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AQA GCSE Physics · Topic 4.2 Electricity
The National Grid
Triple students: the National Grid is tested in Paper 2, in the
Electromagnetism topic, which includes this information and additional content. This page is for
Paper 1 Combined Science students.
In this lesson you'll learn to: describe the National Grid, explain what step-up and
step-down transformers do, explain why transmitting at a high voltage and low current is an
efficient way to transfer energy, and use Vp × Ip = Vs × Is for a
100% efficient transformer.
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Prefer to watch? Scroll to the bottom for the video on this topic
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A video walkthrough is coming soon — it will appear at the bottom of this page
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Start here — have a play
Don't read anything yet. On tab 1, look at the voltage and current in each part of the grid, then press Take away the step-up transformer and watch what happens to the cables. On tab 2, work through the transformer questions.
The chain to learn: step-up transformer → voltage up → current down (the power stays the same) → less energy wasted heating the cables and the surroundings → a more efficient way to transfer energy.
For a 100% efficient transformer the power in = the power out: Vp × Ip = Vs × Is. p = primary (in), s = secondary (out). If the voltage goes up, the current goes down by the same factor.
Transformers are treated as 100% efficient. The cables are given a resistance of 5 Ω.
▲ Mains is a.c., so the charges vibrate back and forth: a bigger wobble where the current is bigger. The power is the same in every part.
Seen the cables glow? Now explain why.
The exam wants one chain of reasoning, written in the right order, and one equation. The notes below have both.
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Revision notes
What the National Grid is
The National Grid is a system of cables and transformers that links power stations to consumers (homes, schools and businesses).
| Part | What it does | Typical voltage |
|---|---|---|
| Power station | Generates the electricity | 25 000 V |
| Step-up transformer | Increases the voltage and decreases the current | 25 000 V → 400 000 V |
| Transmission cables | Carry the energy across the country on pylons | 400 000 V |
| Step-down transformer | Decreases the voltage and increases the current | 400 000 V → 230 V |
| Homes | Use the electricity | 230 V |
⭐ Why step the voltage up?
A transformer doesn't change the power (P = I V). So if the voltage goes up, the current goes down.
A lower current means less energy is wasted heating the cables and the surroundings. So transmitting at a high voltage is an efficient way to transfer energy.
⭐ Why step it down again?
400 000 V would be far too dangerous in a home. The step-down transformer lowers it to 230 V, which is safer, though still dangerous.
The transformer equation
=
Vp × Ip = Vs × Is
power in = power out, for a 100% efficient transformer
p = primary coil (in) · s = secondary coil (out)↕
Up one, down the other
If the voltage is multiplied by 16, the current is divided by 16.
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How to write it in the exam
The chain for "why does the grid use a high voltage?"
The step-up transformer increases the voltage,
so the current decreases (the power stays the same),
so less energy is wasted heating the cables, so the grid is more efficient.
voltagecurrentenergy wasted
Worked example · current in the cables · 2 marks
A power station sends 92 MW along cables at 400 000 V. Calculate the current.
- I = P ÷ V = 92 000 000 ÷ 400 000
- I = 230 A
Worked example · transformer · 3 marks
A step-up transformer: Vp = 25 000 V, Ip = 1600 A, Vs = 400 000 V. Calculate Is.
- Vp × Ip = Vs × Is
- 25 000 × 1600 = 400 000 × Is
- Is = 100 A
Check: voltage up 16 times, so current down 16 times: 1600 ÷ 16 = 100 A.
Exam-style question · explain · 3 marks
Explain why electricity is transmitted at a very high voltage.
- A high voltage means a low current for the same power (1)
- A low current means less energy is wasted heating the cables (1)
- So the National Grid is more efficient (1)
Where marks go missing: "to make it travel further". Always link high voltage → low current → less heating.
📘 Now do it in your workbook
Label the National Grid with its three voltages, work out the current at each part, complete the "why step up?" chain, then answer the exam-style question.
AQA GCSE Physics Workbook · ELECTRICITY · page 15The full workbook covers the whole of Topic 4.2 Electricity, with exam-style questions and worked answers.
✅ Can you do it? Tick as you go
0% complete🎉 Nice work! You've ticked off every objective for this spec point.
Don't forget to hit “Mark complete” at the bottom of the lesson.
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Prefer to watch? Here's the whole thing
Video walkthrough — coming soon
Everything above, explained out loud — useful for a last-minute recap, or if you'd rather hear it than read it.
Everything above, explained out loud — useful for a last-minute recap, or if you'd rather hear it than read it. The video is on its way.
🎬
Video coming soon
The walkthrough for this lesson is being filmed and will appear right here. Until then, the animation and the notes above cover every mark.
Now put it into practice
Page 15 of the workbook has the grid to label, the current calculations and the exam-style question, with full answers at the back.
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