Static electricity is both useful and dangerous. It allows photocopiers to work and is also the cause of of electric storms. Sometimes an enormous amount of charge builds up in storm clouds. Eventually, the charge gets too great and a giant spark is created sometimes to the Earths surface.
3.5 Charging by rubbing
What's the attraction?
When a piece of polythene is rubbed with a cloth, it becomes charged. Electrons are 'rubbed' off the cloth and onto the polythene by friction. This makes the polythene negatively charged and leaves the cloth positively charged.
The rod and the cloth now have opposite charges. We can hang the rod by a piece of thread and will then see that rod and cloth attract each other. We have found that:
opposite charges attract each other
This force of attraction is the same force that holds the electrons close to the nucleus in an atom.
3.5 Repelling charges.
Similarly, when a piece of Perspex is rubbed with a cloth it becomes positively charged. Electrons have been rubbed from the Perspex onto the cloth. Again, the rod and the cloth have opposite charges and will attract each other.
However, what happens if we bring up another piece of rubbed Perspex (which has a positive charge)? Now the two rods repel each other.
The two pieces of Perspex have the same charge. We have found that: