Electric Charge
This page is about how electricity can build up on insulating material - since the charge doesn’t move anywhere we call it static electricity. Static charges can accumulate in lots of places, even in clouds, and it is responsible for bolts of lightning. Electrifying stuff..
Conductors and Insulators
Any material which allows electricity to pass through it is called a conductor. Metals, such as copper, aluminium and iron are good conductors so they are useful for making things like electrical wiring. Materials which do not allow electricity to pass through easily are referred to as insulators and include material such as plastic and wool. We use insulating material such as plastic to cover electrical wiring and prevent people from getting an electric shock.
Static Charge
Generating friction by rubbing two insulating materials together, such as a cloth on a plastic rod, can cause electrons to be rubbed off one material and be transferred to the other. The material which has lost electrons now has a positive electrostatic charge and the one which gains electrons has a negative electrostatic charge. The accumulated positive or negative charges cannot move so they are referred to as static charge.
If you’ve ever rubbed a balloon to your head and stuck it to the wall, you’ve already discovered static electricity. The friction causes electrons to move between yourself and the balloon, resulting in the balloon becoming negatively charged. Even though the wall itself is neutrally charged, the charges within the wall can rearrange themselves so that the positive charges are closer to the surface than the negative ones. For a short time, the oppositely charges between the wall and the balloon will keep the balloon stuck to its surface.
Conductors cannot become charged in this way because the electrons are able to travel through them. A build up of static charge on an insulating material can be discharged by connecting the object to the earth using a metal rod. The electrons will either flow from the object to the ground (if the object has a negative electrostatic charge) or from the ground to the object (if it has a positive electrostatic charge). This process is called earthing.
As static charge builds up on an object, so does its voltage. If the voltage becomes very large, electrons can ‘jump’ from the object to the earth causing a spark. This is how lightning occurs - friction between particles of ice and rain droplets causes the accumulation of electrons within clouds. The increase in voltage causes the electrons to jump to the ground, which is seen as a bolt of lightning.
Sparks resulting from static electricity is a serious problem, since it can cause a fire. Static electricity can also develop when refuelling aircraft and tankers due to friction between the fuel and the pipes which causes the movement of electrons. To prevent the possibility of a spark forming, aircraft are earthed with a metal rod during refuelling.
Attraction and repulsion
A charged object will experience a non-contact force if placed near another charged object. If the two objects have the same charge, they will repel one another. If they have opposite charges, they will attract.
The repulsion of like charges is responsible for your hair standing on end when touching a Van de Graaff generator. Friction beneath the metal dome causes electrons to accumulate on the dome - it has a static charge. If a person places their hand on the dome whilst standing on an insulating material (to prevent earthing), the electrons will move from the dome through your body. The electrons will even travel along your hair and since like charges repel, each strand of hair moves away from one another for a (literally) hair-raising effect.
Uses of static electricity
Spray painting: cars and other objects can be painted evenly using electrostatic charges. The paint is given a positive electrostatic charge and as the paint covers the car, the individual paint droplets will repel one another (since they all have the same charge). This gives the car a smooth, even finish and avoids over-using paint.
Photocopiers: the image to be photocopied is placed onto an image plate which has a positive electrostatic charge. Parts of the image without ink will let the light through, allowing the charge to leak away. This leaves only the darker parts of the image with charge and these positive charges attract negatively charged black powder. The paper is passed through heated rollers, causing the black powder to stick to the paper.