What you'll learn
This revision guide covers energy resources and energy transfer as examined in Pearson Edexcel International IGCSE Physics. You will master the differences between renewable and non-renewable energy sources, understand how electricity is generated from various resources, calculate efficiency and power, and analyse energy transfer processes. These topics are essential for both Paper 1 and Paper 2 examinations.
Key terms and definitions
Renewable energy source — an energy resource that is naturally replenished and will not run out, such as solar, wind, hydroelectric, tidal, wave, geothermal and biofuels
Non-renewable energy source — an energy resource that will eventually be depleted and cannot be replenished on a human timescale, including fossil fuels (coal, oil, natural gas) and nuclear fuel (uranium, plutonium)
Efficiency — the ratio of useful energy output to total energy input, expressed as a percentage or decimal; efficiency = (useful energy output ÷ total energy input) × 100%
Sankey diagram — a visual representation showing energy transfers where the width of each arrow is proportional to the amount of energy it represents
Power — the rate of energy transfer or the rate of doing work, measured in watts (W); power = energy transferred ÷ time taken
Generator — a device that converts kinetic energy into electrical energy using electromagnetic induction
National Grid — the network of cables and transformers that distributes electricity from power stations to consumers across a country
Fossil fuels — coal, oil and natural gas formed from the remains of dead organisms over millions of years; burned to release chemical energy stored within them
Core concepts
Non-renewable energy resources
Non-renewable resources provide the majority of global energy supply but have significant environmental and sustainability concerns.
Fossil fuels (coal, oil, natural gas):
- Formed over millions of years from compressed plant and animal remains
- Burned in power stations to heat water, producing steam that drives turbines
- Turbines rotate generators to produce electricity
- Release carbon dioxide (contributing to climate change) and sulfur dioxide (causing acid rain)
- Relatively cheap and reliable but supplies are finite
- High energy density makes them efficient for transport and storage
Nuclear fuel (uranium-235, plutonium-239):
- Nuclear fission occurs when nuclei split, releasing large amounts of energy
- Heat produced converts water to steam, driving turbines and generators
- No greenhouse gas emissions during operation
- Produces radioactive waste requiring safe disposal for thousands of years
- Risk of nuclear accidents (though extremely low with modern safety systems)
- High construction and decommissioning costs
- Very high energy density — small amounts of fuel produce enormous energy
Renewable energy resources
Renewable resources are increasingly important for sustainable energy supply, though each has advantages and disadvantages.
Solar energy:
- Solar panels (photovoltaic cells) convert light energy directly to electrical energy
- Solar heating panels use sunlight to heat water directly
- No fuel costs and no pollution during operation
- Unreliable — depends on weather and time of day
- Manufacturing panels requires energy and raw materials
- Suitable for individual homes and large-scale solar farms
Wind energy:
- Wind turbines convert kinetic energy of moving air into electrical energy
- Blades rotate due to wind, turning generators
- No fuel costs, no pollution, relatively cheap to maintain
- Unreliable — depends on wind speed
- Visual and noise pollution concerns
- Can harm bird populations
- Requires large land areas for wind farms
Hydroelectric power:
- Water stored in elevated reservoirs flows downhill through turbines
- Gravitational potential energy converts to kinetic energy then electrical energy
- Reliable and can respond quickly to demand changes
- Can be used for energy storage (pumped storage schemes)
- Requires flooding of valleys (habitat destruction, population displacement)
- High initial construction costs
- Limited suitable locations
Tidal energy:
- Tidal barrages trap water at high tide, releasing it through turbines at low tide
- Predictable and reliable (tides follow regular patterns)
- No fuel costs, no pollution
- Very high construction costs
- Limited suitable locations (requires large tidal range)
- Environmental impact on estuaries and marine ecosystems
Wave energy:
- Floating generators move with wave motion, driving turbines
- No pollution, no fuel costs
- Unreliable — wave height varies
- Technology still developing; high maintenance costs
- Can withstand harsh ocean conditions requires robust engineering
Geothermal energy:
- Hot rocks underground heat water to produce steam
- Steam drives turbines connected to generators
- Reliable in volcanically active regions
- No fuel costs, minimal environmental impact
- Very limited suitable locations
- High drilling costs
Biofuels:
- Organic matter (wood, crop waste, animal dung, specially grown crops) burned to release energy
- Considered carbon-neutral (carbon dioxide released was recently absorbed by plants)
- Can be used in existing power stations with modifications
- Growing fuel crops may compete with food production
- Still produces air pollution when burned
- Requires large land areas
Energy efficiency and calculations
Efficiency measures how effectively energy is converted from one form to another. No energy transfer is 100% efficient; some energy is always wasted, usually as heat.
