Edexcel GCSE Physics Topic 3 Conservation of energy: a complete overview of energy stores, GPE and kinetic energy, dissipation, insulation and efficiency

What Topic 3 actually demands

Conservation of energy is a Paper 1 topic that combines calculation (the energy equations and efficiency) with clear conceptual explanation (stores, transfers, dissipation and conservation). Examiners reward both fluent maths and the precise modern language of energy stores and pathways.

This guide walks through all five dot points of the topic, then sets out the exam patterns Pearson repeats. Each dot point has a matching page with practice questions; this overview ties them together.

Gravitational and kinetic energy

The change in gravitational potential energy is $\Delta GPE = m \times g \times \Delta h$ (using the vertical height), and the kinetic energy of a moving object is $KE = \frac{1}{2} \times m \times v^2$ (always squaring the speed). As an object falls, GPE transfers to the kinetic store; as it rises, the reverse happens. Setting $\Delta GPE = KE$ links a falling object's speed to the height fallen.

Energy stores and transfers

Energy is held in stores (kinetic, gravitational potential, elastic, thermal, chemical, nuclear, magnetic, electrostatic) and moved by pathways (mechanically, electrically, by heating, by radiation). An energy transfer diagram names the store that loses energy, the store that usefully gains it, and the store (usually thermal, in the surroundings) that gains the wasted energy.

Conservation and dissipation

The principle of conservation of energy states that energy cannot be created or destroyed, only transferred or dissipated, and the total in a closed system is constant. Dissipation spreads energy into less useful stores, almost always the thermal store of the surroundings. Mechanical processes waste energy because friction and resistance raise the temperature.

Reducing unwanted transfers

Lubrication reduces friction in machines, so less energy is dissipated as heat. Thermal insulation slows the transfer of thermal energy out of a warm space. A building cools more slowly with thicker walls and walls of lower thermal conductivity, because both slow the flow of thermal energy to the outside.

Efficiency

Efficiency $= \frac{\text{useful energy transferred}}{\text{total energy supplied}}$ (or the power form), with no unit, multiplied by $100$ for a percentage. It is always below $100\%$ for a real device because some energy is always dissipated. Sankey diagrams show the useful and wasted energy flows, whose widths add up to the input.

How Topic 3 is examined

A typical Edexcel profile for Conservation of energy:

• Calculations. GPE, kinetic energy and efficiency, sometimes combined by equating GPE lost to KE gained.
• Stores and transfers. Describing or drawing energy transfer diagrams using store-and-pathway language.
• Explanations. Why real outcomes fall short of ideal calculations, and how to reduce unwanted transfers.
• Efficiency and Sankey diagrams. Reading useful and wasted energy and calculating efficiency.

Check your knowledge

A mix of recall and calculation questions covering Topic 3. Attempt them under timed conditions, then check against the solutions.

1. Calculate the change in GPE when a $4\,\text{kg}$ mass is raised $2\,\text{m}$ ($g = 10\,\text{N/kg}$). (2 marks)
2. Calculate the kinetic energy of a $3\,\text{kg}$ object moving at $4\,\text{m/s}$. (2 marks)
3. State the principle of conservation of energy. (2 marks)
4. Name the energy store that dissipated energy usually ends up in. (1 mark)
5. Name one way to reduce unwanted energy transfer in a machine. (1 mark)
6. State the effect of lower thermal conductivity walls on a building's rate of cooling. (1 mark)
7. A device transfers $40\,\text{J}$ usefully from a total of $160\,\text{J}$. Calculate the efficiency as a percentage. (2 marks)
8. State why a real device can never be $100\%$ efficient. (1 mark)