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Energy is one of the most important concepts in physics. In this lesson you will learn about the different energy stores, how energy is transferred between stores, and how to describe changes within a system. This topic forms the foundation of the Energy chapter of the AQA GCSE Physics specification (Section 4.1).
Energy is a quantity that is conserved in every process. It cannot be created or destroyed, only transferred from one store to another. The total energy in a closed system always remains the same.
In physics, we describe energy in terms of stores and transfers. A store is where energy is held, and a transfer is the process by which energy moves between stores.
Exam Tip: AQA no longer uses terms like "light energy" or "sound energy." Instead, you must describe energy in terms of stores and the pathways by which energy is transferred. Make sure your vocabulary matches the specification.
There are eight energy stores you need to know for AQA GCSE Physics. Each store describes a different way in which energy can be held within a system.
| Energy Store | Description | Example |
|---|---|---|
| Kinetic | Energy of a moving object | A car driving along a road |
| Internal (thermal) | Total kinetic and potential energy of the particles in an object | A hot cup of tea |
| Gravitational potential | Energy of an object raised above the ground | A book on a high shelf |
| Elastic potential | Energy stored in a stretched or compressed object | A stretched rubber band |
| Chemical | Energy stored in chemical bonds | Food, batteries, fuels |
| Magnetic | Energy due to the interaction of magnets or magnetic fields | Two repelling magnets held close together |
| 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 in a nuclear reactor |
Exam Tip: You must be able to name and describe all eight energy stores. A common exam question gives you a scenario and asks you to identify the relevant energy stores and transfers. Practise listing them from memory.
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 trolley along a bench |
| Electrically | By charges moving through a circuit | A current flowing through a filament lamp |
| By heating | Due to a temperature difference between objects | A hot pan warming cold water |
| By radiation | By waves such as light, infrared, or sound | The Sun warming the Earth |
graph LR
A[Energy Store 1] -->|Mechanical / Electrical / Heating / Radiation| B[Energy Store 2]
B -->|Some energy is always dissipated| C[Internal energy of surroundings]
style C fill:#ffcccc,stroke:#cc0000
Exam Tip: When answering questions about energy transfers, always state the starting energy store, the pathway of transfer, and the final energy store. For example: "Chemical energy in the battery is transferred electrically to the kinetic energy store of the motor."
A system is a defined group of objects. When a system changes, energy is transferred within or between stores.
| System | Change | Energy Transfer Description |
|---|---|---|
| An object projected upwards | Object rises and slows down | Kinetic store decreases, gravitational potential store increases |
| A vehicle braking | Vehicle slows down | Kinetic store decreases, internal (thermal) energy store of brakes increases |
| Bringing water to a boil | Water is heated on a stove | Chemical store of gas decreases, internal (thermal) energy store of water increases |
| A ball falling | Ball accelerates downwards | Gravitational potential store decreases, kinetic store increases |
A closed system is one where no matter or energy enters or leaves. In a closed system, the total energy is constant. Energy can be transferred between stores within the system, but the total remains unchanged.
Exam Tip: The concept of a "closed system" is used frequently in exam questions. If a question tells you the system is closed, this means total energy is conserved and you can set up equations where energy before equals energy after.
When describing energy changes in a system, follow this structure:
Scenario: A battery-powered toy car is switched on and moves across the floor.
| Step | Answer |
|---|---|
| System | Battery, motor, wheels, car body |
| Initial store | Chemical energy store (battery) |
| Transfer pathway | Electrically (through the circuit) and mechanically (by the motor) |
| Final useful store | Kinetic energy store (car moves) |
| Wasted energy | Internal (thermal) energy store of motor, gears and surroundings (by heating); sound (by radiation) |
graph TD
A[Chemical store of battery] -->|Electrically| B[Motor]
B -->|Mechanically| C[Kinetic store of car]
B -->|By heating| D[Internal energy of motor and surroundings]
B -->|By radiation| E[Sound to surroundings]
style D fill:#ffcccc,stroke:#cc0000
style E fill:#ffcccc,stroke:#cc0000
The law of conservation of energy states that energy can be transferred usefully, stored, or dissipated, but it cannot be created or destroyed. The total energy of a closed system is always constant.
