Overview
What you’ll learn
- Name the energy stores and describe transfers by forces, currents, heating and waves.
- Apply conservation of energy to flow diagrams and multi-stage chains (Supplement for Sankey diagrams).
- Calculate kinetic energy and gravitational potential energy changes (Supplement).
- Use W = Fd = ΔE for mechanical work and link work to energy transferred.
- Compare energy resources for renewability, reliability, scale and environmental impact.
- Calculate efficiency and power, in joules, watts and per cent.
Syllabus checklist (0625)
Statements are paraphrased and grouped by tier. Core is studied by everyone; Supplement is examined on the Extended papers only.
Core — every candidate
- 1.7.1State that energy may be stored as kinetic, gravitational potential, chemical, elastic (strain), nuclear, electrostatic and internal (thermal).
- 1.7.1Describe how energy is transferred between stores during events and processes, including examples of transfer by forces (mechanical work done), electrical currents (electrical work done), heating, and by electromagnetic, sound and other waves.
- 1.7.1Know the principle of the conservation of energy and apply this principle to simple examples including the interpretation of simple flow diagrams.
- 1.7.2Understand that mechanical or electrical work done is equal to the energy transferred.
- 1.7.2Recall and use the equation for mechanical working W = Fd = ΔE.
- 1.7.3Describe how useful energy may be obtained, or electrical power generated, from chemical energy stored in fossil fuels, chemical energy stored in biofuels, water (waves, tides and hydroelectric dams), geothermal resources, nuclear fuel, light from the Sun (solar cells), and infrared and other electromagnetic waves from the Sun (solar panels and wind energy) — including references to a boiler, turbine and generator where they are used.
- 1.7.3Describe advantages and disadvantages of each method in terms of renewability, availability, reliability, scale and environmental impact.
- 1.7.3Understand, qualitatively, the concept of efficiency of energy transfer.
- 1.7.4Define power as work done per unit time and also as energy transferred per unit time; recall and use the equations P = W/t and P = ΔE/t.
Supplement — Extended candidates only
- 1.7.1Recall and use the equation for kinetic energy Ek = ½mv².
- 1.7.1Recall and use the equation for the change in gravitational potential energy ΔEp = mgΔh.
- 1.7.1Know the principle of the conservation of energy and apply this principle to complex examples involving multiple stages, including the interpretation of Sankey diagrams.
- 1.7.3Know that radiation from the Sun is the main source of energy for all our energy resources except geothermal, nuclear and tidal.
- 1.7.3Know that energy is released by nuclear fusion in the Sun.
- 1.7.3Know that research is being carried out to investigate how energy released by nuclear fusion can be used to produce electrical energy on a large scale.
- 1.7.3Define efficiency as (useful energy output ÷ total energy input) × 100% and (useful power output ÷ total power input) × 100%; recall and use these equations.
In 30 seconds
- 1Energy is stored as kinetic, gravitational potential, chemical, elastic (strain), nuclear, electrostatic or internal (thermal) — and is transferred by forces, currents, heating and waves.
- 2Energy is never created or destroyed: the total stays constant, so a flow diagram must balance (Supplement: read a Sankey diagram the same way).
- 3W = Fd = ΔE: work done is the energy transferred by a force acting along a distance, in joules.
- 4Ek = ½mv² and ΔEp = mgΔh — the two great motion-and-height stores (Supplement).
- 5Efficiency = useful output ÷ total input × 100% (energy or power) — no real transfer is 100% efficient (Supplement for the equation).
- 6Power = work done ÷ time = ΔE ÷ t, in watts: 1 W = 1 J/s.
Notes · 1 of 5
Energy stores and transfers
Energy sits in stores and moves between them by forces, currents, heating and waves.
| Store | Everyday example |
|---|---|
| Kinetic | Any moving object: a rolling ball, a flowing river |
| Gravitational potential | Water behind a dam, a book on a shelf |
| Chemical | Food, batteries, fuel, muscles |
| Elastic (strain) | A stretched spring, a bent bow |
| Nuclear | Uranium fuel rods, hydrogen in the Sun |
| Electrostatic | Charged balloon, lightning between clouds |
| Internal (thermal) | A hot cup of tea, the warmth of your body |
Energy transfers between stores in four broad ways: a force doing mechanical work (lifting, pushing); an electrical current doing electrical work (circuits); heating (conduction, convection, radiation); and waves — electromagnetic (light) and sound. A falling ball transfers gravitational potential to kinetic by gravity doing work; a kettle transfers electrical store to the internal store of the water by heating; your ears take in energy carried by sound waves.
