Energy Stores and Systems

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).

What Is Energy?

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.

Energy Stores

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 Transfer Pathways

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."

Systems

A system is a defined group of objects. When a system changes, energy is transferred within or between stores.

Examples of Energy Changes in Systems

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

Closed Systems

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.

Describing Energy Transfers

When describing energy changes in a system, follow this structure:

Identify the system — what objects are involved?
Identify the initial energy store(s) — where is the energy stored at the start?
State the transfer pathway — how is the energy transferred?
Identify the final energy store(s) — where is the energy stored at the end?
Mention dissipation — some energy is always transferred to the internal energy of the surroundings (wasted).

Worked Example

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

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

Dissipation of Energy

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:

Useful energy transfer: electrical to light (by radiation)
Wasted energy transfer: electrical to internal (thermal) energy of the bulb and surroundings (by heating)

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.

Summary

Energy is stored in eight energy stores: kinetic, internal (thermal), gravitational potential, elastic potential, chemical, magnetic, electrostatic, and nuclear.
Energy is transferred between stores by four pathways: mechanically, electrically, by heating, and by radiation.
A system is a defined group of objects; when a system changes, energy is transferred.
In a closed system, the total energy remains constant.
The law of conservation of energy states that energy cannot be created or destroyed.
In every transfer, some energy is dissipated to the internal energy of the surroundings.
When describing energy changes, always state the stores, pathways, and any dissipation.

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."