Electricity will not flow through a circuit which has a “break” or gap in it. A Switch is a device which can be used to complete or break a circuit so that devices can be switched ON or OFF.
To make appliance work, or light glow you need to close the circuit, you must have a complete unbroken path so electric current can flow from one side of your electrical source to the other. When a conductor is connected to the battery terminals, the electrons flow from the negative to the positive terminal.
1. There is a “break” (switch open/OFF) in the circuit. The bulb is switched off.
2. The circuit is now complete with “no break” (switch closed/ON).
The bulb is switched on.
You light a bulb when you turn ON the switch allowing the electricity to flow from the battery. If you disconnect any one of the wires anywhere in the circuit afterwards, you will break the circuit and the light will go out. Switch is very useful since it saves electricity.
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Resources: The Essentials of Science KS2 & Go Circuit