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

This guide covers 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). It explains how electrical power travels from power stations to homes, why the potential difference is changed on the way, and why this wastes so little energy. Statement P8.2f is in bold in the specification, so it is tested only in the Higher Tier papers. Statements P8.2c, P8.2d and P8.2e can be tested in both tiers.

Course pages: the hub and the printable checklist. Companion pages: the revision notes and the practice questions. Check where you stand with the free 10-minute diagnostics. The coil-and-core detail of how a transformer works is on transformers, microphones and loudspeakers. Energy sources and the household supply are on energy resources, trends in use and mains electricity.

What this guide covers

Statement	What you must be able to do	Tier
P8.2c	Recall that the grid transfers power at high potential differences from power stations, then at lower potential differences in each locality for domestic use	Both tiers
P8.2d	Recall that step-up and step-down transformers change the potential difference as power is transferred from power stations	Both tiers
P8.2e	Explain how the national grid is an efficient way to transfer energy	Both tiers
P8.2f	Link the potential differences and numbers of turns of a transformer to the power transfer involved, and relate this to the advantages of transmission at high voltages	Higher Tier only

The specification lists one equation for this topic: the primary p.d. times the primary current equals the secondary p.d. times the secondary current, written Vp Ip = Vs Is. You also need power = potential difference × current (P = VI) and power = current² × resistance (P = I²R), which come from the electricity topic.

P8.2c: The route from power station to home

The national grid is the network of cables, pylons, underground lines and transformers that links power stations to the places that use electricity. Power is the energy transferred each second, in watts.

The key fact is that the potential difference changes along the route.

Stage	Potential difference	What happens
Power station generator	Moderate	Electrical power is produced
Step-up transformer	Raised to a very high value	Prepares the power for long-distance transfer
Main transmission lines	Very high, for example 400 kV	Power crosses the country
Step-down transformers	Lowered in stages	Power reaches towns and streets
Homes and businesses	About 230 V	Power is used safely

The examples of potential difference are illustrative. If a question gives different values, use those.

Many learners think the high potential difference is “pushed along” the cable and used up. It is not. The specification warns about this: high potential differences are applied across the power lines (between a line and the ground), not along them. The potential difference along a cable is only the small amount IR needed to push the current through the cable’s resistance. For a 400 kV line carrying 30 A through a total resistance of 8.0 Ω, the potential difference along the line is 30 × 8.0 = 240 V, a tiny fraction of 400 kV.

P8.2d: Step-up and step-down transformers

A transformer changes the size of an alternating potential difference. The grid uses two kinds.

• A step-up transformer increases the potential difference. It is used at the power station, before the power enters the long transmission lines.
• A step-down transformer decreases the potential difference. It is used at substations near towns and streets, so the potential difference is lowered before it reaches homes.

Transformers need an alternating current, because they work through a changing magnetic field. This is one reason the grid uses a.c. The mechanism is on the transformers page linked above. For P8.2d you only need to recall what each type does and where it sits.

Quick test: look at where the arrow of power is going. Leaving a power station, the potential difference goes up, so step-up. Arriving at a town, it goes down, so step-down. Far from the power station you may see several step-down stages in a row.

P8.2e: Why the grid is efficient

An efficient way to transfer energy wastes little of it. The energy wasted in the grid is mostly thermal energy in the cables, because any current in a wire heats it. This is dissipated to the surroundings and cannot be usefully recovered.

The argument has four steps, and good answers include all four.

1. The power transferred is P = VI. For a fixed power, a higher potential difference means a smaller current.
2. The power wasted as heat in a cable is P = I²R, where R is the resistance of the cable.
3. The wasted power depends on the square of the current. Cutting the current by a factor of 10 cuts the wasted power by a factor of 100.
4. So transmitting at very high potential difference wastes much less energy than transmitting the same power at low potential difference.

Worked example 1: the same power at two voltages

A power station sends 12 MW along a cable of total resistance 8.0 Ω. Compare the power wasted when the transmission potential difference is 40 kV and 400 kV.

At 40 kV:

I = P / V = 12 000 000 / 40 000 = 300 A
Power wasted = I²R = 300² × 8.0 = 720 000 W = 720 kW
Fraction wasted = 720 000 / 12 000 000 = 0.060 = 6.0%

At 400 kV:

I = 12 000 000 / 400 000 = 30 A
Power wasted = 30² × 8.0 = 7200 W = 7.2 kW
Fraction wasted = 7200 / 12 000 000 = 0.00060 = 0.060%

The potential difference rose by a factor of 10, the current fell by a factor of 10, and the wasted power fell by a factor of 100. At 400 kV about 99.94% of the power arrives, compared with 94% at 40 kV.

