Edexcel IGCSE Physics 4(d) Energy Resources and Electricity Generation — Notes
Written for Pearson Edexcel International GCSE Physics (4PH1), linear and modular. This sub-topic is not part of Science Double Award (4SD0) or Science Single Award (4SS0).
Which parts do I need?
The whole page is Physics only and is examined on Paper 2P. Single Award and Double Award candidates can skip it entirely. Some worked examples reuse GPE, KE and efficiency from 4(b) and 4(c).
What this page covers
| Spec | Modular no. | You need to be able to… | Route |
|---|---|---|---|
| 4.18P | 3.18P | describe the chain of energy transfers when electricity is generated from wind, water, geothermal resources, solar heating, solar cells, fossil fuels and nuclear fuel | Physics only · 2P |
| 4.19P | 3.19P | give the benefits and drawbacks of generating electricity on a large scale from renewable and non-renewable resources | Physics only · 2P |
Modular candidates: Topic 4 is Topic 3 in the modular specification, so these are statements 3.18P and 3.19P of 4XPH1 Unit 1 (4WPH1). They are not in 4XSD1.
In one minute
- Most power stations heat water to make steam, which spins a turbine connected to a generator. The heat source can be a fossil fuel, nuclear fuel, hot rocks (geothermal) or concentrated sunlight (solar heating).
- Wind and water turn the turbine directly using the kinetic store of moving air or water — no steam is needed.
- Solar cells transfer the energy carried by light from the Sun electrically, with no moving parts and no turbine.
- Renewable resources (wind, water, geothermal, solar) will not run out; non-renewable ones (fossil fuels, nuclear fuel) are finite.
- Every method has trade-offs: reliability, greenhouse gases and other pollution, land use and location, start-up time, and cost per unit of electricity.
How does a thermal power station generate electricity? Physics only · 2P
Fossil fuel, nuclear, geothermal and solar heating power stations differ only in how they heat the water. After that, the chain is the same.
- The heat source transfers energy by heating to water in the boiler, which becomes high-pressure steam (thermal store of the steam).
- The steam pushes on the turbine blades, transferring energy mechanically to the kinetic store of the turbine.
- The turbine turns the generator, which transfers the energy electrically to the grid.
- The steam is cooled and condensed back to water to be reused. Much of the input energy is transferred to the surroundings here, by heating the cooling water and the air.
What energy transfers happen with each energy resource? Physics only · 2P
| Resource | Energy transfers to electricity |
|---|---|
| fossil fuels (coal, oil, gas) | chemical store of the fuel → burning heats water (thermal store of the steam) → kinetic store of the turbine → generator → electrically |
| nuclear | nuclear store of the uranium → fission in the reactor heats water (thermal store of the steam) → kinetic store of the turbine → generator → electrically |
| geothermal | thermal store of hot rocks underground → water pumped down is heated to steam → kinetic store of the turbine → generator → electrically |
| solar heating systems | radiation (light and infrared) from the Sun is focused by curved mirrors onto a receiver → thermal store of a fluid that boils water → kinetic store of the turbine → generator → electrically |
| solar cells | radiation (light) from the Sun → solar cell → electrically, directly (no turbine or generator) |
| wind | kinetic store of the moving air → kinetic store of the turbine blades → generator → electrically |
| water: hydroelectric | gravitational store of water in a high reservoir → kinetic store of the falling water → kinetic store of the turbine → generator → electrically |
| water: tides and waves | kinetic store of moving sea water (tides rise and fall twice a day; waves are driven by wind) → kinetic store of a turbine → generator → electrically |
At every stage some energy is wasted, mostly to the thermal store of the surroundings (friction in the turbine and generator, hot gases, warm cooling water) and some as sound.
Describe the energy transfers that take place when electricity is generated in a nuclear power station.
Energy in the nuclear store of the uranium fuel is released by fission in the reactor.
It is transferred by heating to water, producing high-pressure steam.
The steam drives the turbine: energy is transferred mechanically to the kinetic store of the turbine.
The turbine turns the generator, which transfers the energy electrically. Energy is wasted to the thermal store of the surroundings, mainly through the cooling water.
How efficient are power stations? Physics only · 2P
Efficiency and Sankey diagrams work exactly as in 4(b). For a power station it is usual to give energies per second (in MW), which works the same way because every value is divided by the same time.
