OCR GCSE Physics A: The national grid and transformers (J249) – Revision Notes

These notes condense statements P8.2c to P8.2f of topic P8.2, Powering Earth, in the OCR GCSE (9-1) Gateway Science Physics A (J249) specification, Version 5.0 (July 2026). P8.2f is in bold in the specification, so it is Higher Tier only. P8.2c, P8.2d and P8.2e are for both tiers. For full explanations, use the study guide, and test yourself with the practice questions.

Course pages: the hub and the printable checklist. Not sure where to start? Try the free 10-minute diagnostics. Related: transformers, microphones and loudspeakers for how a transformer works, and dissipation, efficiency and reducing energy loss.

The route in one picture

Power station -> STEP-UP -> high-p.d. transmission lines -> STEP-DOWN -> homes
  (generator)     (raise p.d.)   (small current)           (lower p.d.)   (about 230 V)

Statement	Fact to recall
P8.2c	The grid transfers power at high potential differences from power stations, then at lower potential differences in each locality for domestic use
P8.2d	Step-up transformers raise the p.d. leaving the power station. Step-down transformers lower it near where the power is used
P8.2e	Explain why the grid is efficient: high p.d., so low current, so little heating in the cables
P8.2f (Higher Tier only)	Link p.d.s, turns and power to the advantages of high-voltage transmission

Key definitions

• National grid: the network of cables and transformers linking power stations to users.
• Step-up transformer: raises the potential difference. More turns on the secondary coil than the primary.
• Step-down transformer: lowers the potential difference. Fewer turns on the secondary coil than the primary.
• Power: energy transferred per second, in watts (W).
• Dissipated energy: energy spread to the surroundings as thermal energy, usually not useful.

Equations

Equation	Symbols and units	Used for
P = V I	Power (W), potential difference (V), current (A)	Finding the current in a line
P = I²R	Power wasted (W), current (A), resistance (Ω)	Heating loss in a cable
Vp / Vs = Np / Ns	Primary and secondary p.d.s, turns	Higher Tier only: output p.d. or turns
Vp Ip = Vs Is	p.d.s (V) and currents (A)	Currents, where no energy is wasted

The specification lists the power equation Vp Ip = Vs Is as a mathematical outcome for P8.2. Its link to the turns of a transformer is in the Higher Tier statement P8.2f.

Method: why high voltage wastes less energy (P8.2e)

1. Write down P = VI. The power is fixed, so raising V lowers I.
2. Write down P = I²R. The wasted power depends on the current squared.
3. Cut the current by a factor of n and the wasted power falls by a factor of n².
4. Conclude that high potential difference means less thermal energy in the cables, so the grid is efficient.

Worked reminder

6.0 MW is sent along a line of resistance 5.0 Ω. At 200 kV, the current is 6 000 000 / 200 000 = 30 A, and the wasted power is 30² × 5.0 = 4500 W. At 20 kV, the current is 300 A and the wasted power is 300² × 5.0 = 450 000 W. The potential difference is 10 times smaller, the current is 10 times larger and the wasted power is 100 times larger.

Method: transformer calculations (Higher Tier only, P8.2f)

1. Decide step-up or step-down by comparing the turns or the potential differences.
2. Use Vp / Vs = Np / Ns to find the missing potential difference or number of turns.
3. Find the power from P = VI on the side where you know both values.
4. Use Vp Ip = Vs Is, or P = VI again, to find the other current.
5. Check the direction: step-up means current goes down; step-down means current goes up.

	Step-up	Step-down
Turns	Ns > Np	Ns < Np
Potential difference	Vs > Vp	Vs < Vp
Current (no energy wasted)	Is < Ip	Is > Ip
Grid position	Power station end	Town and street end

Worked reminder (Higher Tier only)

A step-up transformer has 1200 turns on the primary coil. It takes 25 kV and delivers 275 kV, with 30 MW passing through and no energy wasted. The secondary turns are 1200 × 275 / 25 = 13 200. The primary current is 30 000 000 / 25 000 = 1200 A. The secondary current is 30 000 000 / 275 000 = 109 A (3 s.f.). The potential difference rises by a factor of 11 and the current falls by a factor of 11. Say this in words in an explanation: more turns on the secondary coil, so higher potential difference, so lower current for the same power.

Must-know distinctions

• Across, not along: 400 kV is the p.d. between the line and the ground. The p.d. along the cable is only I × R.
• Power versus potential difference: a transformer changes the potential difference. It does not increase power.
• Current versus potential difference: high potential difference in the line means low current in the line.
• Why step up: to transmit efficiently. Why step down: so the supply reaches homes at a safe, usable value.
• Real versus ideal: a real transformer wastes a little energy as thermal energy, so output power is slightly less than input power.

Model power line

The specification suggests a model power line to show the energy losses at lower voltage and higher current. Remember the pattern: for the same power entering the line, a lower supply p.d. gives a larger current, more heating in the thin wires and a dimmer lamp at the end. In a data table, if the supply p.d. doubles, the current halves and the wasted power falls to a quarter.

Quick self-test

1. What does a step-up transformer do to the potential difference?
2. Where in the grid are step-down transformers used?
3. Give the reason the grid transmits power at high potential difference.
4. A line carries 2.0 MW at 50 kV. Calculate the current.
5. The line has resistance 3.0 Ω. Calculate the power wasted as heat.
6. The same 2.0 MW is sent at 100 kV. Calculate the power wasted, and say how it compares with your answer to question 5.
7. (Higher Tier only) A transformer has a 12 kV primary on 600 turns and a 240 kV secondary. How many turns are on the secondary coil?
8. (Higher Tier only) The primary of that transformer carries 500 A at 12 kV. Assuming no energy is wasted, calculate the current in the secondary coil.
9. (Higher Tier only) A step-down transformer changes 132 kV to 33 kV. The secondary current is 60 A. Calculate the primary current, assuming no energy is wasted.
10. Explain why “400 kV is lost along the cable” is wrong.

Answers

1. It increases the potential difference.
2. Near towns, streets and homes, to lower the potential difference for local use.
3. High potential difference means low current for the same power, so there is less heating in the cables and less energy is wasted.
4. I = P / V = 2 000 000 / 50 000 = 40 A.
5. P = I²R = 40² × 3.0 = 4800 W.
6. I = 2 000 000 / 100 000 = 20 A, and P = 20² × 3.0 = 1200 W. The wasted power is a quarter of the earlier value, because the current has halved.
7. Ns = Np × Vs / Vp = 600 × 240 / 12 = 12 000 turns.
8. Power = 12 000 × 500 = 6.0 MW. Is = 6 000 000 / 240 000 = 25 A.
9. Vp Ip = Vs Is, so Ip = 33 000 × 60 / 132 000 = 15 A. (Step-down: current is smaller on the primary side.)
10. The 400 kV is the potential difference between the line and the ground. The potential difference along the cable is only the current times the cable resistance, which is small.

Where marks are usually lost

• Saying high voltage is used “to make electricity travel faster or further”, rather than to reduce the current.
• Stating that the current is reduced but not linking it to less thermal energy in the cables.
• Writing that wasted power is proportional to current instead of current squared, and so getting the factor wrong.
• Forgetting to convert MW and kV to W and V before using the equations.
• Turning the turns equation upside down and getting a step-up answer from a step-down transformer.
• Giving a step-up transformer’s current as larger than the primary current.
• Saying a transformer “creates” power, or that the output power is higher than the input power.
• Using the wrong tier language: if a question is Higher Tier only, it needs the link between turns, potential difference and power, not just a recall of what each transformer does.

Official syllabus

These notes are aligned to the OCR GCSE (9-1) Gateway Science Physics A (J249) specification, Version 5.0 (July 2026), for first assessment in 2018, published by OCR.