Notes · 2 of 5
Kinetic and gravitational potential energy
The two stores you can calculate: speed on one side, height on the other.
Supplement: a moving object stores kinetic energy: Ek = ½mv², in joules. Notice the speed is squared — doubling a car’s speed quadruples its kinetic energy, which is why braking distance grows so quickly with speed.
Lifting an object through a height stores gravitational potential energy: ΔEp = mgΔh, in joules, where Δh is the change in height. Take g = 9.8 N/kg near the Earth’s surface.
The two connect in drops and swings: a ball dropped from a height h reaches the ground (ignoring air resistance) with kinetic energy equal to the potential energy it lost — mgΔh becomes ½mv².
Notes · 3 of 5
Work done
Work is force × distance moved along the line of the force — the energy transferred.
When a force moves an object, the force does work. Mechanical work done — and electrical work done — is the energy transferred: W = Fd = ΔE, in joules, where d is the distance moved in the direction of the force.
Notes · 4 of 5
Energy resources
Fossil, renewable and nuclear: how each one delivers useful energy or electricity.
Useful energy — especially electrical power — comes from a short list of resources. Most thermal stations share the same chain: a boiler or core releases energy, a turbine spins, and a generator converts that rotation into electrical energy.
| Resource | How it works | Advantages | Disadvantages |
|---|---|---|---|
| Fossil fuels | Chemical energy burned in a boiler; steam drives a turbine and generator | Reliable, high power output, established technology | Non-renewable; CO₂ emissions and pollution; finite supply |
| Biofuels | Chemical energy from recently grown material burned in a boiler | Renewable; can use waste | Needs land and water; still emits CO₂ when burned |
| Hydroelectric | Water behind a dam falls through turbines | Renewable; reliable where water is guaranteed; fast response | Needs large valleys; floods habitats; high build cost |
| Waves and tides | Wave machines or tidal barrages drive turbines | Renewable; predictable (tides) | Harsh environment; costly; tidal sites are rare |
| Geothermal | Hot rocks heat water into steam for turbines | Renewable; reliable; no fuel cost | Only in geologically suitable places; drilling is expensive |
| Nuclear fission | Nuclear store of uranium releases heat in a reactor | Huge energy per kg; reliable; no CO₂ at generation | Radioactive waste; costly decommissioning; fuel is finite |
| Solar cells | Light energy converted straight to electrical | Renewable; no moving parts; scales from rooftops to farms | No output at night; variable with weather; panel manufacture costs |
| Solar heating and wind | Infrared heats water panels; moving air turns wind turbines | Renewable; low running cost | Wind is unreliable; turbines need many units and open land or sea |
Supplement: trace nearly every resource back far enough and you reach the Sun: sunlight grows biofuels, drives the water cycle behind hydroelectric dams, powers the winds and waves, and — via ancient plants — created fossil fuels. The exceptions are geothermal (Earth’s internal store), nuclear (elements formed before the Solar System) and tidal (the Moon’s gravity). The Sun itself shines because nuclear fusion in its core releases energy, and research projects are working to harness fusion reactions for large-scale electricity generation here on Earth.
Notes · 5 of 5
Efficiency and power
How much of the input ends up useful — and how fast the energy is delivered.
No transfer is perfect: a lamp warms the room, a car engine heats the exhaust. Efficiency compares the useful part with the total. Qualitatively, a more efficient machine wastes a smaller fraction of its input. Supplement:
Power is the rate of energy transfer: P = W/t = ΔE/t, measured in watts (1 W = 1 J/s). A 2 kW kettle transfers 2000 J every second; a human climbing stairs might deliver a useful 300 W for a short burst.
Revise
Key terms
The words examiners expect you to use precisely.
- Energy store
- A way energy is held: kinetic, gravitational potential, chemical, elastic (strain), nuclear, electrostatic or internal (thermal).
- Energy transfer
- Energy moving between stores by forces (mechanical work), currents (electrical work), heating or waves.
- Conservation of energy
- Energy cannot be created or destroyed; the total energy of a closed system stays constant.
- Kinetic energy
- The energy store of a moving object, Ek = ½mv², in joules.Supplement
- Gravitational potential energy
- The energy store of an object raised in a gravitational field, ΔEp = mgΔh.Supplement
- Work done
- The energy transferred by a force acting along a distance: W = Fd = ΔE, in joules.
Show 6 more key termsShow fewer key terms
- Efficiency
- Useful energy (or power) output divided by total input, often × 100%.Supplement for the equation
- Power
- The rate of energy transfer: P = W/t = ΔE/t, in watts (1 W = 1 J/s).
- Watt
- The unit of power: one joule transferred per second.
