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A complete circuit

Electricity and magnetism · level 2, lesson 1 of 6

A question to start

Four children each join a battery, a bulb and wires. Only two of the bulbs light. What did the other two get wrong?

The idea

A bulb lights only in a complete circuit: an unbroken loop from one end of the battery, through the bulb, and back to the other end.

A circuit is broken if:

  • there is a gap anywhere in the loop;
  • both wires go to the same end of the battery;
  • a wire touches only the glass of the bulb, not its metal parts.

A bulb has two metal contacts, usually the tip and the side of its base. The electricity must go in one and out of the other.

complete: lightsgap: stays darkgap

Worked examples

1. Both wires to one end. Wires from the + end to both sides of the bulb make no loop through the battery, so the bulb stays dark.

2. Wire on the glass. Glass does not carry electricity. The wire must touch the metal base.

3. Fixing a gap. Find the loose wire, twist or clip it back on, and the bulb lights.

Try it

  1. What is a complete circuit?
  2. Both wires go to the minus end of the battery. Will the bulb light?
  3. Where on a bulb should the wires touch?
  4. A bulb lit yesterday but not today. Name two things you could check.
Answers
  1. An unbroken loop from the battery, through the bulb and back.
  2. No, there is no loop through the battery.
  3. Its two metal contacts: usually the tip and the side of its base.
  4. Any two: a loose wire, a flat battery, a broken bulb, a wire on the wrong part of the bulb.

Common mistake

"Electricity only needs to reach the bulb." It must also get back to the battery. No way back, no light.

Where it leads