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

This guide covers part of P8.2 Powering Earth in the OCR GCSE (9-1) Physics A (Gateway Science) specification (J249, Version 5.0, July 2026, first assessment in 2018): statements P8.2a, P8.2b and P8.2g to P8.2j. None of these statements is in bold, so all of them can be tested in both the Foundation and Higher Tier papers. The national grid and transformers (P8.2c to P8.2f) are taught on separate pages.

Use it with the revision notes and the practice questions. The course hub is at OCR GCSE Physics, the printable list is the checklist, and the free 10-minute diagnostics show which topics to start with. For related forces and matter content, see OCR GCSE Physics: Forces and OCR GCSE Physics: Matter.

What this guide covers

Statement	What you must be able to do
P8.2a	Describe the main energy sources (fossil fuels, nuclear fuel, biofuel, wind, hydroelectricity, tides and the Sun), compare how they are used, and tell renewable from non-renewable
P8.2b	Explain patterns and trends in the use of energy resources, including the changing use of different resources over time
P8.2g	Recall that the UK domestic supply is a.c. at 50 Hz and about 230 volts
P8.2h	Explain the difference between direct and alternating voltage
P8.2i	Recall the functions of the live, neutral and earth wires, and the potential differences between them
P8.2j	Explain why a live wire may be dangerous even when a switch is open, and the dangers of any connection between live and earth; include the protection from insulation

P8.2a: The main energy sources

An energy resource is a store of energy we can use to generate electricity, heat buildings or power transport. Resources are sorted by whether they can run out on a human timescale.

• A non-renewable resource is used faster than it forms, so it will run out.
• A renewable resource is replaced as fast as we use it, or is never used up.

Resource	How the energy is released	Renewable?	Main points
Fossil fuels (coal, oil, natural gas)	Burned to heat water; the steam turns a turbine joined to a generator. Oil also fuels transport, and gas also heats homes	No	Reliable output on demand. Burning releases carbon dioxide
Nuclear fuel (uranium, plutonium)	Nuclear reactions heat water to make steam for a turbine	No	Reliable output, no carbon dioxide from the reaction itself, but radioactive waste must be stored safely
Biofuel	Plant or animal material burned, or turned into liquid or gas fuel. Used for transport and for power stations	Yes	Can be regrown, but needs land and takes time to grow
Wind	Moving air turns the blades of a turbine joined to a generator	Yes	No fuel needed. Output depends on the wind
Hydroelectricity	Water stored at height runs downhill through turbines	Yes	Output can be adjusted quickly. Needs suitable land, and valleys are flooded
Tides	Rising and falling sea water turns turbines	Yes	Predictable, but few suitable sites
The Sun	Solar cells give electricity directly. Solar panels (solar heating) heat water	Yes	Output depends on daylight and cloud

Comparing the ways resources are used

Learn to compare, not just list. Three useful contrasts:

1. Reliability. Fossil, nuclear and biofuel stations can run whenever demand needs them. Wind, solar and tidal output follows the weather or the tide. Tides are predictable even though they vary.
2. Running cost and set-up. Wind and solar need no fuel once built. Fossil and nuclear stations need a constant fuel supply.
3. Environmental effect. Burning fossil fuels and biofuels releases carbon dioxide. Hydroelectric schemes change river valleys. Wind turbines change the look of the landscape.

Worked example 1. A wind farm has 12 turbines, each rated at 2.5 MW. Over one day the farm delivers 30% of its maximum possible output on average. Find the energy delivered in the day.

Maximum power = 12 × 2.5 MW = 30 MW. Average power = 0.30 × 30 MW = 9 MW. Energy = power × time = 9 MW × 24 h = 216 MWh. At full output it would have given 30 MW × 24 h = 720 MWh, so the farm needs other sources to fill the gap when the wind is weak.

P8.2b: Patterns and trends in use

You may be given a table or graph and asked what has changed and why. Work through it in a fixed order.

1. Describe the trend with figures. Quote start and end values.
2. Compare totals and shares. A resource can grow in output while its share of the total falls, if the total grows faster.
3. Give a reason. Common reasons are: fossil fuels are non-renewable and release carbon dioxide; governments and energy companies have invested in renewable resources; costs of wind and solar have fallen; demand has changed.
4. Note limits. Data from one country cannot prove a worldwide pattern.

The table below is made-up data for one country, in terawatt-hours (TWh) of electricity generated.

Source	2005	2015	2025
Coal	120	60	8
Gas	150	140	110
Nuclear	80	70	65
Wind	10	45	100
Solar	1	12	35
Biofuel	5	20	22
Hydroelectricity	8	8	8
Total	374	355	348

Worked example 2. Describe and explain the trend in coal and wind from 2005 to 2025, using the data.