Efficiency formula:
Efficiency = useful energy output / total energy input × 100%
Or using power:
Efficiency = useful power output / total power input × 100%
Improving efficiency:
- Lubrication reduces friction (less energy wasted as heat)
- Insulation reduces unwanted heat transfer
- Streamlining reduces air resistance
- Using more efficient components (LED bulbs instead of filament bulbs)
Sankey diagrams show energy transfers clearly:
- Input arrow on the left represents total energy input
- Useful energy output arrow continues straight
- Wasted energy arrows branch off (usually downward)
- Arrow width proportional to energy quantity
- Total input width equals total output widths
Power calculations and energy transfer
Power is the rate of energy transfer, linking energy, power and time.
Key equations:
Power = energy transferred ÷ time
P = E/t
Where:
- P = power (watts, W)
- E = energy (joules, J)
- t = time (seconds, s)
Rearranging: E = P × t
Common units:
- 1 kilowatt (kW) = 1000 W
- 1 megawatt (MW) = 1 000 000 W
- 1 kilowatt-hour (kWh) = energy used by 1 kW appliance in 1 hour = 3 600 000 J
The kilowatt-hour is used by electricity companies for billing as it represents practical amounts of domestic energy use.
Electricity generation and distribution
Power station operation:
- Energy source (fuel or renewable) heats water or drives turbines directly
- Steam or moving water rotates turbine blades
- Turbine shaft rotates coils in a magnetic field (generator)
- Electromagnetic induction produces alternating current
- Transformer increases voltage for transmission
The National Grid:
- High voltage transmission (400 000 V or 275 000 V) reduces current
- Lower current means less energy wasted as heat in cables (P = I²R)
- Step-up transformers at power stations increase voltage
- Step-down transformers near homes reduce voltage to safe levels (230 V in UK)
- Allows electricity to be distributed efficiently across large distances
- Connects multiple power stations for reliability
Base load and peak demand:
- Base load: constant minimum demand met by nuclear and coal stations (slow to start/stop)
- Peak demand: additional demand during high-use periods met by gas or hydroelectric (quick response)
Environmental and economic considerations
Comparing energy resources:
Environmental impact:
- Greenhouse gas emissions (fossil fuels high, renewables/nuclear low)
- Habitat destruction (hydroelectric, wind farms, biofuel crops)
- Waste products (nuclear waste, ash from coal)
- Visual and noise pollution (wind turbines)
Economic factors:
- Start-up costs (nuclear and hydroelectric very high, wind and solar moderate)
- Running costs (renewables very low, fossil fuels ongoing fuel costs)
- Decommissioning costs (nuclear very high)
- Reliability and availability (fossil fuels and nuclear reliable, most renewables intermittent)
Future trends:
- Increasing renewable energy use to reduce carbon emissions
- Development of energy storage technologies (batteries, pumped storage)
- Improving efficiency of energy conversion and use
- Diversifying energy mix for security of supply
Worked examples
Example 1: Efficiency calculation
Question: A petrol engine has a total energy input of 5000 J. It produces 1200 J of useful kinetic energy. The rest is wasted as heat and sound. Calculate the efficiency of the engine. (3 marks)
Solution:
- Total energy input = 5000 J ✓
- Useful energy output = 1200 J ✓
- Efficiency = (1200 ÷ 5000) × 100% = 24% ✓
Mark scheme notes: 1 mark for correct substitution into efficiency formula, 1 mark for correct calculation, 1 mark for answer with unit (%). Accept 0.24 if answer given as a decimal.
