Science Experiments

How to make a forcefield

Sat, 16th Sep 2006

Part of the show Peruvian Mummies, Ancient Environments and the Sahara

What you Need

CanEmpty coke can
Piece of polystyrene
A full head of hair

What to do

1 - Take the polystyrene and rub it on your hair for about one minute.

2 - Wave the polystyrene near the fizzy drink can and see what happens.

What may happen

When the polystyrene is placed near to the can, the can moves towards you, you will have to be careful to pull the polystyrene away fast enough once the can starts moving.

Why does it happen?

All materials contain electrons; tiny particles that are part of an atom, when these electrons move we call it electricity. Electrons are negatively charged and the centre (nucleus) of the atom they are part of is positively charged. Positive and negative charges  repel themselves and attract each other very strongly so normally there are the same number of each in an object and it isn't charged.

If you touch two materials together sometimes some electrons will get transferred from one material to the other and get lost in it. This will leave both objects charged one positive and one negative. This depends on the material some will tend to gain and others loose electrons. Hair doesn't really mind, rubber likes loosing electrons, and polystyrene likes gaining them.

Although you don't need to rub the two materials together to have this effect, it often helps to increase the area that has been in contact.

To start off the can is uncharged

Charge on a Can

So when you rub the polystyrene on your head it becomes negatively charged. The can is made of a metal (either alumninium or steel), in metals the electrons can move around easily - they are good conductors - so if you put the negatively charged polystyrene next to the can the electrons will be repelled and move to the far side of the can.

Can with charge nearby

So near to the polystyrene there are lots of positive nuclei which are strongly attracted to the polystyrene and on the far side there are negative electrons
 which are further away so less strongly repelled, so overall the can is pulled towards the polystyrene.

If you used something insulating like a lemonade bottle instead of the can, the electrons can't move between molecules, so they really can't move very far and the attraction is much weaker.


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