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A circuit must be complete — Physics, 4–6 years

See why a battery and a bulb need a complete path for the bulb to light. Physics, 4–6 years.

Idea

Electricity can travel around a complete loop, called a circuit. A battery supplies energy, wires make the path, and a bulb changes some of that energy into light. If the loop has a gap, the bulb stays dark because the path is not complete.

Why

People wanted lamps and toys to work when they needed them, and to stop without removing the battery. This led to the useful idea of a switch that opens or closes the path. The problem is not simply “having electricity”; it is giving it a complete route through the device.

Try it

Connect one wire from the battery’s plus end to one bulb contact, and another wire from the second contact to the minus end. The bulb lights because the loop is complete. Put a switch into one wire and open it: the bulb goes out; close it again and the bulb lights.

Common mix-up

It is reasonable to think that touching a wire to the battery should be enough, because the battery is the part that provides energy. But electricity needs a route out and a route back through the bulb. One loose end breaks the loop, just as a gap breaks a ring.

Where it helps

A torch uses a battery, wires, a bulb and a switch in a small circuit. The switch opens the path when the torch is off and closes it when the torch is on. The same simple idea helps toys, doorbells and many other battery-powered objects work.

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