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Conductors and insulators

Electricity and magnetism · level 2, lesson 3 of 6

A question to start

Wires are made of metal on the inside and covered in plastic on the outside. Why both?

The idea

A conductor lets electricity flow through it. An insulator does not.

Conductors Insulators
copper, aluminium, steel, gold plastic, rubber, wood, glass
the lead in a pencil (graphite) paper, cloth, dry air

So a wire has a copper core to carry the electricity, and a plastic coat so it does not leak out into anything it touches, like your hand or another wire.

You can test things with a gap in a circuit: put the object across the gap. If the bulb lights, the object is a conductor.

plastic coat: an insulatorcopper core: a conductor

Worked examples

1. A spoon test. A metal spoon across the gap lights the bulb. A plastic spoon does not.

2. A pencil. A pencil's wood is an insulator, but its lead is graphite, which conducts. Sharpen both ends and it can light a bulb, a little dimly.

3. Water and safety. Pure water conducts poorly, but tap water, and wet skin, conduct well enough to be dangerous near the mains. Keep electricity and water apart.

Try it

  1. What is a conductor?
  2. Is a rubber band a conductor or an insulator?
  3. Why do wires have a plastic coat?
  4. How could you test a coin to see if it is a conductor?
Answers
  1. Something that lets electricity flow through it.
  2. An insulator.
  3. So the electricity stays in the copper and does not flow into things it touches.
  4. Put it across a gap in a circuit and see if the bulb lights.

Common mistake

"Insulators are only for keeping warm." A coat insulates against heat. In electricity, an insulator is something that stops the flow of electricity. Plastic does both.

Where it leads