Power supplies
When you flick a switch in the home electrons start to move almost instantaneously to work for you. They may light a bulb, boil a kettle or make your computer work. Where do the electrons get their energy from? The answer could be from sunlight that reached the Earth 275,000,000 years ago!

At that time large areas of the Earth were covered in forests and swamps. The trees in the forests collected some of the energy in sunlight in their leaves to make food. This energy was stored in materials, such as wood, that the tree made. When trees died many fell into the swamps and did not rot away completely because there were not enough microbes present to do the job. Instead these trees formed a black rock called coal. The energy that the trees had stored was also stored in the coal.

Millions of years later, people began to use coal as a fuel. They used it to provide the energy to heat water in steam engines. You may be surprised to find out that coal is still used to provide energy for a type of steam engine today. These steam engines are used at many power stations. The coal is crushed to dust and sprayed into a furnace where it burns fiercely. The heat from the burning coal is used to heat water and turn it into steam. The steam is directed over a series of turbines. These look like windmills with many small sails, called blades, which spread out like the segments in a Victorian lady's fan. As the steam rushes though the blades, it spins the turbine round three thousand times a minute.

At one end of the turbine is a huge electromagnet. It spins at the same rate as the turbine. Around the electromagnet are coils of wire. The electromagnet and the coils make up an electrical generator. When the electromagnet spins it causes an electrical current to form in the wires. The voltage of this electricity is about twenty five thousand volts. Although this seems very high, scientists have found that they need electricity at a higher voltage if they are going to move it cheaply. If the voltage is not high enough a great deal of heat is lost and a very thick wire is needed to conduct the electricity. If the voltage is made very high less energy is lost as heat and a thinner wire can be used to carry the current.

When the electricity leaves the generator it passes to a step-up transformer. This is a device which increases or steps up the voltage. By using step-up transformers electrical engineers can raise the voltage to four hundred thousand volts. It is at this voltage that the electricity may cross the countryside on cables supported by pylons. When the cables reach the town or city where you live, they deliver the current to a step-down transformer. This reduces the voltage and prepares it for distribution to various places. For example, if there is heavy industry such as a steel works, which needs large quantities of electricity to melt steel, a cable carrying electricity at thirty three thousand volts will be sent to it. Lighter industries, such as shoemaking, need electricity at eleven thousand volts to work the machinery. When electricity is passed into a town it usually travels on cables under the ground. At various places along the cable's path there are substations which distribute the electricity to street cables. It is along one of these street cables that the electricity from a power station approaches you home. When it enters your home it passes into the various circuits that you have and gives the push to the electrons when a switch is turned on.

Although the newest piece of electrical equipment you are using belongs to the technology of the twenty first century, the energy to make it work comes from a time before the Age of the Dinosaurs.

Do all power stations use coal?
No, but many do. Some power stations use oil. A few power stations use the energy in radioactive materials. These are called nuclear power stations. Some power stations use the power in running water. They have turbines, which are spun by water.

What is a wind farm?
This is a small power station, which has perhaps thirty or more aerogenerators or wind turbines. The wind turbine is mounted on a tall tower. As the wind makes it spin a generator is turned and electricity is produced. Wind farms are becoming more widely used to provide electricity because unlike coal or oil-fired power stations they do not cause air pollution.

What pollution is caused by coal or oil-fired power stations?
In the past they have produced gases which have made acid rain. They also produce large amounts of carbon dioxide, which seems to be causing the temperature of the Earth to warm up, in the process called global warming.

How is the current kept on overhead cables?
The towers of metal which hold the power cables overhead are called pylons. You may think that the electricity from the cable would pass through them to the ground. This does not happen because of the way the cable is attached to the pylon.
The metal of the cable and the pylon never meet. They are connected together by columns of clay or ceramic pots. The current cannot pass through the pot because pot is an insulator.

If the current is at such a high voltage in the transmission lines why does it not spark to the ground?
The air between the cables and the ground acts as an insulator. This prevents a spark forming.

Why is it dangerous to fly kites by overhead power cables?
If the kite came into contact with the cable the current could flow down it and kill whoever was at the other end of the string.

Why can birds perch on power lines and not be harmed?
The current has to flow to the Earth to cause damage. While a bird is sitting on the cable it is completely insulated owing to the action of the ceramic pots. As long as the bird does not touch anything that is in contact with the ground it is safe.

What insulation is used in an underground power cable?
The wires carrying the current are enclosed in plastic. Plastic is an insulator. The insulated wires are also enclosed in a metal tube. This protects the wires from accidental damage when roads are dug up.

Are the overhead power cables on railways dangerous?
Yes, they are. They carry electricity at twenty five thousand volts. This provides the power to work the huge electric motors in the locomotives.

Are substations dangerous?
Yes, they are. Any place where electricity is being transmitted is a dangerous place if instructions are not followed. All these places carry a notice warning people to keep away and that is exactly what they should do.