A coal-fired power station receives 2500 MJ from the chemical store of its fuel every second. Each second it supplies 900 MJ electrically and transfers 250 MJ to the surroundings in hot flue gases; the rest goes into the cooling water. (a) Calculate the energy transferred to the cooling water each second. (b) Calculate the efficiency of the station.
(a) By conservation of energy: 2500 − 900 − 250 = 1350 MJ each second.
(b) efficiency = \(\dfrac{900}{2500}\times100\%=\) 36%
The largest waste is to the cooling water, which is why the Sankey diagram’s widest wasted arrow goes there.
How do you calculate the energy from wind and falling water? Physics only · 2P
For hydroelectric power, the energy each second comes from the GPE lost by the water: GPE = m × g × h, where m is the mass of water passing through the turbine each second. For wind, the energy comes from the kinetic store of the air: KE = ½ × m × v². Pearson papers tell you to assume \(g=10\ \text{N/kg}\) (9.8 N/kg is also accepted).
Each second, 200 000 kg of water falls through a height of 50 m from a reservoir to the turbines of a hydroelectric station. The station is 85% efficient. Calculate (a) the GPE lost by the water each second and (b) the electrical power output.
(a) GPE = \(m\times g\times h=200\,000\times10\times50=\) 100 000 000 J each second (100 MW).
(b) useful output = 0.85 × 100 MW = 85 MW
Each second, 9600 kg of air moving at 12 m/s passes through the blades of a wind turbine. The turbine supplies 240 000 J electrically each second. Calculate the efficiency of the turbine.
KE of the air each second = \(\tfrac12\times9600\times12^2=\tfrac12\times9600\times144\) = 691 200 J
efficiency = \(\dfrac{240\,000}{691\,200}\times100\%=34.7\%=\) 35%
A turbine cannot transfer all the kinetic energy of the wind: the air must still be moving after it passes the blades.
What are the advantages and disadvantages of each method? Physics only · 2P
| Resource | Advantages | Disadvantages |
|---|---|---|
| fossil fuels (non-renewable) | reliable, whatever the weather; output can be increased quickly to meet demand (especially gas); one station gives a large output from a small site | burning releases carbon dioxide, a greenhouse gas that contributes to climate change; coal and oil release sulfur dioxide (acid rain); supplies will run out |
| nuclear (non-renewable) | no carbon dioxide while operating; a small mass of fuel releases a very large amount of energy; reliable, constant output | produces radioactive waste that must be stored safely for a very long time; risk of a serious accident; high building and decommissioning costs; slow to start up |
| wind (renewable) | no fuel cost; no greenhouse gases while operating | unreliable — no output in calm weather or in very strong wind; many turbines and a large area needed for the output of one power station; noise near homes |
| hydroelectric (renewable) | reliable; can start up very quickly to meet sudden demand; no greenhouse gases while operating | needs a suitable valley and large rainfall; flooding the valley destroys habitats and may displace people |
| tides and waves (renewable) | tides are predictable; no fuel cost; no greenhouse gases while operating | only a few suitable coastal sites; tidal barrages affect estuary habitats; wave output depends on the weather |
| geothermal (renewable) | reliable, works day and night; small land area | only possible where hot rocks are near the surface (volcanic regions); some gases, including carbon dioxide and hydrogen sulfide, can be released from underground |
| solar cells and solar heating (renewable) | no fuel cost; no greenhouse gases while operating; cells can supply remote places | no output at night and less in cloudy weather; large areas needed for large-scale generation; solar heating needs strong, direct sunlight |
When you compare methods in an answer, make each point specific and say which resource it applies to. Useful headings: reliability and ability to meet demand, effect on the environment, availability of suitable sites, and running cost.
A small island has strong, steady winds, frequent cloud cover and an active volcano. All its electricity currently comes from an oil-fired power station. Evaluate replacing the oil-fired station with wind turbines or a geothermal power station.
Wind: the island has strong, steady winds, so output would be good; there is no fuel to import and no carbon dioxide while operating. But on calm days there is no output, so a back-up supply is still needed, and many turbines are needed for a large output.
Geothermal: the active volcano means hot rocks are close to the surface, so the site is suitable. Geothermal gives a reliable output day and night and needs little land. Some gases from underground may be released.
