GCSE Science/Static Electricity
Have you ever noticed a crackle when you pull off a sweater? Have you ever had your hair stick to your face after combing? What about seeing a balloon stick to the wall after it has been rubbed? All these effects are caused by static electricity. Static electricity is the imbalanced charges of matter. It was the Greeks who first noticed that when they rubbed a piece of amber, it attracted small pieces of paper. The Greek word for amber is elektron, and it is from this that we get our words electron and electricity.
What causes static electricity?
Usually materials are electrically neutral. This means that they are not electrically charged. However this doesn't mean that materials have no charges inside them. All materials are made of atoms. Atoms are made of particles called electrons, protons, and neutrons. Electrons have a negative charge, protons have a positive charge, and neutrons (as you can probably guess by the name) have no charge. Materials are partly made of charged particles. Materials have no overall charge whenever they have the same number of electrons and protons. The negative charge of the electrons exactly cancels out the positive charge of the protons.
Test YourSelf: Q1) The element lithium has three protons. How many electrons must a neutral lithium atom have?
Inside an atom the protons and neutrons are held firmly at the very centre in a structure called the nucleus. The electrons however are held much more loosely. Some can be found at the surface, and if the surface is rubbed, they can be rubbed away. This removal of electrons will leave the atom with more protons than electrons. It will have a positive charge because the protons are positively charged. Atoms that are charged are called ions.
When you rub a balloon on your sweater some of the electrons are transferred from the sweater to the balloon. The sweater becomes positively charged.
Test YourSelf: Q2) What charge will the balloon have?
How do the two types of charge behave?
The sweater is positively charged, the balloon negative. The balloon sticks to the sweater. This is because unlike charges attract. If you were to take two balloons, both of which had been negatively charged and brought them together you would find that they tried to push apart. This is because like charges repel.
Test Yourself Q3) Look at the balloons above. The pair on the left are being repelled. The pair on the right are being attracted. Notice that one out of each pair of balloons is negatively charged. What is the sign (positive or negative) of the charges on the other two balloons?
Sparks can fly!
When a large enough charge is built up sparks can fly. A spark is just a flow of electrons through the air. This can sometimes be very dangerous. For example, petrol flowing through a pipeline can pick up a charge by friction. If a spark flies it could ignite the petrol. For this reason, pipes are earthed. Earthing means connecting the pipe to the ground by means of a copper wire. Copper, unlike air, is a very good conductor of electricity. The electrons can flow easily and safely to the ground without sparking.
Some devices make use of sparks. A Van de Graaf generator consists of a large metal dome that is in contact with a rubber or plastic looped belt. As the belt moves, friction causes it to become charged; the charge is spread out over the metal dome where it builds up until it is large enough to cause a spark.
In the diagram above a girl is standing on a washing up bowl and touching a Van de Graaf machine.
Test Yourself Q4) What do you think is the purpose of the washing up bowl?
Test Yourself Q5) Explain why the girl's hair is standing on end (no it's not her style, it's to do with the charge)