Conservation and Dissipation of Energy

This guide covers sub-topic 4.1.2 Conservation and dissipation of energy, the second of three sub-topics in Topic 4.1 Energy, from the AQA GCSE Physics (8463) specification (first teaching September 2016).

Before studying this

Read Energy Changes in a System first — this guide builds directly on energy stores and the equations introduced there.

Syllabus coverage

AQA GCSE PHYSICS (8463) — Sub-topic 4.1.2

Energy can be transferred usefully, stored, or dissipated, but never created or destroyed. Students should be able to describe, with examples, how in all system changes energy is dissipated, so that it is stored in less useful ways. This energy is often “wasted” — dissipated to the surroundings, becoming increasingly spread out.

Energy is not destroyed but ends up in the surroundings, becoming more difficult to make use of. Students should be able to explain ways of reducing unwanted energy transfer, for example through lubrication and the use of thermal insulation. Students should be able to explain that thermal energy transfer is reduced by materials with low thermal conductivity.

Required practical activity 2 (physics only): investigate the effectiveness of different materials as thermal insulators and the factors that may affect the thermal insulation properties of a material.

Students should be able to describe how the rate of cooling of a building is affected by the thickness and thermal conductivity of its walls. The higher the thermal conductivity of a material, the higher the rate of energy transfer by conduction across the material.

Students should be able to calculate the efficiency for the transfer of energy by a device using: efficiency = useful output energy transfer ÷ total input energy transfer, and efficiency = useful power output ÷ total power input. (HT only) Students should be able to describe ways to increase the efficiency of an intended energy transfer.

Conservation of energy

Energy can be usefully transferred, stored, or dissipated, but it can never be created or destroyed — this is the principle of conservation of energy. In every system change, some energy is dissipated, meaning it becomes stored in less useful ways and spreads out into the surroundings. This is often described as “wasted” energy — not lost, but harder to make use of again.

Reducing unwanted energy transfer

Unwanted energy transfer can be reduced by lubrication (reducing friction between moving surfaces), by streamlining (reducing air resistance, so less energy is dissipated by drag), and by thermal insulation (reducing the rate of energy transfer by heating). Thermal energy transfer is reduced by using materials with low thermal conductivity. The higher a material’s thermal conductivity, the faster energy is transferred through it by conduction — which is why building walls with thick, low-conductivity insulation cool (or heat) more slowly.

Cavity wall insulation and double glazing work by trapping a layer of air, which itself has low thermal conductivity, and by preventing convection currents from forming within the gap — without the insulating material, warm air would simply circulate within the cavity and carry energy across it far more quickly (convection is background from earlier study; the specification itself focuses on thermal conductivity). The insulation is not “stopping” energy transfer altogether, only reducing its rate.

Required practical activity 2 (physics only) investigates the effectiveness of different materials as thermal insulators and the factors that may affect the thermal insulation properties of a material. Identical containers of hot water are wrapped in different materials, or in different thicknesses of the same material, and the temperature of each is measured at regular time intervals as it cools. The material producing the smallest temperature drop over the same time is the most effective insulator — comparing several materials, or the same material at different thicknesses, this way is what the practical tests.

Efficiency

Efficiency describes how much of the total energy (or power) input to a device ends up as useful output, rather than being dissipated:

efficiency = useful output energy transfer ÷ total input energy transfer
efficiency = useful power output ÷ total power input

Efficiency has no units and is often expressed as a percentage (multiply by 100). (HT only) Ways to increase the efficiency of an intended energy transfer include reducing friction (lubrication) and reducing unwanted thermal transfer (insulation).

No real device is 100% efficient. An electric heater comes close, because its intended output is thermal energy and almost all of the input ends up in the thermal store it is meant to heat. In every other device, some energy is always dissipated by friction, sound or unwanted heating, so useful output is always less than total input.

Worked example. A lamp is supplied with 100 J of electrical energy and usefully transfers 20 J as light (the rest is transferred to the thermal energy store of the surroundings). Calculate its efficiency.

efficiency = useful output energy transfer ÷ total input energy transfer
efficiency = 20 ÷ 100 = 0.2 = 20%

The same equation works with power. An electric motor has an input power of 500 W and a useful output power of 400 W:

efficiency = useful power output ÷ total power input
efficiency = 400 ÷ 500 = 0.8 = 80%

Common mistakes

Saying energy is “lost” — energy is never destroyed, only dissipated to the surroundings in a less useful form. Forgetting efficiency has no units and cannot exceed 1 (or 100%) — a calculated value above this signals an arithmetic error. Confusing thermal conductivity with thermal insulation — a low thermal conductivity material is a good insulator.

Quick revision checklist

• State the principle of conservation of energy and explain what “dissipated” means.
• Explain how lubrication and thermal insulation reduce unwanted energy transfer.
• Describe Required practical activity 2 (effectiveness of insulating materials).
• Use efficiency = useful output ÷ total input (for both energy and power).
• (HT only) Describe ways to increase efficiency of an energy transfer.

Related resources

• Conservation and Dissipation of Energy practice questions — worked exam-style questions on this sub-topic
• Conservation and Dissipation of Energy revision notes
• Energy Changes in a System — the previous sub-topic
• National and Global Energy Resources — the next sub-topic
• AQA GCSE Physics hub

This guide is intended to support, not replace, engagement with the official AQA specification and your own teacher’s guidance. Always check the current version of the specification for authoritative detail.