- Sankey diagram
- A flow diagram whose arrow widths show, to scale, where the input energy ends up.Supplement
- Turbine and generator
- The spinning turbine turns the generator, which converts the rotation into electrical energy.
- Nuclear fusion
- The joining of light nuclei that releases energy — the process that powers the Sun.Supplement
Revise
Common mistakes
Where students most often lose marks in this topic — and what to do instead.
Mistake: Listing "heat" as an energy store.
The syllabus calls it the internal (thermal) store; "heat" names a transfer, not a store.
Instead: Say internal (thermal) energy for the store; heating is one way energy transfers.
Mistake: Forgetting to square the speed in Ek = ½mv².
Only the mass is linear; speed enters as v².
Instead: Ek = ½mv²: at 30 m/s the store is four times the value at 15 m/s for the same object.
Show 8 more mistakesShow fewer mistakes
Mistake: Using W = Fd with the distance measured perpendicular to the force.
d must be along the line of action of the force.
Instead: Only displacement in the direction of the force counts as work done by that force.
Mistake: Saying "energy is used up" in a flow diagram.
Energy is conserved; what disappears is the useful fraction.
Instead: Input = useful output + wasted output, usually ending in the internal store of the surroundings.
Mistake: Giving efficiency as a fraction when the question asks for a percentage (or the reverse).
0.7 and 70% are different notations for different mark schemes.
Instead: Read the command: "calculate the efficiency" often accepts either, but "as a percentage" needs × 100%.
Mistake: Confusing power with energy.
A kWh of electricity is energy; a 2 kW kettle is a rate.
Instead: Energy in joules is the amount; power in watts is the amount per second.
Mistake: Claiming solar panels and solar cells are the same device.
One heats water with infrared; the other converts light directly to electricity.
Instead: Solar heating panels warm water; solar (photovoltaic) cells generate electrical power from light.
Mistake: Saying wind and hydroelectric power come from nowhere.
Both are indirect solar energy — the Sun drives the weather and the water cycle.
Instead: Only geothermal, nuclear and tidal do not trace back to solar radiation (Supplement).
Mistake: Calculating power without converting minutes to seconds.
P = ΔE/t needs time in seconds to give watts.
Instead: Convert first: 2.0 min = 120 s, then divide.
Mistake: Assuming a more powerful machine is always more efficient.
Power says how fast energy is delivered; efficiency says what fraction is useful.
Instead: A 3 kW heater can be less efficient than a 1 kW heat pump — the two numbers measure different things.
Check yourself
Quick quiz
20 questions with instant explanations. About 5 minutes.
Question 1 of 3
- Question 1: current question
- Question 2: not answered
- Question 3: not answered
Identify energy stores
A stretched bow about to fire an arrow stores energy mainly as
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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 topic, in three tiers, each with a model answer, mark scheme and examiner tip.
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Practice paper
A 30-mark paper set out like a Cambridge paper, with a separate mark scheme. Give yourself 40 min.
Question paper
Topic 4 Practice Paper
30 marks · 40 min · 5 pages
10 multiple-choice questions and structured questions with answer lines. Topic 4: Energy, work and power (stores, transfers, work, resources, efficiency, power).
Mark scheme
PROTopic 4 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.
Which paper covers this topic?
Paper 1 / 2
Multiple choice (45 min, 40 marks)
Core and Extended versions
Paper 3 / 4
Theory, short and structured (1 h 15, 80 marks)
Core and Extended versions
Paper 5 / 6
Practical test or Alternative to Practical (40 marks)
Experimental skills
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Questions
Frequently asked questions
What equations do I need for IGCSE Physics energy, work and power?
W = Fd = ΔE for work; Ek = ½mv² and ΔEp = mgΔh for the two calculable stores (Supplement); efficiency = useful ÷ total × 100% (Supplement); and P = W/t = ΔE/t for power, in watts.
What is the difference between energy and power?
Energy (joules) is the total amount transferred. Power (watts) is how fast it is transferred — 1 watt means 1 joule every second. A kettle and a phone charger may transfer the same total energy, but at very different rates.
Why is no machine 100% efficient?
Some input energy always ends up in useless stores — usually the internal (thermal) energy of the machine and its surroundings, caused by friction, resistance or turbulence. Conservation of energy says the wasted part still exists; efficiency says how little of it there is.
Which energy resources come from the Sun?
Almost all: fossil fuels (ancient plants), biofuels, hydroelectric, waves, wind and solar cells or panels. The exceptions are geothermal (Earth’s interior), nuclear (the fuel’s nuclear store) and tidal (mainly the Moon’s gravity).
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