How does the energy of a system change when forces act, and how is energy dissipated?
Energy stores and system changes: the ways the energy of a system can change, energy transfers in a closed system, and how energy is dissipated when forces act.
A focused answer to Edexcel GCSE Physics 8.1 to 8.4 and 8.10 to 8.11, covering the ways the energy of a system can be changed, energy transfer diagrams, conservation of energy in a closed system, and how energy is dissipated and wasted as heating when forces do work.
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What this dot point is asking
Edexcel statements 8.1 to 8.4 and 8.10 to 8.11 want you to describe the ways the energy of a system can be changed, to draw and interpret energy transfer diagrams, to explain that the total energy of a closed system does not change, and to explain how energy is dissipated and wasted (raising temperature) when forces do work.
Ways to change a system's energy
A "system" is just the object or group of objects you are considering. Its energy changes when energy is transferred in or out by one of these pathways. For example, an electric kettle's water gains energy by electrical work followed by heating; a lifted box gains energy by mechanical work (a force doing work against gravity).
Energy transfers in a closed system
The closed-system idea lets you account for energy precisely: whatever one store loses, another gains, with the total unchanged. A swinging pendulum (treated as closed, ignoring friction) keeps swapping energy between the gravitational and kinetic stores, with the total constant. Real systems are rarely perfectly closed, which is why some energy escapes as heating.
Dissipation when forces do work
Mechanical processes are never perfectly efficient because friction and resistance always transfer some energy to thermal energy. A thrown ball returns with slightly less kinetic energy because some was dissipated by air resistance; a car coasting slows because friction dissipates its kinetic energy. The energy is not destroyed, just transferred to a less useful store.
How Edexcel examines this
This dot point is examined on both tiers and overlaps with Topic 3, so the ideas of stores, pathways, conservation and dissipation recur. Common questions ask you to describe the energy transfers in a real change (a thrown ball, a pendulum, a car braking) and to explain why the outcome falls short of an ideal (some energy dissipated by friction or air resistance). The mark scheme rewards naming the stores and the direction of transfer, identifying the dissipation to the thermal store of the surroundings, and stating that energy is still conserved overall. A recall question may ask for the ways a system's energy can be changed (work done, electrical, heating, radiation) and the meaning of conservation in a closed system (the total stays constant). Examiners reward the modern store-and-pathway language and penalise saying energy is "lost" or "used up". Linking dissipation to a rise in temperature and to reduced efficiency connects this dot point to the efficiency statement that follows.
Try this
Q1. Name two ways the energy of a system can be changed. [2 marks]
- Cue. By work done by forces (mechanically) and by heating (also electrically or by radiation).
Q2. State what happens to the total energy of a closed system. [1 mark]
- Cue. It stays the same (energy is conserved).
Exam-style practice questions
Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Edexcel 20204 marksA ball is thrown upwards, rises, and falls back to the thrower's hand. Describe the energy transfers during the rise and fall, and explain why the ball returns with slightly less kinetic energy than it left with.Show worked answer →
As the ball rises, energy is transferred from the kinetic store to the gravitational potential store (1 mark). As it falls, energy is transferred back from the gravitational potential store to the kinetic store (1 mark). Throughout, some energy is transferred to the thermal store of the surroundings because of air resistance (work done against air resistance) (1 mark). So when the ball returns, some of the original kinetic energy has been dissipated, and it has slightly less kinetic energy than it started with, although energy is still conserved overall (1 mark). Markers reward the kinetic and gravitational transfers in both directions and the dissipation to the thermal store reducing the returning kinetic energy.
Edexcel 20223 marksState the different ways the energy of a system can be changed, and explain what is meant by saying energy is conserved in a closed system.Show worked answer →
The energy of a system can be changed by work done by forces (mechanically), by electrical work (a current), by heating, or by radiation (1 mark for two or more ways). Energy is conserved in a closed system means that the total energy of the system stays the same: energy can be transferred between stores within the system but none is created or destroyed (2 marks). Markers reward naming ways to change a system's energy (work done, heating, electrical, radiation) and the statement that the total energy of a closed system is constant.
Related dot points
- Work done and energy transfer: the work done equation, the link between work done and energy transferred, and how work done by friction raises temperature.
A focused answer to Edexcel GCSE Physics 8.5 to 8.7, covering the work done equation, the idea that work done by a force equals the energy transferred, the joule as a newton metre, and how work done against friction raises temperature, with worked calculations.
- Power: power as the rate of energy transfer or work done, the power equation, the watt as a joule per second, and the core practical measuring personal power.
A focused answer to Edexcel GCSE Physics 8.12 to 8.14, covering the definition of power as the rate of energy transfer or work done, the power equation, the watt as a joule per second, comparing devices by power, and the core practical measuring personal power, with worked calculations.
- Efficiency of forces: calculating efficiency for a machine, why machines waste energy by heating, and reducing wasteful transfers by lubrication and streamlining.
A focused answer to Edexcel GCSE Physics 8.15 (and 8.10 to 8.11), covering the efficiency equation for a machine doing work, why mechanical processes waste energy as heat, and how lubrication and streamlining reduce wasteful energy transfers, with worked calculations.
- Energy stores and transfers: the named energy stores, the ways energy is transferred, and drawing and interpreting energy transfer diagrams for everyday systems.
A focused answer to Edexcel GCSE Physics 3.3 to 3.5, covering the named energy stores, the four pathways by which energy is transferred, drawing energy transfer diagrams, and analysing the energy changes when systems such as a falling object or a kettle change.
- Conservation and dissipation of energy: the principle of conservation of energy in a closed system, how energy is dissipated to less useful stores, and why mechanical processes waste energy by heating.
A focused answer to Edexcel GCSE Physics 3.4 and 3.6 to 3.8, covering the principle of conservation of energy, why the total energy in a closed system does not change, how energy is dissipated to less useful stores, and why mechanical processes waste energy by heating the surroundings.
Sources & how we know this
- Pearson Edexcel GCSE (9-1) Physics (1PH0) specification — Pearson (2016)