4.3 CONDUCTORS AND INSULATORS

Conductors allow electricity to flow through them but insulators do not. But how do we know if something is a conductor or an insulator? The illustration below shows a very simple way to find out.

Testing conductors and insulators. First make a simple circuit as shown above. The circuit is not complete, so of course the bulb does not light! To check that the circuit is working properly, touch the ends of the two wires together. That completes the circuit and the bulb should light.

To find out if something is a conductor, touch it with the ends of the two wires at the same time. If the object conducts electricity it will complete the circuit. The electric current will flow through it and the bulb will light. If the object is an insulator, the electric current cannot flow through it and the bulb will not light. Test as many objects as you can. Classify each object as a conductor or an insulator.

Object
/ What is it
made from? /
Conductor or
insulator?
wire / copper / conductor
covering on wire / plastic / insulator
coin / mixed metals / conductor
pen or comb / plastic / insulator
pencil (outside) / wood / insulator
pencil lead / graphite / conductor
nail or needle / iron or steel / conductor
window / glass / insulator
saucepan / aluminium / conductor
box or carton / cardboard / insulator

The table opposite lists some common objects that we might test. It also shows the materials that each object is made of, and the results of the test.

Study the results carefully. What do you notice about the things that are classified as conductors? Nearly all of them are made of metals such as copper, iron and aluminium. The only exception is the pencil lead. Pencil leads are not made of lead! They are made of a soft material called graphite, which is not a metal.

After testing all known materials, scientists can generalise and say that all metals and graphite are conductors. Most other materials are insulators.

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  • 1. Which of the following will make the bulb light if we put them in the circuit above? a bush knife, a book, a food tin, a rock, iron from a roof, a cooking pot, a bucket, a gold ring.
  • 2. What is an insulator? What are insulators made of?
  • 3. Why is the covering on an electric wire made of plastic?

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