Energy stores and transfer pathways — energy is always conserved
Energy is held in named stores and moved between them by transfer pathways — keep the two separate. Stores: chemical, kinetic, gravitational, elastic, thermal, magnetic, electrostatic, nuclear. Pathways (how energy moves): mechanically (a force doing work), electrically (a current), by heating, by radiation. Conservation of energy: energy cannot be created or destroyed, only transferred between stores (or dissipated), so total energy input = useful output + wasted energy.
Efficiency = useful output / total output x 100%
Efficiency is the fraction of the total energy supplied transferred usefully: . By conservation the total output equals the input (useful + wasted), so it also works with power. It is a ratio with no unit (or %). A real device is never 100% efficient — some energy is always wasted as thermal energy — so any answer above 100% is wrong.
Conduction, convection and radiation transfer thermal energy
Thermal energy transfers three ways. Conduction: vibrating particles (and free electrons in metals) pass on kinetic energy by colliding, without moving along — metals conduct best. Convection: a heated fluid expands, becomes less dense and rises while cooler fluid sinks (a convection current; fluids only). Radiation: infrared waves that need no medium, so cross a vacuum. A dark matt surface absorbs and emits infrared well, a shiny one poorly; hotter objects radiate faster.
Drawn from real examiner reports.
Dark surface ≠ better conductor
On emission/absorption questions, weaker answers explain a dark surface heating up by calling it a better conductor — wrong mechanism. A dark, matt surface is a good absorber (and emitter) of infrared radiation. Link the surface property to the energy transfer: the property alone scores 1 mark; the link to absorbing radiation / raising the thermal store earns the second.
Jun 2024 1P Q3d; Jun 2023 1P Q8a
Convection current ≠ electric current
Convection transfers thermal energy by the bulk movement of a fluid, not by electricity — do not confuse a convection current with an electric current. Describe the full chain: the fluid is heated, expands, becomes less dense and rises, and cooler denser fluid sinks to replace it. The top mark needs this linked to the rate of energy transfer, which many candidates omit.
Jun 2023 1P Q8c
Name the pathway, not just the store
A store is where energy is held; a pathway is how it moves. On Sankey questions candidates label the wasted store thermal but drop the mark for the transfer pathway. A battery driving a motor transfers energy electrically (there is no "electrical store"); the wasted branch transfers by heating. The four pathways: mechanically, electrically, by heating, by radiation.
Jun 2023 1P Q7c–d
Efficiency can never exceed 100%
Efficiency = useful energy output / total energy output; because energy is conserved and some is always wasted, it must be below 100% and has no unit. To find the wasted energy, work out the total input first, then wasted = total − useful — do not guess. An answer above 100%, or one carrying a unit like joules, must be wrong.
Jun 2024 1PR Q12a
Radiation through a vacuum ≠ needs particles
Radiation is the only thermal transfer that crosses a vacuum, because it travels as infrared electromagnetic waves and needs no medium. Asked why energy reaches a satellite from the Sun, candidates wrongly invoke conduction or convection, or just repeat the question. The correct point: there are no particles between them, so only radiation can transfer the energy.
Jun 2024 1PR Q4c(i)
Energy is transferred, not "used up"
Conservation of energy means energy is never created, destroyed or "used up" — saying it "runs out" loses the mark. Wasted energy is still there; it has been dissipated (spread out) to less useful stores, usually as thermal energy warming the surroundings. Define conservation as: energy cannot be created or destroyed, only transferred from one store to another.
Efficiency scaffold
Read the useful output and total input from the data or Sankey, use conservation for any missing value (wasted = total − useful), then apply efficiency = useful / total × 100%. Substitute before evaluating, and sanity-check the answer is below 100% with no unit.
Draw Sankey arrows to scale
Make Sankey arrow widths proportional to the energy values: the input is widest, the useful output continues across, the wasted arrow branches off labelled thermal. Outputs must add to the input (useful + wasted = input) or conservation breaks. Name the input pathway if asked.
Emission/absorption: heat direction first
For emission or absorption questions, first work out the direction of heat transfer (hotter → cooler), then discuss the relevant surface's absorption or emission, referring explicitly to infrared radiation. A hotter object emits faster; a good absorber is also a good emitter.
| Quantity | Formula | Unit |
|---|---|---|
| Efficiency | none (or %) | |
| Efficiency (power form) | none (or %) | |
| Conservation of energy | total energy input useful energy output wasted energy | J |
Where energy is measured in joules (J) and power in watts (W). Total energy output equals the total energy input (by conservation), i.e. useful + wasted energy. Efficiency is a ratio, so it is unitless and is always less than 100% for a real device.
Energy stores (8): chemical, kinetic, gravitational, elastic, thermal, magnetic, electrostatic, nuclear.
Transfer pathways (4): mechanically, electrically, by heating, by radiation (light and sound).
Thermal-transfer definitions (mark-scheme form):
Define efficiency and give the formula.
An electric lamp is supplied with 80 J of energy. Of this, 24 J is transferred usefully as light.
Calculate the efficiency of the lamp.
