| Energy & Forces - Conversion & transfer of energy - Electric circuits - G20 |
  Teacher's
Guide
EF-D2.2 |
ISE 5-14 Curriculum Support Materials Overview adviceGroup 20 exemplar Energy & Forces - Electric Circuits (Word) |
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1. Electrical energy is
carried from power stations by a network of cables called the
National Grid. Since electrical
energy of this amount cannot be stored it has to be made on demand. Often one
area or another may not have sufficient supply locally
 and
so via the grid they can get what they need from elsewhere around the country.
For example about 30% of the electricity produced in Scotland is exported south
to England and Wales.As the demand increases during the day - for example there is often a surge of power being used when there is an advert break on a favourite TV programme as people go and put on the kettle for a cup of tea - the power stations increase their output accordingly.
Power leaving a power station is at a very high voltage. Voltage is the amount of ‘push’ the electricity has. It is maybe easier to compare electricity to water -if you have several thousand gallons of water stored in a tank it is of little danger. If the tank burst and comes flooding through your living room you are likely to drown. The amount of electricity is not so dangerous it is the amount of push or voltage it has which is dangerous. Transformers increase the voltage and the electricity leaves the power station at either 400,000 volts or 275,000 volts. Thick cables carried on tall pylons carry the power across the countryside. Near to the towns and cities, the power is taken through another set of transformers where some of the voltage is reduced to 132,000 volts. More cables and pylons carry the power into towns and cities where again it enters transformers.
Many children will be familiar with small local transformers which have danger signs on them warning them to keep out. The reason for these high voltages is to reduce losses as the power is carried through the cables. At low voltages too much is lost. To show this as a diagram you should have something like this :-
This is probably a good time to mention the dangers of high powered cables, transformers and pylons to the children. High power cables are obviously lethal to humans. The reason why birds can sit on the wires without being electrocuted is that they are not touching anything else. You are safe if you are only touching the one cable. If in any way you have contact with a second cable, the ground or anything which is linked to the earth you will be electrocuted. That is why sometimes parachutists land on cables but are not injured so long as no-one gets a ladder to rescue them while the power is still on! If such an accident occurs, you must not go near the stranded person until the power has been turned off - telephone number given at the bottom of the pylon. Transformers are often found in estates and housing areas - they are highly dangerous and should not be entered for any reason. |
Heavy
industries are supplied with power at 33,000 volts, light industries at 11,000
volts. By the time the power reaches us in our homes it has been stepped down to
240 volts. 
