Switches: breaking the current
When you switch on a light the bulb starts to shine. When you press a doorbell a buzzer or bell sounds. When you switch on a CD player a motor starts to turn the disc and you hear music. When you switch something off it stops working immediately. What goes on when a switch is turned on and off? To understand this we have to think about what materials are made of.

The first person to think about what materials are made of was called Democritus. He lived in Greece over two thousand years ago. Democritus knew that if you cut something in two you would get two pieces half the size of the original. He knew that if you
cut one of these pieces in two, you would get two smaller pieces but he took the idea further. He wondered what would happen if you kept cutting up the material until you came to a piece that was so tiny that it could not be cut up. He called this tiny particle an atom. The word atom means indivisible.

The idea of atoms was passed down through the centuries and in time scientists began to use it to explain their observations. For example, scientists explained that solids have a fixed shape because their atoms are firmly linked together, while liquids can flow because their atoms slide over one another.

After the invention of the battery scientists began to study electric currents in wires but had difficulty understanding what a current really was. Some thought that it was a kind of liquid that flowed through the wires in some way. As scientists spent more time studying electrical currents they made more devices to test their ideas. One scientist, called William Crookes, even investigated what happened when a glass tube from which air had been removed was put in the circuit. The tube contained two wires that were not connected. When the current was switched on rays were found to flow through the tube. Some scientists thought they were rays of light but when Crookes brought a magnet near the tubes the rays were bent. This meant the rays could not be made of light but were made of tiny particles. Further experiments showed that the particle was very tiny and carried a small charge of electricity. At the time, other scientists had worked out the size of atoms but these particles were much smaller. It was decided that the particles must be part of atoms that had been split away from them. The particles became known as electrons and they were later found to be arranged round the centre of an atom a little like the way the planets in the Solar System are arranged around the Sun.

Earlier in your science course you may have done some experiments to find out which materials conduct electricity and which do not. If you did these experiments you would have found that metals conducted electricity while most other materials did not. The reason why metals conduct electricity is that some of the electrons in metal atoms are free to move about, a bit like a roving planet which could travel from one solar system to the next. These wandering electrons are the key to understanding what happens at a switch.

When you turn on a switch metal contacts meet. The batteries push on the electrons and they move round the circuit crossing the point where the contacts meet. When you turn off the switch the metal contacts are separated by a gap filled with air. The atoms of the gases of the air do not have roving electrons so cannot conduct electricity. This means that the electrons at one of the contacts cannot move across the gap like people who cannot move across a river when a bridge is destroyed.

Next time you switch something on, think of the billions of electrons that are moving round the circuit you have created. When you flick the switch off they simply stop moving.

How big are atoms?
You can get some idea of how big atoms are by taking a ruler and drawing a line one millimetre long. A row of ten million atoms could fit along the line.

If electrons are in the outer part of an atom what is in the centre?
The centre of the atom is called the nucleus. In nearly all atoms it is made from two other kinds of particles called protons and neutrons.

What are materials called that do not conduct electricity?
They are called insulators. They do not conduct electricity because they do not have electrons, which are free to roam like the electrons in metals.

When you turn on a switch why doesn't the current come onto your fingers?
The parts of the switch that you touch are made from insulators. The electrons in their atoms cannot move and make a current.

Why should you not touch a switch with wet hands?
The water off your skin will contain some dissolved substances such as the salt in sweat. These substances can carry electrical charges like electrons and cause a current to flow through the liquid. If some of the water from your skin flowed to a contact the current could travel to your skin and give you an electric shock. If this occurred at a switch connected to the mains in your home or school it could kill you.

Is air an insulator?
Yes, it is. In most conditions it will not allow a current of electricity to pass through it.

When does a current of electricity pass through air?
When the push on the electrons is very high. This occurs in the spark plug in petrol engines where the spark is used to heat the petrol, and make it explode to drive the engine. It also occurs in a storm cloud. The charge, which makes the push on the electrons, is made inside the cloud by the rubbing together of ice, dust and water particles. You can create a charge on a pen by rubbing it on your sleeve. The charge may be strong enough to pick up tiny pieces of paper. In a storm cloud the charge is so strong that it pushed electrons into the air and a current of electricity is made which we call lightning.

When you turn a switch on does current flow out of the battery?
It does not gush down the wires like water down a pipe. Think of the electrons in all the wires in the circuit as it they were lined up to move. The moment the switch is turned on, the push or voltage of the batteries makes the electrons at the switch cross the contacts and the electrons in all the other parts of the wire move too. This is why a light comes on almost the instant you switch it on. If the current was to flow out of the battery like a liquid, there would be some delay before it passed right round the circuit and lit a lamp or made a motor turn.

Are electrons held around the centre of the atom by gravity?
No. They are held by an electrical force because the electrons have a negative charge and the centre or nucleus has a positive charge. The idea of the atom being like a tiny Solar System helped scientists think about the structure of the atom. Before this idea it was thought that the electrons were arranged in an atom like currants in a pudding! Today we do not use the Solar System idea as more things have been found out about the structure of the atom. For example, the electrons move around the centre of the atom at nearly the speed of light - very much faster than any planet.