Overview
What you’ll learn
- Name the eight energy stores and describe the transfers in four everyday systems.
- Quote and apply the conservation of energy to explain where "lost" energy goes.
- Pick and use the ½mv², mgh and ½ke² equations with correct units.
- Convert between energy and power with P = E ÷ t, including appliance ratings.
- Calculate efficiencies as fractions and percentages, and suggest design improvements.
- Describe the specific-heat-capacity practical: method, variables, hazards, error sources.
Specification checklist (AQA Trilogy 8464 · Edexcel 1SC0 · OCR Gateway J250)
Statements are paraphrased from the shared DfE combined science subject content and grouped by tier. Foundation content is studied by every student; Higher statements are examined on Higher-tier papers only. This chapter covers only what Combined Science (double award) examines — separate-science extras live on the GCSE Biology, Chemistry and Physics hubs. QuizQuip is not affiliated with AQA, Pearson Edexcel or OCR.
Foundation — every student
- 21.1Identify the energy stores — kinetic, gravitational potential, elastic potential, thermal, chemical, magnetic, electrostatic and nuclear — and describe systems changing between them (an object accelerated by a force, a ball thrown upwards, a vehicle braking, a kettle boiling water).
- 21.2State the principle of conservation of energy: energy cannot be created or destroyed, only transferred between stores; in a closed system the total energy stays constant.
- 21.3Calculate kinetic energy (E = ½mv²), gravitational potential energy (E = mgh) and elastic potential energy (E = ½ke²), choosing the right equation and units.
- 21.4Define power as the energy transferred per second, calculate P = E ÷ t, and use appliance power ratings to find the energy transferred over a time.
- 21.5Describe dissipation: unwanted energy transfers end up as less useful stores (usually thermal energy of the surroundings), and explain how lubrication reduces friction and thermal insulation (thickness and low thermal conductivity) reduces heating losses.
- 21.6Calculate efficiency as useful energy output ÷ total energy input (or useful power ÷ total power), as a ratio or percentage, and explain why devices can never be fully efficient and how design improves efficiency.
- 21.7Define specific heat capacity and calculate the change in thermal energy using ΔE = mcΔθ.
- 21.8Carry out the required practical: measure the specific heat capacity of a metal (or water) using a heater and joulemeter or an ammeter–voltmeter circuit, controlling the method, identifying variables, hazards and sources of error.
Key points
- 1Eight stores: kinetic, gravitational potential, elastic potential, thermal, chemical, magnetic, electrostatic, nuclear. Transfers happen by forces (work), heating, electricity and waves.
- 2Conservation: energy is never created or destroyed — a closed system's total stays constant. "Lost" energy is really dissipated to less useful (usually thermal) stores.
- 3Three equations: kinetic ½mv², gravitational mgh, elastic ½ke². Speed appears squared in kinetic energy — double the speed, four× the energy.
- 4Power = energy transferred per second: P = E ÷ t, measured in watts (1 W = 1 J/s). A 2 kW kettle for 30 s moves 60,000 J.
- 5Efficiency = useful output ÷ total input (energy or power), as a ratio or ×100 for %. No device reaches 100% — some energy always dissipates.
- 6Slowing the losses: lubrication cuts friction; thicker, low-conductivity walls and trapped-air insulation cut heating losses.
- 7ΔE = mcΔθ: 1 °C change for 1 kg defines specific heat capacity (water: 4200 J/kg °C). Required practical heats a metal block and measures energy in and temperature rise.
Notes · 1 of 4
Energy stores and systems
Name the store, describe the transfer — the grammar of every energy question.
Physics keeps accounts in stores: kinetic (anything moving), gravitational potential (anything lifted), elastic potential (anything stretched or squashed), thermal (anything hot), chemical (fuels, food, batteries), plus magnetic, electrostatic and nuclear. Between stores, energy moves by mechanical work (forces), heating, electrical currents or waves such as light and sound.
Four systems to narrate. A ball thrown upwards: chemical (arm muscles) → kinetic (leaving the hand) → gravitational potential (at the top), then back on the way down. A vehicle braking: kinetic → thermal (brake discs and road). A kettle boiling: electrical → thermal of the element, then of the water. An object accelerated by a push: chemical or kinetic of the pusher → kinetic of the object.
