Friction can lead to electrons moving from one object to another and therefore to a build-up of charges.
If you walk across a carpet while wearing woollen socks, you can build up a charge in your body. If you then touch an object that allows charge to flow, you will get a shock.
You can use friction to charge objects by rubbing them. This method is usually used to charge insulators.
Electrostatic charging through friction always involves a transfer of electrons:
- As two materials rub against each other, some electrons from one material are transferred to the other material.
- An object becomes negatively charged if it gains electrons.
- An object becomes positively charged if it loses electrons.
- Protons do not move between objects.
When a piece of plastic is rubbed with a cloth, it becomes negatively charged. When a piece of glass is rubbed with a cloth, it becomes positively charged.
As two unlike charges attract, the glass is attracted to the plastic.
A charged object is able to charge a neutral object (usually a conductor) without having any direct contact. This is called electrostatic charging by induction.
A positive insulating rod is placed near a neutral conductor (a metal ball in this case). Electrons in the metal ball move towards the positive rod.
This induces a negative charge on the side nearest the rod and a positive charge on the far side.
- Connecting the metal ball to the earth causes electrons to flow to the ball to neutralise the excess positive charges on the far side of the ball.
When the metal ball is disconnected from the Earth, excess negative charges remain.
When the insulating rod is removed, these charges spread out evenly across the ball.
Electrostatic charging can have dangerous consequences.
Friction causes charges to build up on the interior of fuel pipes when fuel is running through. This can cause a spark that ignites the fuel, causing an explosion.
Lightning is caused by the build-up of negative charges on the underside of storm clouds.
These negative charges can induce charges in (ionise) the air below. The negative charges then flow through the air to the ground below at very high speeds.
Tall buildings are fitted with lightning conductors. These direct the lightning along a wire and into the ground so that it does not damage the building.
Electrostatic charging is used in photocopiers.
- A copying plate is given a positive charge.
- An image of the document to be copied is projected onto the copying plate. Where light hits the plate, the positive charge leaks away.
- Negatively charged black toner is attracted to the areas which remain positive.
- Paper is placed over the copying plate. The toner is then transferred to the paper to create the photocopy.
A smoke precipitator is a device that uses electrostatic charging to remove smoke particles from the air. They are used, for example, to remove soot particles from waste gases in coal-fired power stations.
Smoke particles pick up a negative charge from charged rods, and are then attracted to a positively charged plate where they are removed from the air.
Spray paints are used in car manufacturing to evenly paint the body of cars. Some spray paints use electrostatic charging.
Paint particles are positively charged and so repel each other and spread out evenly. This ensures the object receives an even coat of paint.
The object being painted is sometimes negatively charged.
This means that the paint sticks to the object and none is wasted.