At GCSE
At GCSE you calculate kinetic energy (½mv²), gravitational potential energy (mgh), work done (Fd) and efficiency, and compare renewable vs non-renewable energy resources with their environmental impact.
GCSEA-LevelAQAEdexcelOCR
Energy revision, GCSE & A-Level Physics
Energy is the topic where Physics most clearly meets everyday life, kinetic and gravitational potential energy, efficiency calculations, and the trade-offs between energy resources. UK exams reliably ask for definitions and worked calculations.
At A-Level the energy stores model is extended with work done, power as the rate of doing work, and efficiency for real devices. Linking changes in store to forces and motion answers consistently scores in the top band.
Energy stores
Energy is stored in different ways, called energy stores. The eight stores you need at GCSE are: kinetic, gravitational potential, elastic, chemical, thermal, nuclear, magnetic and electrostatic. Energy can be transferred between stores by forces (mechanically), by heating, by radiation, or by electrical current.
The key idea is that energy is never created or destroyed, it is only transferred from one store to another. A moving car has kinetic energy; when it brakes, that kinetic energy is transferred to the thermal store of the brakes and surroundings.
- Eight stores: kinetic, gravitational potential, elastic, chemical, thermal, nuclear, magnetic, electrostatic.
- Transferred by: forces (mechanically), heating, radiation (including light), electrical current.
- Energy is conserved: it cannot be created or destroyed, only transferred.
Conservation of energy
The principle of conservation of energy states that energy can be transferred usefully, stored or dissipated, but can never be created or destroyed. The total energy of a closed system remains constant.
In any real process, some energy is transferred to thermal stores (often called wasted energy) by friction or air resistance. This energy is not destroyed, it is dissipated to the surroundings and becomes less useful because it is spread out and harder to use.
- Total energy in = total energy out (for a closed system).
- Useful energy: the energy transferred to the desired store.
- Wasted (dissipated) energy: usually thermal, spread to the surroundings.
- Energy is never lost, it is only transferred to less useful stores.
Kinetic and gravitational potential energy
Kinetic energy is the energy a moving object has: Ek = ½mv², where m is mass in kg and v is speed in m/s. Doubling speed quadruples kinetic energy because v is squared. The unit is the joule (J).
Gravitational potential energy is the energy an object has because of its position in a gravitational field: Ep = mgh, where m is mass in kg, g is gravitational field strength (≈ 9.8 N/kg on Earth) and h is height in metres. Lifting an object transfers energy from the chemical store of your muscles to the gravitational potential store.
- Kinetic energy: Ek = ½mv². Doubling speed quadruples KE.
- Gravitational potential energy: Ep = mgh. g ≈ 9.8 N/kg.
- At A-Level: elastic potential energy Ep = ½ke² for a spring.
- A falling object transfers GPE to KE: ½mv² = mgh (ignoring air resistance).
Work done
Work is done when a force moves an object through a distance: W = Fd, where F is force in newtons and d is distance in metres (in the direction of the force). The unit is the joule (J), and 1 J = 1 N·m.
Work done is an energy transfer: when you push a box across the floor, you transfer energy from your chemical store to the kinetic store of the box and the thermal store of the box and floor (via friction). The work done against friction increases the thermal store.
- Work done: W = Fd. Unit: joule (J) = 1 N·m.
- Force and distance must be in the same direction.
- Work done against friction transfers energy to thermal stores.
- At A-Level: work = force × displacement, a scalar quantity.
Power
Power is the rate at which energy is transferred (or the rate at which work is done): P = E/t = W/t. The unit is the watt (W), where 1 W = 1 J/s. A more powerful device transfers the same amount of energy in less time.
At A-Level, power can also be expressed as P = Fv (force × velocity), useful for problems involving constant-speed motion such as a car on a motorway.
- Power: P = E/t. Unit: watt (W), 1 W = 1 J/s.
- A 100 W bulb transfers 100 J of energy every second.
- At A-Level: P = Fv (power = force × velocity).
- Kilowatt (kW) = 1000 W. Megawatt (MW) = 1,000,000 W.
Efficiency
Efficiency measures how much of the input energy is transferred usefully: efficiency = useful energy output / total energy input. As a percentage, multiply by 100. No real device is 100% efficient because some energy is always dissipated to thermal stores by friction or resistance.
For example, an electric motor that uses 200 J of electrical energy and transfers 160 J to kinetic energy has an efficiency of 160/200 = 0.8 or 80%. The remaining 20% is dissipated as heat and sound.
- Efficiency = useful energy output / total energy input (× 100 for %).
- No device is 100% efficient (friction, air resistance, resistance all waste energy).
- Also: efficiency = useful power output / total power input.
- Ways to improve efficiency: lubrication, insulation, streamlining.
Energy resources: renewable vs non-renewable
Non-renewable resources (coal, oil, natural gas, nuclear) are finite: they cannot be replenished once used. They are reliable and can respond quickly to demand, but burning fossil fuels releases CO2, contributing to climate change, and nuclear produces radioactive waste.
Renewable resources (wind, solar, hydroelectric, tidal, geothermal, biomass) can be replenished naturally and produce little or no CO2. However, many are intermittent (wind and solar depend on weather) and can have environmental impacts (hydroelectric floods valleys, tidal barrages affect habitats).