Worked example — narrating a system
Q: A cyclist pedals hard on the flat, then brakes at a junction. Describe the energy transfers. A: Chemical energy in the cyclist's muscles does work, filling the kinetic store of bike and rider; some energy is transferred to thermal stores (air resistance, chain friction) all along. Braking transfers the remaining kinetic energy to the thermal store of the brake blocks and wheel rims. Nothing disappears — every joule is accounted for.
Notes · 2 of 4
The three energy equations
½mv², mgh and ½ke² — and the trap of the squared speed.
Kinetic energy = ½ × mass × speed². Because speed is squared, doubling the speed quadruples the energy — the physics behind braking distances. Gravitational potential energy = mass × gravitational field strength × height (g = 9.8 N/kg on Earth). Elastic potential energy = ½ × spring constant × extension² — the extension, again, squared.
| Situation | Equation | Watch out for |
|---|---|---|
| Object moving at speed v | Ek = ½mv² | Speed in m/s; v is squared |
| Object lifted height h | ΔEp = mgh | g = 9.8 N/kg unless told otherwise |
| Spring stretched by e | Ee = ½ke² | Extension in metres, squared |
Exam tips
- Worked example — kinetic energy. Q: A 60 kg runner jogs at 4 m/s, then sprints at 8 m/s. Find both kinetic energies. A: Jogging: Ek = ½ × 60 × 4² = ½ × 60 × 16 = 480 J. Sprinting: Ek = ½ × 60 × 8² = ½ × 60 × 64 = 1920 J. The speed doubled but the energy quadrupled (2² = 4) — quote that comparison for the analysis mark.
- Worked example — gravitational and elastic energy. Q: A 72 kg climber ascends a 12 m cliff (g = 9.8 N/kg); at the top she stretches a 200 N/m safety lanyard by 0.10 m in a test. Find both energies. A: GPE = mgh = 72 × 9.8 × 12 = 8467 J ≈ 8.5 kJ. Elastic: Ee = ½ke² = ½ × 200 × 0.10² = ½ × 200 × 0.01 = 1.0 J. Squash the extension before squaring — 0.10² is 0.01, not 0.1.
Notes · 3 of 4
Power, conservation and efficiency
How fast the energy moves, where it ends up, and what fraction earns its keep.
Power is the rate of energy transfer: P = E ÷ t, in watts. One watt means one joule every second. Appliance ratings let you budget: a 2000 W kettle for 30 s transfers E = Pt = 2000 × 30 = 60,000 J. In the other direction, a motor that transfers 9000 J over 45 s has a power of 9000 ÷ 45 = 200 W.
Conservation of energy is absolute: in any closed system the total never changes. So where does the "lost" energy of a noisy, warm machine go? It is dissipated — spread into less useful stores, almost always the thermal store of the surroundings. A laptop warms the desk; brakes warm the air. The energy still exists; it is just no longer useful, and it is spread out too thinly to re-concentrate.
Efficiency measures the useful fraction: efficiency = useful output ÷ total input — using energy or power — left as a ratio or multiplied by 100 for a percentage. A motor drawing 900 J and delivering 540 J of useful work runs at 540 ÷ 900 = 60%; the missing 360 J heats the windings. No device is 100% efficient — friction, air resistance and electrical resistance always dissipate some energy. Design fights back: lubrication (less friction), streamlining (less drag), and insulation — thicker walls of low thermal conductivity, cavity-wall foam trapping air, to slow the flow of heat out of buildings.
Worked example — efficiency and the wasted share
Q: A motor is supplied with 900 J and delivers 540 J of useful mechanical work. Find its efficiency and the wasted energy. A: Efficiency = 540 ÷ 900 = 0.60 = 60%. Wasted = 900 − 540 = 360 J (its share: 100% − 60% = 40%). Always subtract to find the wasted energy — never quote efficiency above 100%.
Notes · 4 of 4
Specific heat capacity and the required practical
Why water is such a stubborn substance to warm — and how to measure it.
