Conductors and insulators


You can find out which materials are conductors and which are insulators by using this circuit. In the picture a spoon is being tested. Notice that each end of the spoon is gripped by crocodile clips. These provide a firm contact. If the test object is made from a material that conducts electricity the bulb will light. If the material is an insulator the bulb will not light. In the picture you can see that the bulb is glowing so the spoon is made from a conductor - metal.

Do overhead power cables need an insulating cover?
No. The air surrounding power cables acts as an insulator.



Why does the electricity not come off the overhead cables and go down the pylons?
Pylons are the towers that support overhead cables. They are made of metal but the cables are not directly attached to them.

The cables are held to pylons by insulators of glazed baked clay. In this way the cables are insulated from the ground by air and clay.

Do underground cables have insulation?
Yes. They have a thick coat of plastic insulation to keep the electricity from leaking out into the ground and to keep the cables apart. (They also have a metal sleeve to protect them from people accidentally cutting through them when digging up the street!)



Why is the switch in a bathroom made of string?
The string is an insulator. Water is a conductor. If a wet hand touched a normal switch water could enter the switch and conduct electricity from the mains to the hand - with possibly fatal results. Electricity in the switch in the ceiling cannot travel down to a person holding the cord with a wet hand because the string is an insulator. The string is therefore an important, yet simple, safety device.

Which metal is used in overhead power cables?
Aluminium because it is a good conductor of electricity and also a lightweight metal. This means it doesn't pull down on the pylons as much as a heavier metal like copper or steel.

Why are overhead power cables on railways dangerous?
They are not dangerous unless you get close to them. But they are lower than most high-voltage cables. This is because they have to be touched by the pantograph on the top of a railway locomotive. The pantograph is used to draw the current from the cable to power the locomotive's motor.
When you get close to a high voltage power cable a spark can jump from the power cable. This is why it is so important to stay well away from them.

Do we have conductors of electricity inside us?
Yes. They are the nerves. They send small electric currents to all parts of the body. These currents control our muscles. If we received a large current of electricity our nerves could be damaged and our muscles, including our heart muscle, would stop working.