Edexcel IGCSE Physics: Energy Resources and Energy Transfers — Revision Notes

Condensed for the final weeks. For the full explanation, use the Energy Resources and Energy Transfers study guide.

Stores and transfers

Stores: kinetic, gravitational potential, elastic potential, chemical, thermal, nuclear, magnetic, electrostatic.

Transfer pathways: mechanically, electrically, by heating, by radiation.

“Heat” and “light” are pathways, not stores. Writing “light energy” as a store loses the mark.

Ek = 1/2 m v^2        Ep = m g h
efficiency = (useful energy output / total energy output) x 100%
P = E / t

(The specific heat capacity equation, E = m c Δθ, belongs to Topic 5, outcome 5.13P — Physics-only, Paper 2 — not Topic 4.)

Kinetic energy depends on v², so doubling speed quadruples it — the reason braking distance rises so sharply with speed.

Sankey diagrams represent energy transfers visually, with arrow widths proportional to the amount of energy in each pathway, making the split between useful and wasted energy immediately visible.

Worked example — efficiency. A motor is supplied with 500 J of electrical energy and produces 350 J of useful kinetic energy. Efficiency = (350/500) × 100% = 70%.

Worked example — work and power. A crane lifts a 200 kg load through 15 m in 10 s (g = 9.8 N/kg).

W = mgh = 200 x 9.8 x 15 = 29,400 J
P = W/t = 29,400 / 10  = 2,940 W

Work done equals energy transferred, and P = W/t gives the rate of that transfer.

Conservation and dissipation

Energy cannot be created or destroyed, only transferred between stores.

Dissipated energy is not destroyed — it is transferred to the thermal store of the surroundings, spread out and too dilute to be useful. Saying energy is “lost” or “used up” is penalised.

No device is 100% efficient, because friction, air resistance and sound always dissipate some energy. A resistive heater comes closest — nearly all its electrical energy input converts to the wanted thermal energy — but even it loses a little to sound and to light from the heating element.

Thermal transfer

Method	Mechanism	Occurs in
Conduction	Vibrating particles pass energy to neighbours; in metals, free electrons carry it far faster	Mainly solids
Convection	Heated fluid expands, becomes less dense, rises; cooler fluid sinks to replace it	Fluids only
Radiation	Infrared electromagnetic waves; needs no medium	Any, including vacuum

Metals conduct much better than non-metals because of delocalised free electrons, which move through the structure carrying energy — vibration alone is far slower. That is the explanation, not just the fact.

Convection cannot occur in a solid because the particles cannot move from place to place. Only radiation crosses a vacuum, which is why we receive energy from the Sun.

Surfaces: matt black surfaces are the best emitters and the best absorbers; shiny silver surfaces are the best reflectors and the poorest emitters. Emission and absorption go together — a good absorber is a good emitter.

Reducing transfer in a house: loft insulation and cavity walls trap air (a poor conductor) and prevent convection currents; double glazing does the same; shiny foil behind radiators reflects radiation back into the room instead of letting it escape through the wall.

Energy resources

Renewable	Non-renewable
Solar, wind, hydroelectric, tidal, wave, geothermal, biomass	Coal, oil, gas, nuclear

Nuclear is non-renewable but not a fossil fuel, and produces no CO₂ during generation — it belongs in neither simple category, which is worth stating explicitly in an exam answer rather than assuming the examiner will infer it.

Evaluate every resource on four dimensions: reliability, environmental impact, cost (setup and running), and power output. Wind is clean and renewable but unreliable; gas is reliable and quick to start but emits carbon dioxide; nuclear has high output and no CO₂ but produces long-lived waste and high decommissioning costs. A well-argued evaluation weighs several of these dimensions together rather than settling the question on just one.

Exam traps

• Calling heat or light a store.
• Saying energy is lost or used up.
• Explaining metal conduction without free electrons.
• Saying convection occurs in solids.
• Forgetting that a good absorber is also a good emitter.
• Evaluating an energy resource on only one dimension.

Self-test

1. Why are heat and light not energy stores?
2. What actually happens to dissipated energy?
3. Why do metals conduct heat so much better than non-metals?
4. Why can convection not occur in a solid?
5. What is the relationship between a surface’s absorption and emission?
6. A motor is supplied with 500 J of electrical energy and produces 350 J of useful kinetic energy. Calculate its efficiency.
7. A crane lifts a 200 kg load through 15 m in 10 s (g = 9.8 N/kg). Calculate the work done and the power.

Answers: 1. They are transfer pathways — ways energy moves between stores — rather than stores themselves. 2. It is transferred to the thermal store of the surroundings, where it becomes spread out and too dilute to be useful; it is not destroyed. 3. They contain delocalised free electrons that move through the structure carrying energy, which is far faster than transfer by particle vibration alone. 4. The particles are fixed in position and cannot move from place to place, which convection requires. 5. A good absorber is also a good emitter — matt black surfaces are best at both, shiny surfaces poorest at both. 6. efficiency = (350 ÷ 500) × 100% = 70%. 7. W = mgh = 200 × 9.8 × 15 = 29,400 J; P = W/t = 29,400 ÷ 10 = 2,940 W.