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Energy is one of the most fundamental concepts in physics. In this lesson you will learn about the different energy stores recognised by AQA, the four transfer pathways, and how to describe changes within a system. This topic forms the foundation of the Energy chapter of the AQA GCSE Combined Science Trilogy specification (8464), Section 6.1.
Energy is a quantity measured in joules (J). It cannot be created or destroyed — only transferred between stores. This principle is known as the conservation of energy and applies to every physical process.
In the AQA specification you must describe energy using the language of stores and transfer pathways. Older terms such as "light energy" or "sound energy" are not accepted.
Exam Tip: AQA will penalise answers that use outdated language. Always say "energy is transferred from the kinetic store" rather than "kinetic energy is converted into heat energy." Use stores and pathways consistently.
There are eight energy stores you need to know for AQA GCSE Combined Science (8464).
| Energy Store | Description | Example |
|---|---|---|
| Kinetic | Energy of a moving object | A car travelling along a motorway |
| Internal (thermal) | Total kinetic and potential energy of the particles in an object | A hot cup of coffee |
| Gravitational potential | Energy of an object raised above the ground | A skier at the top of a slope |
| Elastic potential | Energy stored in a stretched or compressed object | A compressed spring in a toy |
| Chemical | Energy stored in chemical bonds | Food, batteries, fossil fuels |
| Magnetic | Energy due to the interaction of magnets or magnetic fields | Two repelling magnets held apart |
| Electrostatic | Energy due to the interaction of electric charges | A charged balloon near a wall |
| Nuclear | Energy stored in the nucleus of an atom | Uranium fuel rods in a reactor |
Exam Tip: Memorise all eight stores. A common 2-mark question gives you a scenario and asks you to identify the relevant stores. Practise naming them until you can list all eight without hesitation.
Energy is transferred between stores by four main pathways.
| Transfer Pathway | Description | Example |
|---|---|---|
| Mechanically | By a force acting on an object (doing work) | Pushing a box across the floor |
| Electrically | By charges moving through a circuit | A current flowing through a kettle element |
| By heating | Due to a temperature difference between objects | A hot radiator warming a room |
| By radiation | By electromagnetic waves (light, infrared, etc.) | The Sun warming the Earth |
graph LR
A["Energy Store A"] -->|"Mechanical / Electrical / Heating / Radiation"| B["Energy Store B"]
B -->|"Some energy always dissipated"| C["Internal energy of surroundings"]
style C fill:#ffcccc,stroke:#cc0000
Exam Tip: When describing a transfer, always state: (1) the starting store, (2) the pathway, and (3) the final store. For example: "Energy is transferred from the chemical store of the battery electrically to the kinetic store of the motor."
A system is a defined group of objects being considered. When a system changes, energy is transferred within it or between it and the surroundings.
| System | Change | Energy Transfer |
|---|---|---|
| Object projected upwards | Rises and slows | Kinetic store → gravitational potential store |
| Vehicle braking | Slows down | Kinetic store → internal (thermal) store of brakes |
| Boiling water on a hob | Water heats up | Chemical store of gas → internal store of water |
| Falling ball | Accelerates downward | Gravitational potential store → kinetic store |
A closed system is one where neither matter nor energy can enter or leave. In a closed system the total energy is constant — energy may transfer between stores but the overall amount does not change.
Exam Tip: If a question states the system is closed, you know total energy is conserved. You can equate the energy before with the energy after to set up equations.
Follow this five-step method in extended-response questions:
A battery-powered fan is switched on.
| Step | Answer |
|---|---|
| System | Battery, motor, fan blades |
| Initial store | Chemical store (battery) |
| Pathway | Electrically (through the circuit), then mechanically (motor turns blades) |
| Useful final store | Kinetic store (fan blades and air) |
| Wasted energy | Internal (thermal) store of the motor (by heating); sound to surroundings (by radiation) |
graph TD
A["Chemical store\n(battery)"] -->|"Electrically"| B["Motor"]
B -->|"Mechanically"| C["Kinetic store\n(fan blades)"]
B -->|"By heating"| D["Internal store\n(motor & surroundings)"]
B -->|"By radiation"| E["Sound to\nsurroundings"]
style D fill:#ffcccc,stroke:#cc0000
style E fill:#ffcccc,stroke:#cc0000
| Mistake | Correction |
|---|---|
| Using "heat energy" or "light energy" | Use "internal (thermal) store" and "radiation pathway" |
| Saying energy is "used up" | Energy is transferred and dissipated, never destroyed |
| Forgetting dissipation | Always mention wasted energy to internal store of surroundings |
| Confusing stores with pathways | A store is where energy is held; a pathway is how it transfers |
A 2 kg rock is dropped from a cliff 45 m high. Describe the energy transfers from release to impact and calculate the kinetic energy just before impact (ignore air resistance, g=10 N/kg).
