Energy Stores and Transfers

Energy is the capacity to do work or cause change. It cannot be created or destroyed — only transferred between stores or dissipated. At GCSE level, you must understand the different energy stores, identify how energy is transferred between them, and describe energy changes in a wide range of scenarios.

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Energy Stores

Energy exists in different stores. The Edexcel specification uses the following energy store model:

Energy Store	Description	Example
Kinetic	Energy of a moving object	A car driving along a road
Gravitational potential	Energy due to height in a gravitational field	A book on a shelf
Elastic potential	Energy stored in a stretched or compressed object	A compressed spring
Thermal (internal)	Energy related to the temperature of an object	A hot cup of tea
Chemical	Energy stored in chemical bonds	Food, fuels, batteries
Nuclear	Energy stored in atomic nuclei	Uranium fuel rods
Electrostatic	Energy due to separated charges	A charged capacitor
Magnetic	Energy due to magnetic fields	Two repelling magnets pushed together

flowchart TD
    E["Energy Stores"]
    E --> K["Kinetic\n(moving objects)"]
    E --> G["Gravitational PE\n(height above ground)"]
    E --> EL["Elastic PE\n(stretched/compressed)"]
    E --> TH["Thermal\n(hot objects)"]
    E --> CH["Chemical\n(fuels, food, batteries)"]
    E --> NU["Nuclear\n(atomic nuclei)"]
    E --> ES["Electrostatic\n(charges)"]
    E --> MA["Magnetic\n(magnets)"]

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Energy Transfer Pathways

Energy moves between stores via four main pathways:

Transfer Pathway	Description	Example
Mechanically (by forces)	A force moves an object, transferring energy	Pushing a trolley (chemical → kinetic)
Electrically	Energy transferred by electric current	A battery lighting a bulb (chemical → light + thermal)
By heating	Energy transferred due to temperature difference	Hot water cooling down (thermal → surroundings)
By radiation	Energy transferred by waves (light, sound, infrared)	The Sun warming the Earth (nuclear → thermal via radiation)

Worked Example 1 — Describing Energy Transfers

Question: Describe the energy transfers when a ball is thrown upward, reaches its highest point, and falls back down.

Solution:

1. Throwing upward: Chemical energy in muscles → kinetic energy of ball (mechanically, via a force)
2. Rising: Kinetic energy store → gravitational potential energy store (ball slows down, gains height)
3. At highest point: Maximum gravitational PE, zero kinetic energy (momentarily stationary)
4. Falling: Gravitational PE → kinetic energy (ball speeds up, loses height)

Throughout, some energy is transferred to the thermal store of the air by heating (due to air resistance).

Worked Example 2 — Electric Kettle

Question: Describe the energy transfers when an electric kettle boils water.

Solution:

Chemical energy in the power station fuel → electrical energy (transferred electrically through the grid) → thermal energy of the water (by heating via the heating element).

Some energy is wasted as thermal energy to the surroundings (heating the kettle body, steam escaping).

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Energy Transfer Diagrams

You can represent energy transfers using flow diagrams or Sankey diagrams (covered in detail in the efficiency lesson).

Worked Example 3 — Identifying Stores in a Scenario

Question: For each scenario, identify the initial and final energy stores.

Scenario	Initial Store	Final Store(s)
A car braking to a stop	Kinetic	Thermal (in brakes and road)
A stretched catapult launching a stone	Elastic potential	Kinetic (→ then gravitational PE)
A battery-powered torch	Chemical	Light (radiation) + thermal
Burning coal in a fireplace	Chemical	Thermal + light (radiation)
A wind turbine generating electricity	Kinetic (wind)	Electrical → various stores
Photosynthesis in a plant	Light (radiation from Sun)	Chemical (in glucose)

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System and Surroundings

A system is the object or group of objects you are studying. Everything else is the surroundings.

When energy is "dissipated" or "wasted," it has not disappeared — it has been transferred to the thermal store of the surroundings (usually by heating), where it spreads out and becomes less useful.

Worked Example 4 — Closed vs Open Systems

Question: A hot cup of coffee sits on a table and cools down over time. Describe this in terms of energy stores and systems.

Solution:

System: the coffee. Surroundings: the cup, the air, the table.

The thermal energy store of the coffee decreases. Energy is transferred by heating to the thermal energy store of the surroundings (air, cup, table). The total energy is conserved — it has not been lost, just spread out.

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Energy and Temperature

The thermal energy stored in an object depends on:

• Its mass (more mass → more thermal energy for the same temperature)
• Its temperature (higher temperature → more thermal energy)
• Its specific heat capacity (materials that absorb more energy per degree per kg)

Material	Specific Heat Capacity (J/kg°C)
Water	4200
Aluminium	900
Copper	390
Iron	450
Glass	840
Air	1000
Concrete	800

Water has an exceptionally high specific heat capacity, which is why it is used in central heating systems and as a coolant.

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Common GCSE Exam Mistakes

Mistake	Correction
Saying energy is "used up" or "lost"	Energy is transferred or dissipated, never destroyed
Confusing energy stores with transfer pathways	"Kinetic" is a store; "mechanically" is a pathway
Listing "heat energy" as a store	The correct term is "thermal energy store"
Listing "light energy" as a store	Light is a transfer pathway (radiation), not a store
Forgetting wasted energy in descriptions	Always mention where the wasted energy goes (usually thermal in surroundings)
Not specifying which object's energy store	Say "the thermal energy store of the water" not just "thermal energy"

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Key Points

• Energy is stored in eight main stores: kinetic, gravitational PE, elastic PE, thermal, chemical, nuclear, electrostatic, and magnetic.
• Energy is transferred between stores by four pathways: mechanically, electrically, by heating, and by radiation.
• Energy is always conserved — it can be transferred but never created or destroyed.
• Dissipated energy is transferred to the thermal store of the surroundings, where it is less useful.
• Always specify which object's energy store you are referring to in exam answers.