Energy Stores and Transfers: KS3 Physics Guide

Energy is stored in objects and systems, and it is transferred between stores by pathways. The eight energy stores include kinetic, thermal, chemical, gravitational potential, elastic potential, magnetic, electrostatic, and nuclear. The four energy transfer pathways are heating, work done by a force, electrical work, and radiation. Understanding this framework is a core KS3 physics requirement.

Why do we talk about energy stores and transfers?

Before around 2016, KS3 and GCSE physics described energy using "types of energy" — kinetic energy, heat energy, light energy, and so on. The AQA, OCR, and Edexcel specifications now use the stores and transfers model, which is more physically accurate because:

Energy is stored in an object or system.

Energy moves between stores via specific transfer pathways (mechanisms).

The previous language — "heat energy" or "light energy" — confused the store with the pathway. Heat (heating) and light (radiation) are transfer pathways, not stores. Energy is stored as thermal energy, not "heat." This distinction matters at GCSE and is worth learning correctly from Year 7.

The eight energy stores

Energy store | Description | Example
Kinetic | Stored in moving objects | A rolling football
Thermal | Stored in the internal energy of a substance | A hot cup of tea
Chemical | Stored in the bonds of chemical substances | Food, fuel, batteries
Gravitational potential | Stored due to position above a reference point | A book on a shelf
Elastic potential | Stored in a stretched or compressed object | A stretched rubber band
Magnetic | Stored in the field between magnets | Two attracting magnets
Electrostatic | Stored in the field between charged objects | A charged balloon near a wall
Nuclear | Stored in atomic nuclei | Uranium in a nuclear reactor

At KS3, the most commonly assessed stores are: kinetic, thermal, chemical, gravitational potential, and elastic potential. Magnetic, electrostatic, and nuclear are encountered more at GCSE and A-level.

The four energy transfer pathways

Energy moves from one store to another through one of four pathways:

Heating — energy transferred due to a temperature difference. Energy flows from a hotter object to a cooler one (never the other way). Example: a hot radiator warms the air in a room.

Work done by a force (mechanical work) — energy transferred when a force moves an object. Example: pushing a bicycle pedal down does work on the chain and transfers energy to the kinetic store of the bicycle.

Electrical work — energy transferred by an electric current. Example: a battery drives a current through a circuit, transferring energy from the battery's chemical store to the kinetic store of a motor.

Radiation — energy transferred by electromagnetic waves (including light, infrared, radio waves). Example: the Sun transfers energy to the Earth's surface via radiation through space.

A worked example: a ball rolling off a table

Track the energy stores and transfers as a ball rolls across a table and falls to the floor.

Starting position: ball sitting still on a high table

The ball has energy in its gravitational potential store (due to its height above the ground).

It has zero kinetic energy because it is stationary.

Ball rolls across the table

Someone applies a force to set it rolling — mechanical work transfers energy from the person's chemical store (from food) into the ball's kinetic store.

The ball also still has gravitational potential energy while it is on the table.

Ball falls off the table edge

As the ball falls, the gravitational potential store decreases and the kinetic store increases — energy is transferred from one store to the other as the ball speeds up. This is NOT a transfer pathway; both stores are within the same system (the ball + Earth).

In a simplified model (ignoring air resistance), all the gravitational potential energy converts to kinetic energy by the time the ball hits the ground.

Ball hits the floor

On impact, the kinetic store decreases rapidly.

Energy is transferred to the thermal store of the ball and the floor (they warm up very slightly) via the heating pathway — the collision generates friction.

If the ball bounces, some energy goes temporarily into the elastic potential store of the ball before returning to the kinetic store.

Sound waves also carry energy away — this is also a form of radiation.

Conservation of energy

The principle of conservation of energy states: energy cannot be created or destroyed; it can only be transferred between stores or pathways. The total energy in a closed system always stays the same.

This is one of the most important principles in all of physics. It means:

You can never get more energy out of a system than you put in.

If energy seems to "disappear," it has been transferred to the thermal store (usually by heating due to friction or resistance) — it has not been destroyed.

In most real systems, some energy is always transferred to the thermal store via heating (friction, air resistance, resistance in wires). This energy is said to be dissipated — not destroyed, but spread out in the surroundings in a way that makes it very difficult to use again.

Example: A wind-up toy has energy in its elastic potential store. When released, that energy transfers to the kinetic store (the toy moves) and to the thermal store of the surroundings (friction in the mechanism and between wheels and floor). If you measure all the stores carefully, the total energy is the same as the original elastic potential energy stored in the wound spring.

Efficiency

Not all energy transferred from a source ends up in a useful store — some always goes to the thermal store via heating. Efficiency measures what fraction of the input energy ends up in a useful output store:

Efficiency = useful energy output ÷ total energy input

Or as a percentage: efficiency (%) = (useful energy output ÷ total energy input) × 100

Example: An electric motor is supplied with 200 J of electrical energy. It transfers 160 J to the kinetic store (useful output) and 40 J to the thermal store (wasted, due to friction and electrical resistance).

Efficiency = 160 ÷ 200 = 0.8, or 80%

No device is 100% efficient in practice — some energy always ends up in the thermal store. The goal in engineering is to minimise wasted energy and maximise useful output.

Sankey diagrams

A Sankey diagram is a visual tool for representing energy transfers. The width of the arrow represents the amount of energy.

Diagram in words: Imagine a wide arrow entering from the left, representing total input energy (say, 200 J). As it passes through a box labelled "Motor," it splits into two arrows going right. The upper arrow is wide (160 J) — the useful kinetic energy output. The lower arrow bends downward and is narrower (40 J) — the wasted thermal energy. The widths add up to the original arrow. This proportional representation makes it easy to see at a glance how much energy is wasted.

At KS3, you may be asked to interpret a Sankey diagram given in a question, or to draw a simple one.

What does the national curriculum say?

The Department for Education's Science Programmes of Study for Key Stage 3 states that pupils should understand "the concept of energy and calculate energy stores and transfers" and "that energy is quantitative, that you can calculate changes in energy stored in a system, and the conservation and dissipation of energy." BBC Bitesize KS3 physics covers energy stores, transfer pathways, conservation, and efficiency as core content.

Frequently asked questions

What are the eight energy stores at KS3?

The eight energy stores are: kinetic, thermal, chemical, gravitational potential, elastic potential, magnetic, electrostatic, and nuclear. At KS3 the most commonly assessed are kinetic, thermal, chemical, gravitational potential, and elastic potential.

What is the difference between an energy store and an energy transfer?

An energy store is where energy is held — for example, in a moving object (kinetic store) or in food (chemical store). An energy transfer is the process by which energy moves from one store to another — the four pathways are heating, mechanical work, electrical work, and radiation.

What does conservation of energy mean?

Conservation of energy means energy cannot be created or destroyed — only transferred between stores or pathways. The total energy in a closed system always remains the same. If energy appears to "disappear," it has been transferred to the thermal store (usually by friction or resistance) rather than destroyed.

How do you calculate efficiency?

Efficiency = useful energy output ÷ total energy input. Multiply by 100 to express it as a percentage. An efficiency of 80% means 80% of input energy goes to a useful output store and 20% is wasted (usually to the thermal store via heating).

Why is no device 100% efficient?

In any real device, some energy is always transferred to the thermal store via heating — because friction and electrical resistance are impossible to eliminate completely. The wasted thermal energy spreads into the surroundings and cannot easily be recovered for use. This is why machines need a continuous energy input and why fossil fuels (or other energy sources) are consumed constantly.

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