# Physics - The National Grid (AQA Syllabus A)

In GCSE Science students will look at electrical energy. This is the last of six quizzes on that subject and it looks at how electricity is transported via the National Grid.

The National Grid is the name given to the network of cables and transformers that transport electricity from the power stations in Britain to homes, factories, offices, shops and the other places that require it. It was created during the 1920s and 1930s in order to give the country a more reliable supply of electricity. Before the National Grid, power stations were owned by private electricity generating companies who had their own local electricity grids. These ran on different voltages so a kettle bought in Birmingham might not work in Lichfield, just 20 miles away!

Generators at the power stations produce electricity at 25,000 volts. Electricity transported, or transmitted, at that voltage over the distances covered by the National Grid would create high currents. The electricity would lose a lot of its energy as heat which would be very wasteful. To get round this, the electricity generated by the power station is passed through a 'step up' transformer, increasing the voltage to as much as 400,000 volts. This means that the current will be a lot lower. To understand how this works, think about the relationship P = V x I where P is power, V is volts and I is current. If you increase the voltage, the current must decrease. With a lower current, there is less of a heating effect and so less energy is wasted.

The high voltage electricity is transmitted along overhead lines on pylons and through underground cables - the supergrid. But this high voltage is no good to consumers so it is reduced by step down transformers to an appropriate level for the end user - higher voltages for factories, car garages etc. than for homes and offices.

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1. The electricity generated at power stations is usually at 25,000 V. Which type of transformer is used to change the voltage required by the supergrid?
You can remember this because the voltage is increased from the power station to the National Grid (it is 'stepped up')
2. When the National Grid was established, the cables were all carried by pylons. Now, some of the cables of the supergrid have been buried underground. Which of the following is not an advantage of buried high voltage cables?
Some people believe that living near overhead power cables causes health issues because of the electromagnetic fields they emit. Underground cables do not emit electromagnetic fields
3. Where would you find a step-down transformer?
They are used to reduce the high voltages of the National Grid ready to distribute to consumers
4. The National Grid is a system of what?
The transformers change the voltage of the electricity in the National Grid. Power stations and consumers are at either end of the National Grid but not parts of it
5. Why is electricity carried by the supergrid at very high voltage?
The higher voltages mean lower currents are flowing so less energy is wasted
6. Why are CHP (combined heat and power) stations more efficient than most fossil fuel burning power stations?
The electricity from CHP stations is usually used locally too. This also makes them more efficient as it avoids the inevitable losses caused by transporting electricity for hundreds of miles
7. Which of the following is not an advantage of the National Grid?
The grid is about having a reliable system that can cope with sudden demands or power station breakdowns without consumers having a power cut. Having a National Grid does not make electricity costs any cheaper
8. About seven to eight percent of the electrical energy is lost in the National Grid, mainly as which type of energy?
When electricity passes through any wire there is a slight resistance. This causes some of the electrical energy to be transferred into heat energy
9. Which of the following contains the missing words from the passage in the correct order?

________ transformers are used at power stations to produce the very high ____________ needed to ensure that electricity is transmitted efficiently through the National Grid. Such high voltages are dangerous to use in the home, so _________ transformers are used in substations near to homes to reduce the voltage to safe levels. The voltage of ________ electricity is about 230 V.
If you associate step-up transformers with power stations, that narrows your choice to two possibilities
10. Which of the following would be a good stand-by power station?
You need a power station with a very short start up time that will definitely be available at times of peak electricity demand