This is one of the most fundamental principles in all of physics. Every energy calculation you perform at GCSE is based on this law.
| Key Point | Detail |
|---|---|
| Energy is conserved | Total energy before = total energy after |
| Energy can be transferred | Between stores and via pathways |
| Energy can be dissipated | Spread out into the surroundings, usually as internal (thermal) energy |
| Energy cannot be created or destroyed | This is an absolute rule in physics |
In every real-world energy transfer, some energy is dissipated — transferred to the surroundings in a way that is not useful. This wasted energy usually ends up in the internal (thermal) energy store of the surroundings.
For example, when a light bulb is switched on:
The dissipated energy has not been destroyed. It has been spread out and is now stored in the thermal energy of many particles in the surroundings, making it very difficult to do anything useful with.
Exam Tip: AQA examiners want to see you use the phrase "internal (thermal) energy of the surroundings" rather than just "heat." This shows you understand that the energy has been transferred to the particles of the surroundings and is now stored, not lost.
Common mistake: Students often write "kinetic energy is transferred to sound energy." Sound is not an energy store — it is a form of radiation pathway. The correct statement is: "the kinetic energy store decreases; energy is transferred by sound waves (radiation) to the internal energy store of the surroundings."
Scenario: A 0.2 kg ball is dropped from a height of 2 m onto the floor. After bouncing, it reaches a maximum height of 1.5 m. (Use g = 10 N/kg.)
Step 1 — Identify the initial GPE: E_p = m x g x h = 0.2 x 10 x 2 = 4.0 J stored in the gravitational potential energy store of the ball and Earth system.
Step 2 — Just before impact: By conservation of energy, all 4.0 J has been transferred (mechanically, by the ball falling) to the kinetic energy store of the ball.
Step 3 — After bouncing to 1.5 m: E_p = 0.2 x 10 x 1.5 = 3.0 J now stored as gravitational potential energy.
Step 4 — Dissipated energy: Dissipated = 4.0 - 3.0 = 1.0 J transferred to the internal (thermal) energy of the ball, the floor and the surrounding air (by heating), and some by sound radiation.
Notice that the total energy is conserved — the 1.0 J has not been lost, merely spread so thinly across many particles that it can no longer be recovered usefully.
A closed system has a boundary across which no energy or matter can flow. In reality, nearly every school-physics scenario is an approximately closed system — we assume it is closed in order to simplify calculations.
| System Type | Definition | Typical Example |
|---|---|---|
| Closed | No energy or matter crosses the boundary | A perfectly insulated flask of water |
| Open | Energy and/or matter can cross the boundary | A kettle boiling with the lid off (steam escapes) |
When answering an exam question, if the examiner says "assume the system is closed" or "ignore air resistance", they are signalling that you should apply strict conservation of energy and equate the stores before and after.
Exam Tip: The phrase "ignore air resistance" is your cue to write an energy-conservation equation, e.g. E_p (at top) = E_k (at bottom), and solve it.
Question: A battery-powered torch is switched on. Describe the energy transfers that take place when the torch is in use. [3 marks]
Grade 4–5 answer:
Chemical energy in the battery turns into light energy and some heat.
This identifies the starting store and mentions the output, but it uses outdated language ("light energy", "heat"), does not name transfer pathways, and does not mention dissipation.
Grade 8–9 answer:
Energy is stored in the chemical energy store of the battery. When the switch is closed, energy is transferred electrically through the circuit to the bulb. At the bulb, energy is transferred by radiation (visible light) to the surroundings — this is the useful transfer. At the same time, some energy is transferred by heating to the internal (thermal) energy store of the bulb, wires and surrounding air — this is the dissipated (wasted) energy. The total energy is conserved: useful light output + wasted thermal output = chemical energy used by the battery.
This response names the initial store, specifies both transfer pathways, distinguishes useful and wasted transfers, uses the phrase "internal (thermal) energy", and explicitly applies conservation of energy.
Exam Tip: Energy stores and systems questions appear on almost every AQA Physics paper. Make sure you can describe transfers using the correct vocabulary — store names, pathway names, and the concept of dissipation. Avoid outdated terms like "heat energy" or "light energy."
AQA alignment: This content is aligned with AQA GCSE Physics (8463) specification section 4.1 Energy — specifically 4.1.1.1 Energy stores and systems, 4.1.1.2 Changes in energy, and 4.1.2.1 Energy transfers in a system. Assessed on Paper 1.