Static means 'still' and is the word we use when we talk about charge that can't move. A good example of this is when you rub a balloon on your hair and it sticks to the ceiling. You use friction to charge up the balloon.
There are two kinds of charge: positive (+) and negative (-).
All objects are made up of atoms, and all atoms contain positive particles (protons) and negative particles (electrons).
And yet atoms don't have an overall charge. They are neutral.
That's because atoms contain the same number of protons and electrons. So the positive and negative charges are cancelled out.
Static electricity is caused by an atom having too many or too few electrons (e-).
When insulating materials are rubbed together, electrons are knocked off one of them and onto the other.
This happens when a polythene strip is rubbed with a duster, the electrons move from the duster to the strip. The strip becomes negative (because it has more electrons than it needs to cancel out the protons) and the cloth becomes positive (because it has less electrons than it needs).
The way you may have seen static generated at school is with a Van de Graaff Generator (see diagram). The motor turns the rubber band and friction between the band and the rollers starts to build up huge amounts of charge.
This charge is then deposited on the metal dome, which is a conductor, but is insulated from everything else (like birds sitting on a power line), so it is able to store the charge.
The charge can build up in a big way, causing voltages as high as 50,000 Volts. A shock from this doesn't kill you, as the actual amount of charged particles stored on the dome is so small.
The basic rule you need to know is:
Like charges repel.
Opposite charges attract.
Press the play button below to see how the charges move when placed together:
This can lead to funny things happening when a charged object is brought near a neutral one. Follow the steps of this example of induction:
|Step 1:||Ball is neutral (no overall charge) as it has the same number of positive and negative charges.|
|Step 2:||A negative strip (excess electrons) is placed near the ball.|
|Step 3:||The negative charge of the strip causes the electrons to move to the opposite side of the ball, leaving the right hand side positive.|
|Step 4:||As opposite charges attract, there is an attractive force between them. If the strip is fixed, the ball will swing towards it.|
If enough charge builds up on an insulator, the charge can leap the gap, causing a spark. This can be prevented by discharging the object, gradually. This is called earthing. This is to do with Planet Earth - it's not just a silly name! The Earth can soak up any excess charge (see the section on Safety as well).
When static comes up in an exam, it is normally through an example. Think about these:
When you rub your hair (e.g. balloon or jumper), or hold onto a Van de Graaff generator while you are insulated yourself, your hair stands on end (unless it's full of super strength gel!). Because each of the hairs has the same charge, they try to get as far away from each other as possible.
Photocopiers and some computer printers want the toner to go in the right place. Give the toner charge and give the paper you want it to appear on the opposite charge. Bingo!
Special powder paints that have been charged, spread out to form a very even coating of paint on an earthed metal object - a car body or a bike frame, for example. It also means paint is not wasted as all the paint is attracted to the object. Less mess too!
Static is also used to remove pollution from smoke-stacks. Electrostatic plates are placed in the chimneys and they attract all the polluted dust.
If clouds get charged up enough, you get lightning, the biggest spark of all.
Static can also be dangerous when refuelling aircraft. The fuel rubs against the side of the hose and lots of charge builds up. If the plane isn't earthed, the spark can blow the plane up quicker than in Die Hard 2. A thin copper wire between the plane and earth is enough to carry the excess charge away.