What is an electrical circuit?

A closed circuit

An electrical circuit is a set of components connected to one another by wires, such that they form a closed loop. It is this loop that allows electric current to flow continuously from the generator to the loads, and then back to the generator.

If the loop is open (a broken wire, an open switch), the current can no longer flow: this is the most common cause of a fault. The circuit is then said to be open. When the loop is complete, the circuit is said to be closed.

The example of the torch

A torch consists of:

• a battery (the generator), which supplies electrical energy,
• a bulb (the load), which converts electrical energy into light,
• connecting wires,
• a switch, which opens or closes the loop.

When the switch is pressed, the loop closes and the bulb lights up immediately.

A common pitfall to avoid

Many pupils think that the current starts at the battery, flows into the bulb, and then stops. In reality, the current must return to the battery for the circuit to work: without a return path, there is no closed loop, and therefore no current at all. A lamp with only a single wire connected to the battery will never light up, even if the bulb and battery are in perfect working order.