Specific heat capacity (c) is the energy needed to raise the temperature of 1 kg of a substance by 1 °C. Water's huge value — 4200 J/kg °C — is why it is used in radiators, coolant systems and hot-water bottles: it drinks energy without warming quickly. The working equation: ΔE = mcΔθ — energy change = mass × specific heat capacity × temperature change.
Required practical — measuring specific heat capacity (method): weigh a metal block (or a insulated container of water); fit an electric immersion heater and a thermometer; connect the heater through a joulemeter (or an ammeter and voltmeter so E = VIt can be totalled); record the starting temperature, switch on for a fixed time, and read the temperature every minute; stir if heating a liquid. Calculate c = energy supplied ÷ (mass × temperature rise).
| Item | In this practical |
|---|---|
| Independent variable | Which material (or the mass) is being heated |
| Dependent variable | Temperature rise after a fixed energy input |
| Controls | Same heater, same insulation, same starting temperature |
| Main error sources | Heat escaping to the room; heater energy warming the block's thermometer hole; thermometer lag — both push the calculated c off the true value |
Hazards in this practical
The heater and block get hot enough to burn — handle with the block's purpose-made handle and switch off before touching. The heater must never be switched on out of the material (it overheats in air). Liquids: use eye protection and no naked flames.
Worked example — a full SHC calculation
Q: A 1.0 kg aluminium block is heated by a 50 W heater for 5.0 minutes; the temperature rises 16.5 °C. Find c and compare with the accepted ~900 J/kg °C. A: Energy: E = Pt = 50 × 300 = 15,000 J. Then c = E ÷ (mΔθ) = 15,000 ÷ (1.0 × 16.5) = 909 J/kg °C — close to the accepted value; the small excess is extra energy lost to the surroundings and the thermometer fitting.
Revise
Key terms
The words examiners expect you to use precisely.
- Energy store
- A way energy is held — kinetic, gravitational potential, elastic potential, thermal, chemical, magnetic, electrostatic or nuclear.
- Conservation of energy
- Energy cannot be created or destroyed, only transferred between stores; a closed system keeps its total constant.
- Dissipation
- Energy spreading into less useful stores — usually warming the surroundings.
- Power
- The energy transferred per second: P = E ÷ t, measured in watts (1 W = 1 J/s).
- Efficiency
- Useful output divided by total input, as a ratio or percentage — always below 100%.
- Specific heat capacity
- The energy needed to raise 1 kg of a substance by 1 °C (water: 4200 J/kg °C).
Show 2 more key termsShow fewer key terms
- Thermal conductivity
- How well a material conducts heat — high values (metals) transfer energy quickly.
- Joulemeter
- A meter that measures the energy transferred through a circuit directly, used in the SHC practical.
Revise
Common mistakes
Where students most often lose marks in this chapter — and what to do instead.
Mistake: Saying energy is "used up" or destroyed
It violates conservation of energy.
Instead: Energy is transferred to other stores — usually dissipated as heat to the surroundings.
Mistake: Forgetting to square the speed in kinetic energy
Ek = ½mv² has v squared.
Instead: Square the speed first, then multiply by ½m — double speed gives four times the energy.
Show 10 more mistakesShow fewer mistakes
Mistake: Quoting efficiency above 100%
No device creates energy.
Instead: Efficiency = useful ÷ input, always less than 1 (or 100%); a result above it signals a slipped decimal.
Mistake: Mixing up power and energy
Watts measure a rate; joules measure an amount.
Instead: Power = energy ÷ time. Convert with P = E ÷ t or E = Pt.
Mistake: Using grams or centimetres in the energy equations
The equations demand kilograms, metres and seconds.
Instead: Convert mass to kg and extension/height to m before substituting.
Mistake: Saying insulation "destroys" heat loss
Heat still escapes, just more slowly.
Instead: Insulation reduces the rate of transfer — thicker, low-conductivity barriers slow conduction.
Mistake: Squaring the extension wrongly in ½ke²
0.10² is 0.01, not 0.1 or 0.2.
