Conductors and insulators

A conductor lets electricity flow through it easily; an insulator does not. Metals are good conductors, which is why wires are made of copper. Plastic, rubber, wood and glass are insulators, which is why they are used to cover wires and make plugs and switches safe to touch.

Key facts

  • Metals (copper, iron, steel, aluminium, gold, silver) are good electrical conductors.
  • Graphite (pencil “lead”) is the only common non-metal that conducts well.
  • Plastic, rubber, wood, glass, wool, paper and air are insulators.
  • Test a material by putting it in the gap of a circuit: if the bulb lights, it conducts.
  • Water with salts in it (tap water, sweat) conducts – never touch electrical things with wet hands.
  • Long, thin wires resist current more than short, thick ones.

Worked examples

Example 1

A steel nail is placed across the gap in this circuit. Will the bulb light?

  1. Steel is a metal.
  2. Metals are conductors, so the gap is bridged and the loop is complete.
  3. The bulb lights.

Answer: Yes – steel is a metal, so it conducts and completes the loop.

Example 2

Why is an electrical cable made of copper wire inside a plastic coating?

  1. The inside must carry the current: copper is an excellent conductor.
  2. The outside must keep the current away from people: plastic is an insulator.

Answer: Copper conducts the electricity; plastic insulates it so you cannot get a shock.

Example 3

Same-sized samples are tested with a 3 V cell. Currents: copper 0.5 A, steel 0.3 A, graphite 0.1 A, plastic 0 A. Rank them from best to worst conductor.

  1. With the same push, a better conductor lets more current through.
  2. Order the currents 0.5 > 0.3 > 0.1 > 0
  3. So: copper, steel, graphite, then plastic (an insulator).

Answer: copper, steel, graphite, plastic