The National Grid — AQA GCSE Physics
Test yourself on The National Grid with AQA GCSE practice questions.
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The National Grid explained
The National Grid is the countrywide network of cables, pylons and transformers connecting every power station to every consumer.
Read the full explanation
Electricity generated at around 25 kV leaves a station through a step-up transformer that raises the potential difference to 275 kV or 400 kV for the long transmission lines, and a chain of step-down transformers at substations brings it back down, first for towns and industry and finally to about 230 V for homes. Because every station feeds the same network, no house depends on a particular power station, and output can be matched to demand as it rises in the evening or falls overnight. Stations are often held below maximum output to keep spare capacity for a surge, a breakdown or a still, cloudy day.
Electrical power is transferred from power stations to consumers using the National Grid.
The grid's job is to move power, and power is the product of potential difference and current, P = V I. A station generating 500 MW can deliver that power as a huge current at a low potential difference or as a small current at a very high one. The cables have resistance, and the power wasted heating them is P = I² R, which depends on the square of the current and not on the potential difference. So the grid transmits at 400 kV: raising the potential difference from 25 kV to 400 kV cuts the current by a factor of 16 for the same transmitted power, and cuts the wasted heating by 16², which is 256. That is why long-distance lines always run at high potential difference and must be held out of reach on pylons.
Step-up transformers are used to increase the potential difference from the power station to the transmission cables then step-down transformers are used to decrease, to a much lower value, the potential difference for domestic use.
A transformer changes the potential difference of an ac supply by having different numbers of turns on its two coils: a step-up transformer has more turns on the secondary coil than the primary, a step-down transformer has fewer. Transformers only work with alternating current, because it is the continually changing magnetic field in the iron core that induces a potential difference in the secondary coil. For an ideal transformer the power out equals the power in, so when the step-up transformer at the power station raises the potential difference, the current in the transmission cables falls in the same proportion and less energy is dissipated heating the cables. At the far end the step-down transformer reduces the potential difference to a safe value, about 230 V, for homes.
Students should be able to explain why the National Grid system is an efficient way to transfer energy.
The argument has three links. First, the transmitted power is P = V I, so the same power can be carried at a high potential difference and a small current. Second, the power wasted heating the cables is P = I² R, which falls with the square of the current, so cutting the current by a factor of 16 cuts the waste by 256. Third, transformers themselves are very efficient, typically above 99%, so little is lost in making the change. Together these mean only a few per cent of the generated energy is dissipated to the surroundings between station and consumer, which is what efficiency measures: useful output energy transfer divided by total input energy transfer. Transmitting at 230 V would need enormous currents and waste far more.
Your focus
- Describe the route electricity takes from a power station to a house, naming each transformer in order.
- Suggest two reasons why fossil fuel power stations are run below their maximum output.
- Explain why a single connected network makes the supply more reliable for consumers.
Show all 12 objectives
- Explain why transmitting at a high potential difference reduces the energy wasted in the cables.
- Calculate the current in a transmission line from a stated transmitted power and potential difference.
- Justify the use of tall pylons and long insulators in terms of the 400 kV transmission potential difference.
- Explain why a step-up transformer at the power station reduces the energy wasted in the transmission cables.
- State the purpose of the step-down transformer at a local substation and the potential difference it produces.
- Identify a transformer as step-up or step-down from the numbers of turns on its primary and secondary coils.
- Explain the efficiency of the National Grid as a chain from high potential difference to small current to small I² R losses.
- Calculate the efficiency of a transmission link from its input and useful output energy transfers.
- Compare the energy wasted when the same power is transmitted at 400 kV and at 230 V.
The National Grid exam tips
Marking Points
- one mark for describing the grid as a national network of cables and transformers linking power stations to consumers
- one mark for a step-up transformer at the power station and step-down transformers near consumers
- one mark for the supply being matched to the demand of the consumers
- one mark for spare capacity kept for an emergency shutdown or for unreliable renewable output
- one mark for the transmitted power being the product of potential difference and current, P = V I
- one mark for a higher transmission potential difference giving a smaller current for the same power
- one mark for the power wasted in the cables depending on the square of the current, P = I² R
- one mark for less energy being dissipated to the surroundings, so the transmission is more efficient
- one mark for the step-up transformer increasing the potential difference
- one mark for the current in the transmission cables decreasing as a result
- one mark for less energy being transferred to the surroundings from the transmission cables
- one mark for the step-down transformer reducing the potential difference to a safe value for domestic use
- one mark for a step-up transformer raising the potential difference so that the current is smaller
- one mark for the power dissipated in the cables depending on the square of the current
- one mark for less energy being dissipated to the surroundings over the transmission distance
- one mark for efficiency stated as useful output energy transfer divided by total input energy transfer
Examiner Tips
- 💡Trace the route in order, station, step-up, transmission lines, step-down, consumer, when asked to describe the grid.
- 💡For 'suggest two reasons' questions about spare capacity, give genuinely distinct reasons rather than two wordings of one.
- 💡Quote 400 kV for transmission and 230 V for homes to show the scale of the change.
- 💡Answer in the order potential difference up, current down, I² R losses down, efficiency up.
- 💡Never write that the step-up transformer reduces potential difference, because mark schemes refuse the whole answer.
- 💡Write 'reduces the energy transferred to the surroundings' rather than 'stops energy being lost'.
- 💡Two marks here belong to the pair 'potential difference increases' and 'current decreases', so write both.
- 💡For the transformer nearest the consumer, the expected answer is a safe potential difference, not efficiency.
- 💡State the transformer type from the turns ratio whenever the numbers of turns are given.
- 💡Build the answer as a chain of causes, because each link is usually worth a mark.
- 💡If numbers are supplied, carry out the I² R comparison rather than describing it in words.
- 💡Give efficiency as a decimal or a percentage, and never above 100%.
Common Mistakes
- describing the grid as a store of electricity rather than a transmission network
- saying each home is supplied by one named power station
- placing the step-down transformer at the power station and the step-up transformer at the house
- claiming stations run at maximum output at all times in order to be efficient
- saying a higher potential difference reduces the wasted energy without mentioning the smaller current
- stating that the potential difference decreases at the step-up transformer
- claiming no energy at all is transferred to the surroundings from the cables
- using P = I R rather than P = I² R when arguing about cable losses
- writing that the step-up transformer increases both the potential difference and the current
- saying a transformer increases the power available, when the power out cannot exceed the power in
- claiming a transformer works with dc
- describing the step-down transformer's purpose as saving energy rather than making the supply safe
- arguing that a high potential difference makes the electricity travel faster along the cables
- saying the grid is efficient because transformers create extra energy
- quoting an efficiency greater than 1, or greater than 100%
- omitting the step that connects the smaller current to the smaller heating loss