At release, the rock has gravitational potential energy only:
Ep=mgh=2×10×45=900 J
As it falls, energy is transferred from the gravitational potential store to the kinetic store mechanically (the force of gravity does work on the rock). In the absence of air resistance, no energy is dissipated, so at impact:
Ek=Ep=900 J
In reality, a small amount is transferred to the internal (thermal) store of the air by friction, so the kinetic energy just before impact is slightly less than 900 J.
A drone with a battery stores 72,000 J of chemical energy. During a flight, 45,000 J of energy is used to lift the drone (gravitational potential), 8,000 J becomes kinetic energy of rotating blades, and the rest is dissipated. How much energy is dissipated and by which pathways?
Dissipated=72,000−45,000−8,000=19,000 J
Dissipation pathways include:
A 0.25 kg cup of tea is left on a table. After 20 minutes it has cooled from 85 °C to 45 °C. Describe the energy transfers using AQA language.
Initially the tea has a large internal (thermal) store. Energy is transferred by heating (conduction through the cup and convection currents in the air) and by radiation (infrared radiation from the cup surface). The energy ends up as an increase in the internal store of the surrounding air, cup, and table — the surroundings warm slightly but, because they are so large, the temperature rise is tiny. No energy has been destroyed — it has simply spread out.
| Concept | Category | Examples |
|---|---|---|
| Kinetic | Store | Moving car, flowing water |
| Electrostatic | Store | Charged Van de Graaff dome |
| Mechanical | Pathway | Pushing a trolley |
| Electrical | Pathway | Current heating a wire |
| Heating | Pathway | Flame warming a pan |
| Radiation | Pathway | Sun heating the ground |
Common mistake callout: Candidates often confuse the store with the pathway. Remember: a store is a noun describing where energy is held, while a pathway is a verb-like description of how energy moves. You cannot "store energy in radiation" — radiation is how it moves between stores.
graph LR
A["Chemical store\n(battery)\n100 J"] -->|"Electrically"| B["Bulb"]
B -->|"By radiation\n(useful light)"| C["Radiation store\n10 J"]
B -->|"By heating\n(wasted)"| D["Internal (thermal)\nstore of surroundings\n90 J"]
style D fill:#ffcccc,stroke:#cc0000
Of 100 J of chemical energy in the battery, only 10 J becomes useful light; 90 J is dissipated to the internal (thermal) store of the surroundings. Even though the energy is "wasted," the total is still 100 J — energy is conserved.
| Context | Initial Store | Pathway | Final Useful Store | Main Dissipation |
|---|---|---|---|---|
| Electric kettle | Chemical (mains supply upstream) | Electrically | Internal (thermal) of water | Internal of kettle casing/air |
| Wind turbine | Kinetic (moving air) | Mechanically | Electrical output (to grid) | Internal (bearings, generator) |
| Rollercoaster cart | Gravitational potential (top of hill) | Mechanically | Kinetic | Internal of track, sound |
| Pole vaulter | Kinetic (runner) | Mechanically (pole bending) | Elastic potential → kinetic → gravitational potential | Internal of pole, air |
Grade 3–4 response: "When the fan turns on, chemical energy in the battery becomes kinetic energy in the blades, and some energy is lost as heat."
Grade 5–6 response: "Energy is transferred electrically from the chemical store of the battery to the motor, and then mechanically to the kinetic store of the fan blades. Some energy is dissipated to the internal (thermal) store of the surroundings by heating."
Grade 7–9 response: "The battery's chemical store supplies energy electrically through the circuit. The motor transfers most of this energy mechanically to the kinetic store of the blades, but some is transferred by heating to the internal (thermal) store of the motor windings (due to electrical resistance) and a small amount is transferred by radiation as sound waves to the surroundings. Because energy is conserved, the chemical store decreases by an amount equal to the total increase in the kinetic store plus the dissipated energy. In a genuinely closed system the total energy would remain constant; in practice the fan is not isolated and heat flows out to the surroundings, spreading the energy across a vast number of particles so that it can no longer be usefully recovered."
Use precise terms throughout: kinetic energy, internal (thermal) energy, gravitational potential energy, elastic potential energy, useful vs wasted energy, and the pathways mechanical / electrical / heating / radiation.
AQA alignment: This content is aligned with AQA GCSE Combined Science: Trilogy (8464) specification section 6.1 Energy — specifically 6.1.1 Energy changes in a system, 6.1.1.1 Energy stores and systems, and the introductory ideas of 6.1.2 Conservation and dissipation of energy. Assessed on Physics Paper 1.