Why not simply use thicker cables to cut R instead? Thicker cables are heavier and more expensive, and need stronger pylons. Raising the potential difference with a transformer is a much cheaper way to cut the loss.

The grid is also efficient in a wider sense: one connected network lets power from many stations reach many places, so supply can be matched to demand. Keep your explanation focused on the heating loss, since that is what the specification asks for.

The model power line

The specification suggests a model power line to show energy losses at lower voltage and higher current. A typical set-up uses a low-voltage supply, long thin wires as the “cables” and a lamp as the “town”.

With the same power entering the line, a lower supply potential difference means a larger current, so the thin wires heat more and the lamp is dimmer. Using a step-up transformer at the start and a step-down transformer at the end makes the line current smaller and the lamp brighter. Here is a made-up set of results for 12 W entering a line of resistance 0.50 Ω.

Supply p.d. to line (V)	Line current (A)	Power wasted in line (W)	Percentage reaching the end
6	2.0	2.0	83%
12	1.0	0.50	96%
24	0.50	0.125	99%

Doubling the supply potential difference halves the current and cuts the wasted power to a quarter. That is the pattern to describe in a data question.

P8.2f: Turns, power and high-voltage advantages (Higher Tier only)

Two relationships from a transformer link the potential differences, the turns and the power. For primary coil values Vp, Ip, Np and secondary values Vs, Is, Ns:

Vp / Vs = Np / Ns        (turns equation)
Vp × Ip = Vs × Is        (power equation)

The second equation says the power into the primary equals the power out of the secondary. That is true for a transformer in which no energy is wasted. Real transformers are very efficient but not perfect, so the output power is slightly below the input power.

Combine the equations. If the secondary has k times as many turns as the primary, then:

• the secondary potential difference is k times the primary,
• the secondary current is 1/k times the primary current.

A transformer trades potential difference against current. It cannot create power, so a step-up transformer gives more potential difference and less current, and a step-down transformer gives less potential difference and more current.

Worked example 2: a step-up transformer at a power station

A transformer at a power station has 500 turns on the primary coil and 10 000 turns on the secondary coil. The primary potential difference is 20 kV and the power is 80 MW. Assume no energy is wasted. Calculate the secondary potential difference and both currents.

Vs = Vp × Ns / Np = 20 000 × 10 000 / 500 = 400 000 V = 400 kV
Ip = P / Vp = 80 000 000 / 20 000 = 4000 A
Is = P / Vs = 80 000 000 / 400 000 = 200 A

Check: the turns ratio is 20, so the potential difference rises by a factor of 20 (20 kV to 400 kV) and the current falls by a factor of 20 (4000 A to 200 A). Vp Ip and Vs Is both equal 80 MW.

Worked example 3: a step-down transformer in a street substation

A street transformer takes 11 kV on a primary coil of 4400 turns and delivers 230 V. The load is 55 kW and no energy is wasted. Calculate the turns on the secondary coil and both currents.

Ns = Np × Vs / Vp = 4400 × 230 / 11 000 = 92 turns
Ip = P / Vp = 55 000 / 11 000 = 5.0 A
Is = P / Vs = 55 000 / 230 = 239 A (3 s.f.)

The step-down transformer has far fewer secondary turns, a smaller potential difference and a much larger current. The thick cables near the substation carry these large currents over short distances, so the heating loss is small.

Relating this to the advantages of high voltage

The step-up transformer lets the station send out the same power as a small current. The small current means:

• much less energy dissipated as thermal energy in the transmission cables (I²R),
• more of the power reaching customers,
• thinner, lighter cables that can be supported on lighter pylons.

At the other end, step-down transformers lower the potential difference to a value suitable for homes. Say that to sum up the two jobs: step up for efficient transmission, step down for safe use.

A full Higher Tier answer links the chain: more secondary turns, so higher potential difference, so (for the same power) lower current, so less heating in the cables, so less energy wasted.

Common errors

• Saying the grid uses high voltage “to make the power stronger” or “to push the current further”. The purpose is a lower current.
• Saying the wasted power depends on the current rather than the current squared, and so missing the factor of 100.
• Calling the potential difference lost along the cable 400 kV. The loss along the cable is IR, which is small.
• Mixing up the two transformer types. Check the turns: more on the secondary means step-up.
• Claiming a step-up transformer gives more power. Power out cannot exceed power in.
• Writing that a transformer works on direct current.
• Forgetting units: 80 MW is 80 000 000 W and 20 kV is 20 000 V before you substitute.

Next steps

Try the practice questions, then use the revision notes for the final weeks. The checklist lets you tick off P8.2c to P8.2f.

Official syllabus

This page is 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.