Edexcel IGCSE Physics · Spec 4.1-4.3
Energy Stores & Transfers
The different energy stores and how energy is transferred between them.
Physics revision video
Energy Stores & Transfers
Prefer to watch on YouTube? Open this video on YouTube.
Explained
Naming the stores, and saying where the energy went
Energy is not created or destroyed. It moves between stores, and describing a situation properly means saying which store emptied, which store filled, and which object each store belonged to.
The stores
Kinetic, for anything moving. Gravitational, for anything raised up. Elastic, for anything stretched or compressed. Chemical, for fuels, food and batteries. Thermal, for anything warm. Magnetic, electrostatic and nuclear complete the set.
Eight stores, and they are a closed list. If a question asks which store has changed, the answer is one of those eight.
The transfers
Energy moves between stores by only four routes: mechanically, when a force does work; electrically, when a current flows; by heating; and by radiation, meaning light or sound.
Light, sound and electricity are therefore not stores. Nothing holds energy as sound. Sound is a way energy travels between one store and another, and calling it a store is the commonest slip in this topic.
Describing a change properly
Name the object as well as the store, and say which way each one went.
A ball thrown upwards: the kinetic store of the ball empties and its gravitational store fills, transferred mechanically.
A torch: the chemical store of the battery empties, energy is transferred electrically to the bulb, and leaves it by radiation and by heating.
A car braking: the kinetic store of the car empties and the thermal store of the brakes and surroundings fills, transferred by the friction between them.
What the examiners say about this topic
A principal examiner report for Edexcel International GCSE Physics explains the vocabulary situation directly. It says the language used to describe energy stores and transfers is concise, that there are eight energy stores and only four energy transfers, and that this is slightly different from the description more prevalent in centres. The mark scheme, it says, has to reflect acceptable answers using either scheme.
That is unusually candid. Many students are taught older phrasing such as light energy and sound energy, and the examiners accept it where they can. But the stores and pathways scheme is the one the specification uses, and it is the one that never runs into trouble.
A later report shows where answers actually fall down. On a conservation of energy question, many learners correctly quoted the principle but failed to state what the energy was transferred into. Some suggested energy was wasted as gravitational potential energy, and few explicitly referred to friction or to energy transfer to the surroundings.
Quoting the principle is the easy half. Saying where the energy actually went, to a thermal store and then to the surroundings, is the half that carries the marks.
Conservation and dissipation
The total energy in a closed system stays the same. That is conservation of energy, and it holds without exception.
But not all of it ends up somewhere useful. Some always spreads into the thermal store of the surroundings, usually through friction or air resistance, and once spread out it cannot be gathered back.
That is dissipation. The energy has not been lost or destroyed, and saying so is the point of the word. Use dissipated rather than lost when you can, because it says what actually happened.
Sankey diagrams
A Sankey diagram shows the transfers as arrows whose widths are proportional to the energy. The input enters from the left, the useful output continues to the right, and the wasted energy branches off, usually downwards, as heating.
The widths must add up, because energy is conserved. A diagram where the branches total more than the input shows energy being created, which is the check to make before you read anything off it.
Spec 4.1-4.3
What you need to know
- Name the main energy stores
- Describe how energy is transferred
- State the law of conservation of energy
Active recall
Quick check
Answer each question before opening the answer.
Name four energy stores.
Any four of: kinetic, gravitational potential, elastic (strain), chemical, thermal (internal), nuclear, electrostatic, magnetic.
State the principle of conservation of energy.
Energy cannot be created or destroyed, only transferred from one store to another.
Physics revision app
Take this topic further in the app
275 guided topics with questions marked as you answer them, section checkpoints and timed practice papers. Fifteen topics are free to try, with no card needed.