Energy Stores and Systems

Energy is a fundamental concept that underpins every area of physics. The AQA specification requires you to understand how energy is stored, transferred between stores, and conserved within a system. This lesson introduces the eight energy stores, the four transfer pathways, and the concept of a system — the foundation for every energy calculation and explanation you will meet in the course.

What Is a System?

A system is an object or a group of objects that you choose to study. Everything outside the system is called the surroundings. When you define a system, you can then describe energy changes within it and energy transfers between the system and surroundings.

Examples of systems:

System	What It Includes
A ball thrown upwards	The ball and the Earth
An electric kettle	The kettle, the water, and the heating element
A car braking	The car, the brakes, and the road surface
A stretched spring	The spring

The Eight Energy Stores

AQA identifies eight energy stores. Energy can be transferred between these stores but the total energy in a closed system is always conserved.

Energy Store	Description	Example
Kinetic	Energy of a moving object	A car driving along a road
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 drawn bowstring
Thermal	Energy related to the temperature of an object (internal energy)	A hot cup of tea
Chemical	Energy stored in chemical bonds	Food, fuels, batteries
Nuclear	Energy stored in the nucleus of an atom	Uranium fuel rods
Electrostatic	Energy of separated or interacting charges	A charged balloon near a wall
Magnetic	Energy of magnets that are attracting or repelling	Two repelling bar magnets held apart

Exam Tip: You must use the correct AQA store names. Do not write "sound energy" or "light energy" — these are transfer pathways, not stores.

The Four Transfer Pathways

Energy moves between stores via four pathways:

Pathway	How Energy Is Transferred	Example
Mechanical (by forces)	A force moves an object — work is done	Pushing a trolley
Electrical	Charge flows through a circuit	A lamp connected to a battery
Heating	Energy moves from a hotter to a cooler region	A radiator warming a room
Radiation	Energy transferred by electromagnetic waves	The Sun warming the Earth

flowchart LR
    CS["Chemical Store\n(battery)"] -->|"Electrical pathway"| KS["Kinetic Store\n(motor spins)"]
    KS -->|"Mechanical pathway\n(friction)"| TS["Thermal Store\n(motor heats up)"]
    TS -->|"Heating pathway"| SU["Surroundings"]
    style CS fill:#10b981,color:#fff,stroke:#059669
    style KS fill:#3b82f6,color:#fff,stroke:#2563eb
    style TS fill:#ef4444,color:#fff,stroke:#dc2626
    style SU fill:#6b7280,color:#fff,stroke:#4b5563

Describing Energy Transfers

A complete energy description must state:

The starting store.
The transfer pathway.
The ending store.

Worked Example 1 — Dropping a Ball

A ball is held above the ground, then released.

Stage	Store	Explanation
Ball held high	Gravitational potential	Ball is above ground level
Ball falling	Gravitational potential → Kinetic	Height decreases, speed increases
Ball hits ground	Kinetic → Thermal (and some sound via mechanical waves)	Ball and ground warm up slightly

Worked Example 2 — An Electric Kettle

Stage	Store	Pathway
Plugged in, switched on	Chemical (power station fuel)	Electrical pathway through the grid
Element heats up	Electrical → Thermal	Heating pathway into the water
Steam rises	Thermal → Kinetic (of steam particles)	Heating to surroundings

Worked Example 3 — A Catapult Firing a Stone

You pull back the elastic band → energy is transferred mechanically from your chemical store to the elastic potential store of the band.
You release → elastic potential → kinetic store of the stone (mechanical pathway).
The stone rises → kinetic → gravitational potential.
Air resistance → kinetic → thermal store of the air (heating pathway).

Closed and Open Systems

Type	Definition	Energy Transfer
Closed system	No matter or energy enters or leaves	Total energy is constant
Open system	Energy (and sometimes matter) can enter or leave	Total energy of the system can change

In GCSE physics, when we say "energy is conserved", we mean the total energy in a closed system does not change — it just moves between stores.

Energy Flow Diagrams

You may be asked to draw or interpret energy flow diagrams (Sankey diagrams are covered later). A simple description looks like this:

flowchart TD
    A["Input energy\n(chemical store of fuel)"] --> B["Useful output\n(kinetic store of car)"]
    A --> C["Wasted output\n(thermal store of engine and surroundings)"]
    style A fill:#3b82f6,color:#fff,stroke:#2563eb
    style B fill:#10b981,color:#fff,stroke:#059669
    style C fill:#ef4444,color:#fff,stroke:#dc2626

Common Exam Mistakes

Mistake	Correction
Writing "light energy" or "sound energy" as a store	Light and sound are transfer pathways (radiation and mechanical waves), not stores
Confusing thermal and chemical stores	Chemical = energy in bonds (food, fuel). Thermal = energy due to temperature
Saying "energy is used up"	Energy is never used up — it is transferred to other stores (usually thermal)
Not naming the pathway	A full answer needs the pathway (mechanical, electrical, heating, or radiation)
Forgetting gravitational potential needs the Earth	GPE exists because of the gravitational field between the object and the Earth — the system must include both

Quick-Fire Practice Table

For each scenario, identify the main energy stores involved.

Scenario	Start Store	End Store(s)
Burning a candle	Chemical	Thermal + Radiation (light)
A wind turbine generating electricity	Kinetic (wind)	Kinetic (turbine) → Electrical → various
Stretching a rubber band	Chemical (your muscles)	Elastic potential
A nuclear power station	Nuclear	Thermal → Kinetic → Electrical
A phone charging	Electrical	Chemical (battery)
A rollercoaster going downhill	Gravitational potential	Kinetic (+ thermal via friction)

Summary

Energy is stored in eight stores: kinetic, gravitational potential, elastic potential, thermal, chemical, nuclear, electrostatic, and magnetic.
Energy is transferred by four pathways: mechanical, electrical, heating, and radiation.
A system is the object(s) you are studying. A closed system has constant total energy.
Always describe energy changes using the correct store names and pathway names.
Energy is never created or destroyed — only transferred between stores.

Worked Example 4 — A Complete Energy Description (6-Mark Style)

Question: Describe the energy transfers when a battery-powered toy car drives up a ramp and then stops.

Answer:

Initially, energy is in the chemical store of the battery.
Energy is transferred via the electrical pathway to the motor.
The motor transfers energy via the mechanical pathway to the kinetic store of the car (the car moves).
As the car moves up the ramp, energy transfers from the kinetic store to the gravitational potential store (the car gains height).
Friction between the wheels and ramp transfers energy from the kinetic store to the thermal store of the wheels, ramp, and surrounding air via the heating pathway.
When the car stops at the top, the remaining energy is in the gravitational potential store and the thermal store of the surroundings.
Throughout the process, total energy is conserved — the energy in the chemical store has been shared between GPE and thermal stores.