Conclusion: geothermal is the better replacement for a reliable supply because it does not depend on the weather; wind turbines could be added to reduce costs further. (Solar cells are a poor choice here because of the frequent cloud.)
An “evaluate” answer needs points on both sides and a justified conclusion.
Exam tips for 4(d)
If a question asks about two resources, use a separate heading or bullet list for each. Points that could apply to either resource cannot be credited to one of them.
If the question already tells you both resources are renewable, “it is renewable” is not a new advantage. Look for a different point: reliability, emissions, land area or location.
“Bad for the environment” or “expensive” on its own earns little. Say what the effect is (“releases carbon dioxide, a greenhouse gas”) or what the cost is (“high building cost but no fuel cost”).
Geothermal energy comes from hot rocks underground. It does not burn any fuel, it is renewable, and it can be built only in certain places. Do not confuse it with burning biofuels or fossil fuels.
For an energy-transfer description, start with the correct store (chemical, nuclear, thermal, gravitational or kinetic) — or, for solar, energy transferred by radiation from the Sun — and end with “transferred electrically by the generator”.
Common misconceptions
| Misconception | Why it is wrong | Correct idea |
|---|---|---|
| Solar cells and solar heating systems are the same. | They use different transfers. | Solar cells transfer light electrically directly; solar heating boils water to drive a turbine. |
| Nuclear power stations release carbon dioxide from burning uranium. | Uranium is not burned. | Energy is released by nuclear fission; no carbon dioxide is produced while operating. |
| Geothermal is non-renewable because the heat will run out. | The rocks are kept hot by processes deep in the Earth. | Geothermal is renewable. |
| Renewable resources cause no environmental effects. | Building and siting them affects land, habitats and people. | They release no greenhouse gases while operating, but still have other impacts. |
| A wind turbine transfers all of the wind’s kinetic energy. | The air must keep moving past the blades. | Only part of the wind’s kinetic energy is transferred. |
Key facts
| Resource | Renewable? | Energy comes from | Uses a turbine? | Greenhouse gases while operating? |
|---|---|---|---|---|
| fossil fuels | no | chemical store of the fuel | yes (steam) | yes |
| nuclear | no | nuclear store of the fuel | yes (steam) | no |
| geothermal | yes | thermal store of hot rocks | yes (steam) | small amounts from underground |
| solar heating | yes | radiation from the Sun | yes (steam) | no |
| solar cells | yes | radiation from the Sun | no | no |
| wind | yes | kinetic store of the air | yes (wind) | no |
| hydroelectric | yes | gravitational store of the water | yes (water) | no |
| tides and waves | yes | kinetic store of the water | yes (water) | no |
No new equations: calculations on this page use efficiency (4(b)), GPE and KE (4(c)), all on the Equation List.
Check your understanding
1. State the energy transfers in a solar cell. Physics only · 2P
Energy carried by light (radiation) from the Sun is transferred electrically by the cell; some is wasted by heating the cell.
2. Describe the energy transfers in a gas-fired power station. Physics only · 2P
Chemical store of the gas → burning heats water to steam (thermal store) → steam turns the turbine (kinetic store) → generator → electrically. Wasted energy heats the surroundings.
3. Each second, 150 kg of water falls 80 m to a small hydroelectric turbine that is 90% efficient. Calculate the electrical power output. (g = 10 N/kg) Physics only · 2P
GPE lost each second = 150 × 10 × 80 = 120 000 J; output = 0.90 × 120 000 = 108 000 W (108 kW).
4. A power station has an efficiency of 40%. It supplies 1200 MW electrically when its fuel supplies 3000 MW. Show that these values are consistent. Physics only · 2P
0.40 × 3000 = 1200 MW ✓
5. Give one advantage and one disadvantage of nuclear power compared with coal. Physics only · 2P
Advantage: no carbon dioxide (greenhouse gas) released while operating. Disadvantage: produces radioactive waste that must be stored safely for a very long time.
6. Explain why a country cannot rely on wind turbines alone for its electricity. Physics only · 2P
Wind speed varies, so output is unreliable: there is no output on calm days, and demand cannot be met at all times. A reliable back-up (for example hydroelectric or gas) is needed.
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