OCR GCSE Physics A: Energy resources, trends in use and mains electricity (J249) – Revision Notes

Quick recall notes for OCR GCSE (9-1) Physics A (Gateway Science), J249, Version 5.0 (July 2026), statements P8.2a, P8.2b and P8.2g to P8.2j. All of these are assessed in both tiers. The study guide has the full explanations and the practice questions test them. See the course hub, the checklist and the free 10-minute diagnostics.

Energy sources (P8.2a)

Resource	Renewable?	Quick comparison point
Coal, oil, gas	No	Reliable on demand; burning releases carbon dioxide
Nuclear fuel	No	Reliable on demand; radioactive waste needs safe storage
Biofuel	Yes	Can be regrown; needs land and time
Wind	Yes	No fuel; output depends on wind
Hydroelectricity	Yes	Output easy to adjust; needs suitable valleys
Tides	Yes	Predictable; few suitable sites
The Sun	Yes	No fuel; output depends on daylight and cloud

Definitions to learn

• Renewable: replaced as fast as it is used, or never used up.
• Non-renewable: used faster than it forms, so it will run out.

Compare using three headings: reliability, cost and set-up, effect on the environment.

Steam route for fossil, nuclear and some biofuel stations: fuel energy heats water, steam turns a turbine, the turbine turns a generator.

Useful relationship: energy = power × time. For example 9 MW for 24 h gives 216 MWh.

Worked reminder: comparing two sources. Wind and gas for electricity.

• Reliability: gas can run whenever demand needs it. Wind output varies with the weather, so other sources must fill the gap when the wind is weak.
• Cost and set-up: wind needs no fuel once built. Gas needs a continuing fuel supply.
• Environment: gas burning releases carbon dioxide. Wind turbines do not, but they change the landscape.
• Conclusion: say which you would choose for a stated purpose and why, for example wind to cut carbon dioxide emissions, gas for output on demand.

Worked reminder: hydroelectricity. Water stored at height has gravitational potential energy. It runs down through turbines, which turn generators. Output can be adjusted quickly by changing how much water flows.

Trends in use (P8.2b)

Method in steps

1. Quote start and end values from the table or graph, with units.
2. Work out the change, then the percentage change: (change ÷ starting value) × 100.
3. Work out shares if totals are given: (source ÷ total) × 100.
4. Give a reason linked to the resource: it is non-renewable, it releases carbon dioxide, a cost has fallen, or investment has grown.
5. Say what the data cannot show.

Must-know distinction: a source can grow in output while its share falls, if the total grows faster. Percentage change and percentage-point change are different measures.

Small worked reminder. Gas generates 150 TWh in a year of 374 TWh in total. Share = 150 ÷ 374 × 100 = 40.1%. If it falls to 110 TWh in a year of 348 TWh, the share is 31.6%.

Worked reminder: percentage change. Wind output rises from 10 TWh to 100 TWh. Change = 90 TWh. Percentage change = 90 ÷ 10 × 100 = 900%. Coal falls from 120 TWh to 8 TWh. Change = 112 TWh. Percentage fall = 112 ÷ 120 × 100 = 93.3%.

Writing a trend answer. A strong answer follows this pattern: “Coal fell from 120 TWh to 8 TWh between 2005 and 2025, a fall of 93.3%. This is because coal is a non-renewable fossil fuel that releases carbon dioxide when burned, so the country replaced it with wind and solar.”

Mains supply (P8.2g, P8.2h)

Feature	UK domestic supply
Type	Alternating current (a.c.)
Frequency	50 Hz (50 cycles each second)
Potential difference	About 230 V
Period of one cycle	1 ÷ 50 = 0.02 s

	d.c.	a.c.
Direction of p.d.	One direction	Repeatedly reverses
Trace	Flat line	Repeating wave
Source	Cell, battery	Mains socket

Reading a trace in steps

1. Find the length of one complete wave in divisions.
2. Multiply that length by the time base setting; the result is the period T.
3. Use f = 1 ÷ T.
4. Check the unit: hertz for frequency, seconds for period.

