Static Electricity

When some insulating materials are rubbed together they become electrically charged. Note that both materials must be insulators. With conductors, like metals, electrical charges move quickly to the ground.

They become electrically charged because electrons (which are negatively charge particles) move from one insulating material to the other. Both materials end up with the same amount of charge but the end charges are opposite:

• – the material which gains electrons becomes negatively charged
• – the material which loses electrons becomes positively charged

Charged objects with different charges will attract each other. Charged objects with the same charge will repel each other.

If an object is charged and a smaller object is not charged then the charged object will attract the smaller, uncharged object.

When a charge flows through a conductor this is called a current. An electric current is the rate at which a charge flows. In a conductor which is solid, like a metal wire, electrons are the charge carriers. Metals are good conductors because they contain free conduction electrons that aren’t held in one atom. The electrons in insulators, on the other hand, are confined in atoms and so can’t conduct electricity.

A conductor must be away from the ground in order to hold a charge. If it’s earthed then the electrons will flow down to the earth and the conductor will be discharged.

The larger the charge on an isolated object the bigger the potential difference between it and the ground. If this potential difference reaches a high enough point then a spark can jump between the object and any earthed conductor brought close to it.

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Electrostatics

We use electrostatics for a number of reasons. In photocopiers the copying plate has a charge. When you want to photocopy an image onto a sheet of paper the image is projected onto the charged plate. The light hits the image and as it does so the charge ‘leaks’ away and leaves a pattern of the image. Ink powder is attracted to the charged regions of the plate and it’s this powder which then gets transferred onto a piece of blank paper. The paper is then heated which makes the powder melt and stick to it, creating a copy.

Electrostatic smoke precipitators are found in chimneys. They prevent dust and smoke particles being released into the air. When the particles pass off the charged grid they become charged. The plates lining the chimney walls carry the opposite charge and so attract the particles which stick to them. The dust and dirt can then be easily collected later.

Electrostatic paint sprayers are popular for painting cars. The paint droplets are positively charged. This means they repel each other and produce a fine cloud. The car, meanwhile, is connected to a negative terminal and so attracts the paint droplets.

As useful as electrostatics is, it also has its dangers. One way to prevent an accident is to use earthing. For example, road tanker filler pipes pump fuel into storage tanks. If they were to become charged and a spark was caused then the fuel vapour would explode. To prevent this, filler pipes are earthed. Antistatic materials are another way to prevent charge.