Example 2: Power and energy
Question: A 2.5 kW kettle is used to boil water for 4 minutes. Calculate the energy transferred in joules. (3 marks)
Solution:
- Convert power to watts: 2.5 kW = 2500 W ✓
- Convert time to seconds: 4 minutes = 240 s ✓
- Energy = power × time = 2500 × 240 = 600 000 J (or 600 kJ) ✓
Mark scheme notes: 1 mark for each correct conversion, 1 mark for correct answer. Accept answer in kJ if clearly stated. Deduct 1 mark if time not converted to seconds and incorrect answer given.
Example 3: Sankey diagram interpretation
Question: The Sankey diagram shows the energy transfers in a light bulb with 100 J input energy. 75 J is wasted as heat and 25 J is useful light energy.
(a) Calculate the efficiency of the bulb. (2 marks) (b) Explain how the diagram would differ for an LED bulb with 90% efficiency using the same energy input. (2 marks)
Solution: (a) Efficiency = (25 ÷ 100) × 100% = 25% ✓✓
(b) The useful light energy arrow would be much wider (90 J instead of 25 J) ✓, and the wasted heat arrow would be much narrower (10 J instead of 75 J) ✓.
Mark scheme notes: Part (a): 2 marks for correct answer with working. Part (b): 1 mark for identifying wider useful arrow, 1 mark for identifying narrower waste arrow (accept quantitative values).
Common mistakes and how to avoid them
Confusing renewable with 'clean' energy — renewable energy sources still have environmental impacts (e.g., hydroelectric flooding, manufacturing solar panels). Renewable means naturally replenished, not zero environmental impact.
Forgetting unit conversions — always convert kW to W (multiply by 1000), kJ to J (multiply by 1000), and minutes to seconds (multiply by 60) before substituting into formulas. Check the units required in the question.
Stating nuclear power is renewable — nuclear fuel (uranium) is finite and non-renewable, despite producing no carbon emissions during operation. Don't confuse 'low carbon' with 'renewable'.
Calculating efficiency greater than 100% — if you get efficiency > 100%, you've made an error. Useful energy output cannot exceed total energy input. Check which value is larger and ensure useful output is the numerator.
Misinterpreting Sankey diagrams — arrow width represents energy quantity, not importance. The total width of output arrows must equal the input arrow width (conservation of energy).
Mixing up energy and power — energy is measured in joules (J), power in watts (W). Power is the rate of energy transfer. Don't use power values when energy values are needed, or vice versa.
Exam technique for Energy Resources and Energy Transfer
Command word 'compare' requires you to state similarities AND differences between energy sources. Writing only advantages or only disadvantages of one source will not gain full marks. Use comparative language: "whereas", "however", "both".
'Describe and explain' questions about power stations need a clear sequence: energy source → heat/movement → turbine rotation → generator → electromagnetic induction → electricity. Explaining how the generator produces electricity (electromagnetic induction) is often required for full marks.
Calculations must show working — even if you can do the calculation mentally, write out the formula, substitution with units, and final answer with correct unit. This allows partial credit if your final answer is incorrect.
Extended response questions on environmental/economic factors require balanced arguments. Mention both advantages and disadvantages, and consider different stakeholder perspectives (cost, reliability, environmental impact, location constraints). Aim for 4-6 distinct points for 6-mark questions.
Quick revision summary
Energy resources divide into renewable (solar, wind, hydroelectric, tidal, wave, geothermal, biofuels) and non-renewable (fossil fuels, nuclear). Power stations convert various energy forms to electrical energy via turbines and generators. The National Grid uses high voltage transmission to reduce energy losses. Efficiency = (useful energy output ÷ total energy input) × 100%, never exceeding 100%. Power = energy ÷ time, with units watts = joules ÷ seconds. All energy resources have environmental and economic advantages and disadvantages requiring evaluation.