Worked reminder: the 50 Hz supply. At 50 Hz the supply completes 50 cycles every second, so one cycle lasts 1 ÷ 50 = 0.02 s, and a minute holds 50 × 60 = 3000 cycles.

Worked reminder: why a d.c. cell and a.c. mains differ on a trace. A cell gives the same p.d. at every moment, so the trace is a flat horizontal line. Mains p.d. changes continually, crossing zero as it reverses, so the trace repeats as a wave.

Live, neutral and earth (P8.2i, P8.2j)

Wire	Colour	Function	P.d. to earth
Live	Brown	Carries the alternating p.d. to the appliance	About 230 V
Neutral	Blue	Completes the circuit	About 0 V
Earth	Green and yellow	Safety wire joined to the metal case	0 V

P.d. between wires: live to neutral about 230 V; live to earth about 230 V; neutral to earth about 0 V.

Dangers

• Touching the live wire puts a large p.d. across your body, so a current passes through you.
• A switch in the live wire isolates the appliance. A switch in the neutral wire leaves the appliance connected to the live wire when it is off, so the live wire can be dangerous even when a switch is open.
• Insulation around wires and the casing stops you touching live conductors. Damaged or wet insulation raises the risk.
• A connection between live and earth gives a large current, which can overheat wires and cause fire or shock.
• If a live wire touches a metal case that is not earthed, the case becomes live and the next person to touch it gives a path to earth.

Worked reminder: faulty case. A live wire comes loose and touches a metal case.

• With an earth wire: the case is joined to earth, so a large current flows through the earth wire, away from the user.
• Without an earth wire: the case is at about 230 V. A person who touches it provides a path to earth and receives a shock.

Plug wiring in order. The live wire is brown, the neutral is blue and the earth is green and yellow. The plug practical in the specification is wiring a plug.

Must-know distinctions

• Renewable is about replacement, not about cost or cleanliness.
• Neutral completes the circuit in normal use. Earth carries no current in normal use.
• Live to neutral is the working p.d. of about 230 V. Neutral to earth is about 0 V.
• Frequency counts cycles each second. Period is how long a single cycle lasts.

Quick self-test

1. State two renewable and two non-renewable energy resources.
2. Why is biofuel counted as renewable?
3. A source provides 60 TWh in a year when the total is 240 TWh. Find its share.
4. Output of a source rises from 20 TWh to 50 TWh. Find the percentage change.
5. State the frequency and approximate p.d. of the UK domestic supply.
6. One wave on a trace takes 5 divisions and the time base is 4 ms per division. Find the frequency.
7. How many mains cycles occur in 2 s?
8. Which wire is a safety wire connected to the metal case?
9. What is the approximate p.d. between neutral and earth?
10. Explain why a switch should be in the live wire.

Answers

1. Renewable: any two of biofuel, wind, hydroelectricity, tides, the Sun. Non-renewable: any two of coal, oil, gas, nuclear fuel.
2. It can be regrown, so it is replaced as fast as it is used.
3. 60 ÷ 240 × 100 = 25%.
4. Change = 30 TWh, so 30 ÷ 20 × 100 = 150%.
5. 50 Hz and about 230 V, alternating (a.c.).
6. T = 5 × 4 ms = 20 ms = 0.020 s, so f = 1 ÷ 0.020 = 50 Hz.
7. 50 × 2 = 100 cycles.
8. The earth wire.
9. About 0 V.
10. Opening a switch in the live wire disconnects the appliance from the 230 V supply, so its parts are not live. A switch in the neutral wire would leave them connected to the live wire.

Where marks are usually lost

• Writing “it will not run out” for renewable without saying it is replaced or never used up.
• Quoting a change in a graph without figures and units.
• Confusing percentage change (divide by the starting value) with share (divide by the total).
• Giving percentage change as 150 when it should be 150%.
• Writing “current” where “potential difference” is meant, for example for the 230 V of the live wire.
• Saying a.c. “has no direction”. Its direction changes repeatedly.
• Stating that the neutral wire is safe to touch. Say it is near 0 V in normal use only.
• Not linking the open switch to the live wire still being connected.
• Treating insulation as stopping current in a faulty circuit, rather than stopping contact with the conductor.

Official syllabus

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