parallel.

Conductors and insulators

Wires are made of copper, a metal that conducts electricity easily, and wrapped in plastic, an insulator that stops the current escaping into your hand. Every electrical cord you see uses both.