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Physics · Key Stage 3 · The School
Stores, not types
Modern teaching says energy is STORED and TRANSFERRED, not that there are 'types of energy'. The stores: kinetic, gravitational potential, elastic potential, chemical, thermal, nuclear, magnetic, electrostatic. A stretched band is an elastic store; a raised book is a gravitational store; food is a chemical store.
Transfers
Energy moves between stores by four pathways: mechanically (a force moving something), electrically, by heating, and by radiation (light and sound). Dropping the book transfers gravitational to kinetic, mechanically. A kettle transfers electrical to thermal by heating.
Conserved, but not always useful
Energy is never created or destroyed. But at every transfer some ends up warming the surroundings, spread out too thinly to use again — DISSIPATION. That is why no machine is 100% efficient, and why 'wasted energy' means 'no longer useful', never 'gone'.
Following the joules through one machine
An electric drill is supplied with 500 J of energy. 400 J leaves as useful movement in the kinetic store of the bit; 80 J warms the motor; 20 J leaves as sound. Check the books balance first: 400 + 80 + 20 = 500, and it must, because energy is conserved. Efficiency is the useful share: 400 ÷ 500 = 0.8, or 80%. A Sankey diagram draws exactly this — one thick arrow in, splitting into arrows whose WIDTHS are proportional to the joules, useful ones carrying straight on and dissipated ones peeling away downwards. If the widths of the arrows leaving do not add up to the width going in, the diagram is wrong before you read a single number off it.