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.