Energy Stores and Transfers

This lesson introduces the concept of energy stores and energy transfers, as required by the Edexcel GCSE Combined Science specification (1SC0). Understanding how energy is stored and moved between stores is the foundation of the entire energy topic.

What Is Energy?

Energy is a quantity measured in joules (J). It cannot be created or destroyed — only transferred from one store to another. This is the principle of conservation of energy.

Energy enables things to happen: heating a room, moving a car, lighting a lamp or powering a phone all require energy transfers.

Exam Tip: You will never be asked to define energy in a single sentence, but you must be able to describe energy in terms of stores and transfers. Avoid outdated language such as "forms of energy" — the specification uses stores and pathways.

The Eight Energy Stores

The Edexcel specification recognises eight energy stores. You must know all of them and be able to give examples.

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 stretched rubber band
Thermal	Energy related to the temperature of an object	A hot cup of tea
Chemical	Energy stored in chemical bonds	Food, batteries, fossil fuels
Magnetic	Energy due to magnets attracting or repelling	Two repelling magnets held apart
Electrostatic	Energy due to charges attracting or repelling	A charged balloon near a wall
Nuclear	Energy stored in the nucleus of an atom	Uranium fuel in a nuclear reactor

Key Points

Every object or system has energy in one or more of these stores at any given time.
When something happens (a change), energy is transferred between stores.
The thermal store of an object increases when its temperature rises.

Energy Transfer Pathways

Energy moves between stores along four pathways (sometimes called transfer mechanisms).

Pathway	How It Works	Example
Mechanically	A force moves an object (work done)	Pushing a trolley
Electrically	A current flows through a circuit	A lamp connected to a battery
By heating	Energy moves from a hotter region to a cooler one	A radiator warming a room
By radiation	Energy is transferred by waves (light, sound, infrared)	The Sun warming the Earth

Exam Tip: In exam answers, always name both the stores involved and the pathway. For example: "Energy is transferred from the chemical store of the battery to the thermal store of the filament electrically, and then to the thermal store of the surroundings by radiation."

Describing Energy Transfers

To describe an energy transfer fully you should state:

The starting energy store (and the object).
The pathway by which energy is transferred.
The end energy store (and the object).

Worked Example 1

A ball is dropped from a height.

Energy is transferred from the gravitational potential store of the ball
mechanically (by the gravitational force)
to the kinetic store of the ball.
As the ball hits the ground, energy is transferred from the kinetic store to the thermal store of the ball and the ground by heating.

Worked Example 2

A battery-powered torch is switched on.

Energy is transferred from the chemical store of the battery
electrically through the circuit
to the thermal store of the filament, which then transfers energy by radiation (light and infrared) to the surroundings.

Energy Transfer Diagrams

Energy transfers can be represented using simple flow diagrams. Arrows show the direction of transfer and can be labelled with the pathway.

flowchart LR
    A["Chemical store\n(fuel)"] -->|Mechanically| B["Kinetic store\n(car)"]
    B -->|By heating| C["Thermal store\n(surroundings)"]
    A -->|By heating| C

How to Draw an Energy Transfer Diagram

Identify the initial store and place it on the left.
Identify the useful end store and place it on the right.
Draw arrows between stores and label each arrow with the transfer pathway.
Include wasted energy stores (usually thermal) below or to the side.

Systems

A system is an object or group of objects that you are studying. When a system changes, energy is transferred. You define the boundary of the system depending on the question.

System	Change	Transfer
A kettle and its water	Water is heated	Chemical store of electricity supply → thermal store of water
A ball and the Earth	Ball is thrown upward	Kinetic store → gravitational potential store
A spring and a trolley	Compressed spring released	Elastic potential store → kinetic store

Exam Tip: When answering a question about a system, first identify which objects are inside the system and which are outside. Energy leaving the system is said to be dissipated to the surroundings.

Common Misconceptions

Misconception	Correction
"Energy is used up"	Energy is transferred, not used up. The total energy is always conserved.
"Heat is a store"	Heat is a pathway (by heating). The store is thermal.
"Light energy"	Light is a pathway (radiation). Energy is in the thermal or nuclear store of the source.
"Sound energy"	Sound is a transfer by mechanical waves. The energy ends up in the thermal store.

Quantifying Energy

Energy is measured in joules (J). Larger quantities use:

Unit	Symbol	Equivalent
kilojoule	kJ	1 kJ = 1 000 J
megajoule	MJ	1 MJ = 1 000 000 J
gigajoule	GJ	1 GJ = 1 × 10⁹ J

Worked Example 3

A heater transfers 3.5 MJ of energy. Express this in joules.

$$3.5 \text{ MJ} = 3.5 \times 10^6 \text{ J} = 3,500,000 \text{ J}$$

Summary

Energy is stored in eight stores: kinetic, gravitational potential, elastic potential, thermal, chemical, magnetic, electrostatic and nuclear.
Energy is transferred between stores along four pathways: mechanically, electrically, by heating and by radiation.
A system is the object or group of objects being studied.
Energy is conserved — it is never created or destroyed, only transferred.
Always describe energy changes using the correct store and pathway language.
Energy is measured in joules (J).