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.
Say the store, not the "type"
The specification uses stores — kinetic, gravitational, elastic, thermal, chemical, nuclear, magnetic, electrostatic — and pathways between them: mechanical work, electrical work, heating, radiation. "Sound energy" and "movement energy" are old vocabulary that costs marks. Name the store it starts in and the store it ends in.
Efficiency, and where the rest went
Efficiency is useful output over total input, and the missing fraction is not destroyed — it has been dissipated, usually as thermal energy to the surroundings. An answer that says energy was lost is wrong on the physics; an answer that says where it went is right.