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Electricity - The National Grid
Birds don't get electrocuted when they stand on pylon wires - why not?

Electricity - The National Grid

Test your knowledge of the National Grid in this GCSE Physics quiz. Electricity is generated in power stations of various kinds - ones that burn fossil fuels, nuclear fuelled power stations, hydroelectric etc. In the early days of electricity generation, it was carried out by companies using small local power stations. If one of these broke down then all of the consumers would be without electricity until it was repaired. Some companies supplied AC and others supplied DC and there was no agreement about what voltage to use.

This changed just after the end of the First World War - in 1919, the government at the time realised that electricity generation and supply needed to be better organised. An Act of Parliament created electricity generating authorities who took over the existing small and inefficient power stations, replacing them with a smaller number of larger ones.

They were also to interconnect their systems so that power could be transferred around the country to where it was most needed. A few years later, another authority was created (the Central Electricity Board) which established the National Grid by connecting together the largest of the power stations.

The National Grid is an electricity transmission network which spans the whole of the United Kingdom. It distributes electricity to most homes and businesses in the UK. Some homes and small businesses are 'off grid' which means that they generate their own electricity, usually by using environmentally sensitive methods. The National Grid also has undersea interconnections to northern France, Northern Ireland, the Irish Republic, the Isle of Man and the Netherlands, with more planned for the future.

Power stations generate electricity at 25,000 V and they are connected to the National Grid via step-up transformers. This is because electricity is sent through the grid at 400,000 V, 275,000 V or 132,000 V. A step-up transformer is one that increases the voltage. If the electricity was sent into homes, offices, shops, factories etc at these voltages, it would be extremely dangerous to use. Where the electricity is distributed to consumers, step-down transformers are used, for example, the last transformer between your school or your home and the National Grid reduces the voltage to 230 V. Transformers will only work using AC, so power stations no longer generate DC.

The reason that electricity is transferred around the country at such high voltages is to reduce energy losses. At high voltages, it requires less current to transfer the electricity through the wires. A lower current means that less of the electrical energy is transferred into heat energy. The further that the electricity travels through the National Grid, the greater the loss of energy.

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1.
What is transferred through the National Grid?
Coal
Petrol
Gas
Electricity
The highest voltage cables are made from aluminium which is light, strong and a good electrical conductor
2.
What voltage is used to transfer electricity through the National Grid?
Up to 300,000 V
Up to 400,000 V
Up to 500,000 V
Up to 600,000 V
There are 132 kV, 275 kV and 400 kV circuits that make up the grid
3.
How is such a high voltage produced?
By using step-up transformers
By using step-down transformers
By using a Darlington pair
None of the above
Step-up transformers can be used to increase voltage whilst decreasing current, conserving the total amount of energy
4.
Why is a high voltage used?
It increases the amount of energy wasted in cables
It reduces the amount of energy wasted in cables
The cables cannot transfer low voltage electricity
So they can transfer the electricity at a high current
Some electricity used in homes and businesses on the south coast could have been generated in Scotland so keeping energy losses down to the minimum possible is really important
5.
What is the mains voltage in homes in the UK?
120 V
170 V
230 V
270 V
Whilst in the UK the voltage is 230 V, in other countries this can vary, for example, in the USA it is 110 V
6.
How is voltage from the National Grid changed into 230 V for home use?
Step-up transformer
A thin cable
A thick cable
Step-down transformer
Step-down refers to the voltage
7.
Electricity is sent through the National Grid using which type of electrical current?
Alternating current
Direct current
Alternating and direct current
Neither alternating nor direct current
If it was sent using DC, it would not be possible to use transformers so a lot more energy would be wasted (or we would need to use extremely high and therefore more dangerous voltages)
8.
What voltage is produced in power stations?
15,000 V
20,000 V
25,000 V
30,000 V
This then needs to be increased for distribution via the National Grid
9.
Why don't birds get electrocuted when they stand on pylon wires?
As they are not connected to any grounded point, the electricity will not flow through the bird and will continue through the cables
They do get electrocuted
Birds do not conduct electricity
None of the above
Electricity will always try to make its way back to the Earth by finding the most efficient path. Air is actually a good electrical insulator and high voltages are required for electricity to be able to pass through it. The birds are not earthed and surrounded by air, so the electricity follows the line of least resistance - the wire
10.
Why does reducing the current and increasing the voltage mean less energy is lost?
A high voltage means they can transfer the electricity at a high current, transferring it at a high current means fewer electrons travelling through the cables which reduces the amount of collisions between electrons and the cables, thus wasting less energy as heat
A high voltage means they can transfer the electricity at a low current, transferring it at a low current means fewer electrons travelling through the cables which reduces the amount of collisions between electrons and the cables, thus wasting less energy as heat
A high voltage means they can transfer the electricity at a low current, transferring it at a low current means more electrons travelling through the cables which reduces the amount of collisions between electrons and the cables, thus wasting less energy as heat
A high voltage means they can transfer the electricity at a low current, transferring it at a low current means fewer electrons travelling through the cables which increases the amount of collisions between electrons and the cables, thus wasting less energy as heat
The heating effect of an electric current increases in direct proportion to the square of the current, so keeping the current as low as possible saves energy losses

 

Author:  Martin Moore

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