Energy stores and transfers
Physics · GCSE · The School
Stores, not "types"
GCSE physics tracks energy in STORES: kinetic (movement), gravitational potential (height), elastic potential (stretch), thermal, chemical (fuels, food, batteries), and a few more. Events shift energy BETWEEN stores along pathways: mechanically (forces), electrically, by heating, or by radiation (light, sound).
Conservation — the master rule
Energy is never created or destroyed, only transferred. A falling apple shifts gravitational potential into kinetic; a braking car shifts kinetic into thermal in the brakes. Track the total and it always balances — that bookkeeping is the whole game.
Efficiency and dissipation
Useful transfers always leak: some energy DISSIPATES to thermal stores of the surroundings (friction, resistance) and cannot be gathered back. Efficiency = useful output ÷ total input. A motor taking 200 J and delivering 150 J of useful work is 0.75 (75%) efficient; the missing 50 J warmed the world slightly.
An efficiency calculation, carried through
A kettle draws 2,300 W and takes 90 seconds to boil. Total energy in = power × time = 2,300 × 90 = 207,000 J. Of that, 184,000 J actually goes into the thermal store of the water; the rest warmed the element, the casing, the worktop and the air. Efficiency = useful ÷ total = 184,000 ÷ 207,000 = 0.889, or 88.9%. Three habits worth fixing now. Efficiency has NO units — it is a ratio, so an answer of "0.889 J" is wrong. It can never exceed 1, so an efficiency above 100% means an arithmetic slip, every time. And "wasted" energy has not vanished: 23,000 J is spread through the kitchen as thermal energy, too dilute to gather back but every joule still present.
Cutting the waste, not just naming it
Lubricating oil keeps moving surfaces from grinding on each other, so less energy dissipates to the thermal store through friction. Loft insulation traps air, which conducts heat poorly, so a house loses its thermal store more slowly. Streamlining a car's shape lets air flow around it with less turbulence, cutting the kinetic energy dissipated by drag. Each fix targets the specific pathway doing the wasting, which is why "insulate it" is not a universal answer.
A second efficiency calculation
An electric motor takes 500 J to lift a load, and the load gains 350 J of gravitational potential energy. Efficiency = useful ÷ total = 350 ÷ 500 = 0.7, or 70%. The other 150 J did not vanish: most of it dissipated as heat in the motor's coils and bearings, and a little as sound. Same formula as the kettle, different pathway for the waste — which is the pattern behind every efficiency question.