Coal fell from 120 TWh to 8 TWh, a fall of 112 TWh. As a percentage of its 2005 value that is (112 ÷ 120) × 100 = 93.3%. Wind rose from 10 TWh to 100 TWh. The change is 90 TWh, and (90 ÷ 10) × 100 = 900%, so output rose by nine times its starting value. Shares of the total: coal went from 120 ÷ 374 = 32.1% to 8 ÷ 348 = 2.3%. Wind went from 2.7% to 28.7%. A reason: coal is a non-renewable fossil fuel and releases carbon dioxide when burned, so the country has replaced it with renewable resources.

Worked example 3. Did the use of renewable resources overtake fossil fuels?

Renewable (wind, solar, biofuel, hydroelectricity) in 2025: 100 + 35 + 22 + 8 = 165 TWh, which is 165 ÷ 348 = 47.4% of the total. Fossil fuels (coal and gas): 8 + 110 = 118 TWh, which is 118 ÷ 348 = 33.9%. Renewable output was greater than fossil-fuel output in 2025. In 2005 it was 24 TWh (6.4%) against 270 TWh (72.2%). Nuclear (18.7%) is neither group.

If you are asked about a long-term trend, say what changed and give a reason linked to the resource’s properties, such as supply, cost or pollution.

P8.2g: The UK domestic supply

The domestic mains supply in the UK is alternating current (a.c.) at 50 Hz and about 230 volts.

• 50 Hz means 50 complete cycles every second.
• The period of one cycle is T = 1 ÷ f = 1 ÷ 50 = 0.02 s (20 ms).
• In 3 s there are 50 × 3 = 150 cycles.

P8.2h: Direct and alternating voltage

	Direct (d.c.)	Alternating (a.c.)
Potential difference	Stays in one direction	Reverses direction repeatedly
Trace on a data logger or oscilloscope	A flat horizontal line	A repeating wave that crosses zero
Example source	Cell or battery	UK mains socket
Charge flow	In one direction around the circuit	Charge carriers move back and forth

The potential difference of a cell is steady, so the trace is flat. A mains p.d. rises, falls through zero, reverses and returns, 50 times a second.

Worked example 4. A data logger records the mains supply. The time base is 10 ms per division and one complete wave takes up 2 divisions. Find the frequency.

Period = 2 × 10 ms = 20 ms = 0.020 s. Frequency = 1 ÷ 0.020 = 50 Hz, which matches the UK domestic supply.

P8.2i: Live, neutral and earth

A three-pin plug has three wires. Each has a different job.

Wire	Colour	Function	P.d. to earth
Live	Brown	Carries the alternating p.d. from the supply to the appliance	About 230 V
Neutral	Blue	Completes the circuit by returning current to the supply	About 0 V
Earth	Green and yellow	A safety wire. It is connected to the metal case and does not carry a current in normal use	0 V

So the p.d. between live and neutral is about 230 V, the p.d. between live and earth is about 230 V, and the p.d. between neutral and earth is about 0 V.

In normal use, current flows in through the live wire, through the appliance, and out through the neutral.

P8.2j: Why the live wire is dangerous

Your body is at about the same potential as earth because you stand on the ground. If you touch a wire at about 230 V, there is a large p.d. across your body and a current passes through you. This can cause a severe shock.

The switch and the live wire

A switch should open the live wire. If the switch is placed in the neutral wire, opening it stops the current, but the appliance stays connected to the live wire at 230 V. A person who touches a live part inside the device, even when it is switched off, completes a path to earth and is shocked. A switch in the live wire makes everything beyond it safe.

In general, parts on the supply side of an open switch stay at the live p.d., so they can be dangerous.

Insulation

The wires, the plug and the device are covered in insulation such as plastic. Insulation stops a person touching the live conductor and stops it touching other conductors. Damaged, frayed or wet insulation makes a shock more likely. Water is a conductor, so wet hands increase the risk.

Live to earth

Any connection from live to earth gives a low-resistance path and a large current. This can happen if a frayed live wire touches a metal case that is earthed, or if a person provides the path. The danger is a sudden heavy current that overheats wires, can start fires and gives severe shocks. If the case is not earthed, the metal case itself becomes live at 230 V, and the next person to touch it provides the connection to earth.

Worked example 5. A faulty toaster has a loose live wire touching its metal case. The case is joined to earth through the earth wire. Explain what happens.

The live wire is at about 230 V and the case is connected to earth at 0 V, so there is a large p.d. across a very small resistance, which gives a large current. (For example, 230 V across 46 Ω would be 5 A, and a lower resistance gives much more.) The large current flows through the earth wire rather than through the user. This protects the user. The same large current is what overheats wires, so it is dangerous if nothing stops it.

Common errors

• Writing that a resource is renewable because it is “free”. The test is whether it is replaced as fast as it is used.
• Calling biofuel non-renewable because it is burned. It can be regrown.
• Mixing up percentage change with a change in share. A rise of 10 TWh and a rise of 10 percentage points are different things.
• Saying the earth wire carries the current “back to the supply” in normal use. That is the neutral wire.
• Saying a switch in the neutral wire is safe. The appliance stays live.

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: statements P8.2a, P8.2b, P8.2g, P8.2h, P8.2i and P8.2j.