Energy Transfer and Conservation

La méthode, étape par étape

1. 1

   Name the energy stores

   kinetic · gravitational · thermal · chemical · elastic · nuclear

   Modern teaching describes stores rather than 'types' or 'forms' of energy. The distinction matters because energy is not converted into a different substance — it is moved from one store to another.

2. 2

   Identify the transfer pathway

   mechanically · electrically · by heating · by radiation

   The pathway is how energy moves, distinct from where it starts and ends. A kettle transfers energy electrically from the mains to the thermal store of the water.

3. 3

   Track a transfer from start to finish

   chemical (fuel) → kinetic (car) + thermal (surroundings)

   Every transfer should be described as store to store. Naming both ends prevents the vague answers — 'energy is used up' — that lose marks despite describing the situation correctly.

4. 4

   Apply conservation of energy

   total energy before = total energy after, always

   Energy is never lost, only transferred to less useful stores — usually thermal energy dissipated to the surroundings. Saying energy is 'lost' is acceptable shorthand only if 'to the surroundings' follows.

5. 5

   Calculate efficiency

   efficiency = useful energy out ÷ total energy in

   Expressed as a decimal or percentage, and never above 1 or 100%. An efficiency above 100% means an arithmetic error, since that would require creating energy.

Stores and pathways are different questions

A store is where energy is; a pathway is how it moves. A stretched spring holds energy in an elastic store, and releasing it transfers that energy mechanically to a kinetic store. Confusing the two produces answers that name a pathway when a store was asked for.

The vocabulary changed in recent specifications for exactly this reason. Older language about 'forms of energy' encouraged students to think of energy changing substance, and stores-and-pathways describes what actually happens more accurately.

• Stores: kinetic, gravitational, thermal, chemical, elastic, nuclear
• Pathways: mechanical, electrical, heating, radiation
• Total energy is always conserved
• Dissipated energy goes to thermal stores in the surroundings
• Efficiency = useful out ÷ total in

Conduction, convection and radiation

Conduction transfers energy through a material as particles vibrate and pass energy to neighbours — effective in solids, especially metals, where free electrons carry energy quickly. Convection moves energy through fluids as warmer, less dense regions rise and cooler ones sink.

Radiation transfers energy as infrared waves and requires no material at all, which is how energy reaches Earth from the Sun. Questions about vacuum flasks and insulation typically require all three to be addressed.

Where the energy goes

Energy described as wasted has almost always been transferred to thermal stores in the surroundings, usually through friction or electrical resistance. It has not disappeared; it has become spread out and too dilute to be useful.

This is why efficiency is never 100% in practice. Reducing waste means reducing friction, improving insulation or lowering resistance, which are the standard answers to 'how could this device be made more efficient'.

How we teach energy

We insist on store-to-store descriptions in every answer. 'Energy is used up' is the phrase that costs the most marks in this topic, and replacing it with a named starting store and a named ending store fixes it.

We also check every efficiency answer against 100%. A figure above it is always an error, and teaching students to notice that themselves saves them from handing in an impossible result.