Charge: if two insulating materials rub agaisnt each other, electrons are rubbed off one material and depostied on the other. The material which gains electrons then becomes negativley charged, becuase electrons have a negative charge, and the material which then loses electrons has a positive charge. Which one gains or loses electrons depends on the materials used.
An example of this would be rubbing a dry cloth on a polythene rod. Electrons are transferred from the dry cloth onto the polythene rod, if the same cloth is then rubbed against a perspex rod, the electrons from the perspecx rod would move onto the dry cloth, making it have a negative charge once again.
Like charges repel and opposite charges attract and the bigger the distance between the forces, the weaker the force is.
When a charge flows through a conductor, there is a current in it. Electric current is the rate of flow of charge. In a solid conductor e.g. a metal wire, the charge is carried by electrons. The reason metals are good conductors is because they have a declocalised outer shell electron which mean they can carry a current as electrons are charged. This is why insulators cannot conduct as all the electrons are held within the atom.
A conductor can only hold charge when it is isolated from the ground otherwise the electrons will flow to/from the earth and discharge it.
The bigger the charge of an isolated object, the higher the potential difference between the object and the earth. If p.d. is high enough, a spark may jump across the gap between the object and the ground.
In a photocopier, a copying plate is given a charge. An image of the page to be copied is projected onto the charged plate. When light hits the image, the charge 'leaks' away leaving behind a pattern of the image. Black ink powder is attracted to the charged parts of the plate and the powder is then transferred into a piece of paper. The paper is heated so that the powder melts and sticks to it producing a copy of the original image.
Electrostatic smoke precipitators are used in chimneys to atteact dust and smoke particles so they aren't released into the atmosphere. The particles pass over a charged grid and pick up a charge, they are then attracted to the plates on the chimney wall with the opposite charfe. The particles stick to these sheets and are then shaken off and collected.
Cars are usually painted using an electrostaic paint sprayer. The spray nozzle is connected to a positive termial. This mean that the paint droplets pass through it and pick up a positive charge. This make the paint droplets repel each other, so they spread out to form a fine cloud. The car will be connected to a negative terminal so the paint droplets are attracted to the car.
Danger of electrostatics
Static electricity can be dangerous. The filler pipes on road tanker that are used to pump fuel into storage tanks are earthed to prevent them from becoming charged, because a spark could occur and cause an explosion of the fuel vapour. Apart from eathing, another way to deal with the dangers of electrostatics is by using a antisatic material