- Non-renewable: coal, oil, gas, nuclear. Reliable, but CO2 (except nuclear) and finite supply.
- Renewable: wind, solar, hydro, tidal, geothermal, biomass. Sustainable, low CO2, but variable output.
- Nuclear: low CO2 but produces radioactive waste and has high decommissioning costs.
- Biomass: renewable but still produces CO2 when burned (though offset by growth).
At A-Level
At A-Level the energy stores model is applied more rigorously: power as the rate of doing work (P = Fv), efficiency of real machines, and energy changes in collisions and oscillations. Calculations involving energy transfer in mechanical and thermal systems are common.
Common exam mistakes
Saying energy is lost in a system.
Energy is never lost. It is transferred to less useful stores, usually thermal. Say dissipated, not lost.
Forgetting that doubling speed quadruples KE.
Ek = ½mv². Speed is squared, so doubling v gives four times the kinetic energy, not two times.
Writing efficiency as useful output divided by wasted input.
Efficiency = useful energy output / TOTAL energy input. Wasted energy is part of the total input.
Confusing energy and power.
Energy (J) is the total amount transferred. Power (W) is the rate of transfer. A 100 W bulb on for 10 s transfers 1000 J.
Saying renewable energy produces no CO2 at all.
Biomass combustion releases CO2, and manufacturing solar panels and turbines has a carbon footprint. Most renewables have very low CO2 compared to fossil fuels, but not zero.
Writing work done = force × time.
Work done = force × distance (in the direction of the force). Force × time gives impulse, which is a different quantity.
Energy exam questions to test yourself
Original exam-style prompts. Answer out loud or in writing first, then open the answer to check your wording.
1. Calculate the kinetic energy of a 2 kg ball moving at 5 m/s. [2 marks]+
Ek = ½mv² = ½ × 2 × 5² = ½ × 2 × 25 = 25 J.
2. A 5 kg object is lifted to a height of 3 m. Calculate its gravitational potential energy. (g = 9.8 N/kg) [2 marks]+
Ep = mgh = 5 × 9.8 × 3 = 147 J.
3. A motor transfers 800 J of useful energy from 1000 J of input. Calculate its efficiency. [2 marks]+
Efficiency = useful output / total input = 800/1000 = 0.8 = 80%.
4. Explain why no machine can be 100% efficient. [3 marks]+
Some energy is always transferred to thermal stores by friction or air resistance; this energy is dissipated to the surroundings; it cannot be recovered as useful energy, so useful output is always less than total input.
5. A car of mass 1200 kg travels at 20 m/s. Calculate its kinetic energy. [2 marks]+
Ek = ½mv² = ½ × 1200 × 20² = 600 × 400 = 240,000 J = 240 kJ.
6. Explain the difference between renewable and non-renewable energy resources, giving one example of each. [3 marks]+
Renewable resources can be replenished naturally and will not run out (e.g. wind); non-renewable resources are finite and cannot be replaced once used (e.g. coal); renewable resources produce far less CO2 than fossil fuels.
7. A 1500 W heater runs for 2 minutes. Calculate the energy transferred. [2 marks]+
E = Pt = 1500 × (2 × 60) = 1500 × 120 = 180,000 J = 180 kJ.
Energy flashcards and quizzes
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Example flashcards
Q: Write the equation for kinetic energy.
A: Ek = ½ × m × v² (mass in kg, speed in m/s, energy in J).
Q: Define power.
A: Power is the rate of energy transfer (or rate of doing work): P = E/t. Measured in watts (1 W = 1 J/s).
Q: Calculate the GPE gained by a 60 kg person climbing 5 m. (g = 9.8 N/kg)
A: Ep = mgh = 60 × 9.8 × 5 = 2940 J.
Q: State the principle of conservation of energy.
A: Energy can be transferred between stores but cannot be created or destroyed; the total energy of a closed system is constant.
Quick summary
Energy is a high-yield Physics topic for GCSE and A-Level students (AQA, Edexcel, OCR). At A-Level the energy stores model is applied more rigorously: power as the rate of doing work (P = Fv), efficiency of real machines, and energy changes in collisions and oscillations. Examiners reward precise definitions and applied explanations, focus on the core ideas and the small set of terms that come up every series.
Key terms
- Kinetic energy
- Gravitational potential energy
- Work done
- Power
- Efficiency
- Joule
- Conservation of energy
- Renewable
- Non-renewable
Energy FAQs
Why can no real machine be 100% efficient?+
Some energy is always transferred to thermal energy stores by friction or air resistance, dissipating to the surroundings and becoming less useful.
What is the difference between energy and power?+
Energy (joules) is the total amount transferred. Power (watts) is the rate at which it is transferred. A 100 W bulb on for 10 s transfers 1000 J.
Are renewable resources always better than non-renewable?+
Renewables produce far less CO2 and won't run out, but they can have higher upfront costs, variable output (wind, solar) and habitat impacts (hydro, tidal).
How does a wind turbine transfer energy?+
Kinetic store of moving air → kinetic store of rotating blades → kinetic store of generator rotor → electrical store via electromagnetic induction.
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