Energy Stores, Transfers and Conservation
Energy is never used up — it is moved from one store to another. This page covers the stores, the transfers between them, and the one principle that governs all of it.
1. What energy is
Energy is the capacity to do work.
Measured in joules (J).
For a one-mark definition, write “the capacity to do work”. Tutors gave this exact phrasing as the safe answer.
Energy is not a force. Confusing the two was a recorded error. A force (newtons) is a push or pull; energy (joules) is the capacity to do work. Similarly, gravity is a force while gravitational potential energy is a store — another recorded confusion.
2. The energy stores
| Store | Description |
|---|---|
| Kinetic | a moving object |
| Gravitational potential | an object at a height |
| Elastic (strain) potential | a stretched or compressed object |
| Chemical | fuels, food, batteries |
| Thermal (internal) | a hot object |
| Nuclear | the nucleus of an atom |
| Electrostatic | separated charges |
| Magnetic | separated magnetic poles |
Geothermal energy is THERMAL energy stored in hot rocks — not chemical. That specific confusion was recorded. Similarly, a battery holds chemical energy, which becomes electrical when it’s connected.
3. Energy transfers
Energy moves between stores by four pathways:
Mechanically — by a force doing work Electrically — by a current By heating — conduction, convection or radiation By radiation — light, sound and other waves
Common examples:
| Situation | Transfer |
|---|---|
| Falling object | gravitational potential → kinetic |
| Ball thrown up | kinetic → gravitational potential |
| Stretched spring released | elastic → kinetic |
| Battery powering a lamp | chemical → electrical → light + thermal |
| Burning fuel | chemical → thermal |
| Electric kettle | electrical → thermal |
| Nuclear reactor | nuclear → thermal → kinetic → electrical |
Use the store names, not vague words like “heat energy” or “movement energy”. Write thermal and kinetic.
4. Conservation of energy
Energy cannot be created or destroyed — only transferred from one store to another. The total energy in a closed system stays constant.
Learn this wording: “energy is neither created nor destroyed, only transferred.” Tutors gave it verbatim as the mark-scheme phrasing.
Applying it: for a falling object, the GPE lost = KE gained (ignoring air resistance). With air resistance, the difference has become thermal energy in the surroundings — still conserved, just no longer useful.
The pendulum is the standard example:
At the highest point: GPE is maximum, KE is zero. At the lowest point: KE is maximum, GPE is minimum.
Know where each is greatest in a pendulum — flagged directly by tutors as an exam staple.
5. Useful and wasted energy
Every transfer produces some energy in a form we don’t want — usually thermal energy (and sometimes sound).
Dissipation is the spreading out of energy to the surroundings, where it becomes too spread out to be useful.
Wasted energy is not destroyed — it is dissipated, warming the surroundings. Saying energy is “lost” or “used up” contradicts conservation; say wasted or dissipated.
Reducing waste: lubricate to cut friction, insulate to reduce thermal transfer, streamline to reduce air resistance.
6. Sankey diagrams
A Sankey diagram shows energy transfers as arrows whose width is proportional to the energy.
- Input enters from the left
- Useful output continues to the right
- Wasted energy branches off, usually downwards
- The total width in = total width out — a visual statement of conservation
Example: a lamp with 100 J input, 20 J as light and 80 J as thermal energy — the light arrow is one quarter the width of the thermal one.
Draw the arrows to scale, and label each with its energy value and store.
7. Mistakes that cost marks
Confusing energy with force.
Confusing gravity with gravitational potential energy.
Calling geothermal energy chemical.
Saying energy is “lost” or “used up”.
Vague store names like “movement energy”.
Reversing a transfer direction.
Sankey arrows not to scale, or unlabelled.
Forgetting that wasted energy still counts in the total.
Frequently asked questions
What is energy? The capacity to do work, measured in joules.
What is the principle of conservation of energy? Energy is neither created nor destroyed, only transferred between stores.
What are the energy stores? Kinetic, gravitational potential, elastic potential, chemical, thermal, nuclear, electrostatic and magnetic.
What is the difference between a store and a transfer? A store is where energy is held; a transfer (mechanical, electrical, heating, radiation) is how it moves.
What happens to wasted energy? It is dissipated to the surroundings, usually as thermal energy.
Is energy destroyed when it is wasted? No — it becomes too spread out to be useful.
Where is KE greatest on a pendulum? At the lowest point; GPE is greatest at the highest point.
What does a Sankey diagram show? Energy transfers, with arrow width proportional to energy.
What type of energy is geothermal? Thermal energy stored in hot rocks.
Quick revision checklist
- I can define energy as the capacity to do work
- I know the unit is the joule
- I don’t confuse energy with force
- I can name all the energy stores
- I can name the four transfer pathways
- I can describe the transfers in common situations
- I can state the conservation of energy in the standard wording
- I can apply it to a falling object and a pendulum
- I know where KE and GPE peak on a pendulum
- I say energy is dissipated, not lost
- I can suggest ways to reduce wasted energy
- I can draw and read a Sankey diagram
These notes cover energy stores, transfers and conservation in the Cambridge IGCSE Physics (0625) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across a large set of one-to-one IGCSE Physics lessons, with particular attention to the errors students make most often and the wording examiners reward. Always check the current syllabus and formula list for your own exam series.
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