Attraction and repulsion

You should know how and why insulators can be electrically charged.

Moving charges

When you rub two different insulating materials against each other they become electrically charged. This only works for insulated objects - conductors lose the charge to earth.

When the materials are rubbed against each other:

• negatively charged particles called electrons move from one material to the other
• the material that loses electrons becomes positively charged
• the material that gains electrons becomes negatively charged
• both materials gain an equal amount of charge, but the charges are opposite

Detecting charge

If two charged objects with the same type of charge are brought close together, they will repel each other - that is, if they are both positive or both negative. They will attract each other if they have opposite charges. You can check this by clicking on the sentences in the diagram.

The only way to tell if an object is charged is to see if it repels another charged object. This is because charged objects will also attract small uncharged objects. You can check this by clicking on the third sentence in the diagram.

Discharge

A charged object can be discharged by connecting it to earth with a metal wire or other conductor. If the potential difference (voltage) is very large, a spark may jump across the gap between the charged object and the conductor. This can be dangerous. For example, it could cause an explosion in a petrol station.