Instead: Square the extension in metres, then halve the product of k and that value.
Mistake: Thinking a powerful machine is automatically efficient
Power says nothing about the useful fraction.
Instead: A 2 kW heater can be less efficient than a 1 kW one — efficiency compares output with input.
Mistake: Blaming the SHC practical's overshoot on "experimental genius"
The usual cause is systematic heat loss.
Instead: Energy escapes to the room and warms the thermometer fitting, so the temperature rise lags the true value.
Mistake: Interpreting specific heat capacity as "how hot something is"
Temperature and energy are different quantities.
Instead: SHC is the energy needed per kilogram per degree — water's high value makes it a superb coolant.
Mistake: Naming "heat" as an energy store
The stores list has thermal, not heat.
Instead: Thermal energy is the store; heating is one of the transfer pathways.
Mistake: Using g = 10 without checking the paper
Values differ between questions and boards.
Instead: Use the g given in the question (9.8 N/kg is the default) and state it in the working.
Check yourself
Quick quiz
20 questions with instant explanations. About 5 minutes.
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Practise
Topical questions
12 exam-style questions with model answers and mark schemes.
A set of structured and calculation questions matched to this chapter, in three tiers, each with a model answer, mark scheme and examiner tip. Higher-tier questions are tagged.
Practise topical questionsExam practice
Practice paper
A 30-mark paper in the style of a Combined Science paper, with a separate mark scheme. Give yourself 40 min.
Question paper
Chapter 21 Practice Paper
30 marks · 40 min · 5 pages
6 multiple-choice questions and structured questions with answer lines. Energy stores and systems, conservation of energy, kinetic, gravitational and elastic energy equations, power, dissipation and insulation, efficiency calculations, specific heat capacity and its required practical.
Mark scheme
PROChapter 21 Practice Paper — mark scheme
Answers · Marking points · 3 pages
Model answers with a mark for each creditworthy point — for marking at home or in class.
How does this chapter fit the real exams?
Physics Paper 1
1 h 15 · 70 marks (Trilogy)
Energy, electricity, particle model, atomic structure (chapters 21–26)
Physics Paper 2
1 h 15 · 70 marks (Trilogy)
Forces, waves, magnetism and electromagnetism (chapters 27–30)
Tiers
Foundation (grades 1-1 to 5-5) · Higher (4-3 to 9-9)
Two grades are awarded together; the 21 required practicals are examined on the written papers
Want a full paper? Try the Biology Paper 1-style (Foundation), Biology Paper 1-style (Higher), Biology Paper 2-style (Foundation), Biology Paper 2-style (Higher), Chemistry Paper 1-style (Foundation), Chemistry Paper 1-style (Higher), Chemistry Paper 2-style (Foundation), Chemistry Paper 2-style (Higher), Physics Paper 1-style (Foundation), Physics Paper 1-style (Higher), Physics Paper 2-style (Foundation), Physics Paper 2-style (Higher), Required practicals practice sets.
Questions
Frequently asked questions
Which energy equations do I need for Combined Science?
Three: kinetic energy ½mv², gravitational potential energy mgh, and elastic potential energy ½ke² — plus ΔE = mcΔθ for heating and P = E ÷ t for power. Learn what each symbol means, not just the letters.
Why does doubling speed quadruple kinetic energy?
Speed is squared in ½mv². Double v and v² becomes 2² = 4 times as large — the same squaring that makes braking distances grow so fast.
What is the difference between power and efficiency?
Power is how fast energy is transferred (watts). Efficiency is what fraction of it is useful (a ratio or percentage). A powerful machine can be inefficient and vice versa.
How do I answer "where does the wasted energy go?"
Name the pathway (friction, electrical resistance, air resistance) and the destination (thermal store of the surroundings, plus sound if noisy). Never say it disappears — dissipation means spread out, not gone.
What ruins the SHC practical result?
Escaped heat. Energy that leaks to the room or warms the thermometer fitting never reaches the material, so the measured temperature rise is too small and the calculated specific heat capacity comes out too high. Insulate, stir liquids, and start below room temperature if you can.
Up next
Chapter 22: Energy supplies and resources