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 grid's job is to move power, and power is the product of potential difference and current, P = V I.
Read the full explanation
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.
Your focus
- 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.
The National Grid exam tips
Marking Points
- 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
Examiner Tips
- 💡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'.
Common Mistakes
- 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