💪 Mechanically
A force moves something. Pushing, pulling, lifting, stretching and squashing are all mechanical transfers. Example: your hand pulls back a bowstring.
Energy Transfers » Energy Stores and Transfers
What you'll learn this session
Study time: 30 minutes
Edexcel spec: 4.1, 4.2
- The eight energy stores, with an example of each
- The four ways energy is transferred between stores
- How to describe any situation as store, transfer, store
- The units used in this topic, starting with the joule
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Unlock This CourseEnergy is kept in stores
Energy is not a substance you can see or hold. It is more like money in a set of bank accounts. Physics keeps track of where the energy is (the stores) and how it moves from one store to another (the transfers). When something happens, one store empties a little and another store fills up.
You have met the joule (J) already in the electricity lessons. It is the unit for every energy store and every energy transfer. A joule is small: one small bar of chocolate holds roughly a million joules in its chemical store.
Key terms:
- Energy store: a way that an object or a material holds energy, for example because it is moving, hot, stretched or high up.
- Energy transfer: energy moving from one store to another.
The eight energy stores
The specification names eight stores. Learn all eight names, and for each one learn what tells you the store is filling up.
| Store | What has energy in this store | The store gets bigger when... | Example |
|---|---|---|---|
| Chemical | Fuels, food and batteries | there is more fuel, food or a fuller battery | A tank of petrol, a slice of toast |
| Kinetic | Anything that is moving | the object moves faster | A cyclist riding along a road |
| Gravitational | Anything lifted up above the ground | the object is lifted higher | A book on a high shelf, water behind a dam |
| Elastic | Anything stretched, squashed or bent that can spring back | the object is stretched or squashed more | A drawn bow, a wound-up clockwork toy |
| Thermal | Every object, because of its temperature | the object gets hotter | A mug of hot tea |
| Magnetic | Magnets that are attracting or repelling | repelling poles are pushed closer, or attracting poles are pulled apart | Two bar magnets held with their north poles facing |
| Electrostatic | Electric charges that are attracting or repelling | like charges are pushed closer, or unlike charges are pulled apart | Charged socks clinging together out of a tumble dryer |
| Nuclear | The nuclei of atoms | (it empties when nuclei split or join) | Uranium fuel in a nuclear power station |
Notice what is not in the table: light, sound and electricity. These are not stores. They are ways of moving energy, as you will see next.
The four energy transfers
Energy can move between stores in four ways. The specification uses these exact words, so use them in your answers.
⚡ Electrically
A current flows in a circuit. Example: a battery makes a current in the motor of a toy car.
🔥 By heating
Energy moves from a hotter object to a cooler one because of the temperature difference. Example: a hot pan warms the soup inside it.
💡 By radiation
Energy is carried by waves. The specification lists light and sound here. Example: a lamp lights up a room, a loudspeaker fills it with music.
Describing a situation: store, transfer, store
Exam questions ask you to describe the energy transfers in a device or an event. Every good answer follows the same three steps.
The three-step method
1. Start: which store has energy at the beginning? This store decreases.
2. Transfer: how does the energy move? Choose from mechanically, electrically, by heating or by radiation.
3. End: which store increases?
If there are several stages, repeat the steps to make a chain.
Here are five everyday situations set out this way. Read each row from left to right.
| Situation | Store that decreases | Transfer | Store that increases |
|---|---|---|---|
| A battery torch is switched on | Chemical store of the battery | Electrically to the lamp, then by radiation (light) | Thermal store of the surroundings |
| An electric kettle heats water | (energy arrives from the mains) | Electrically to the heating element, then by heating | Thermal store of the water |
| A ball falls from a window | Gravitational store of the ball | Mechanically (the force of gravity pulls the ball down) | Kinetic store of the ball |
| A catapult fires a stone | Elastic store of the stretched rubber | Mechanically (the rubber pushes the stone) | Kinetic store of the stone |
| A car brakes to a stop | Kinetic store of the car | Mechanically (friction in the brakes) | Thermal store of the brakes |
Worked example 1: the torch in full
Question: Describe the energy transfers when a battery torch is switched on.
Step 1: the energy starts in the chemical store of the battery.
Step 2: it is transferred electrically by the current to the lamp. The lamp transfers energy to the surroundings by radiation (light). The lamp also gets warm, so some energy is transferred by heating.
Step 3: the energy ends up in the thermal store of the surroundings. The things the light shines on, and the air around the lamp, become very slightly warmer.
Worked example 2: the catapult from the very start
Question: A girl pulls back the rubber of a catapult and lets go. Describe the energy transfers.
Pulling back: energy from the chemical store of her muscles is transferred mechanically to the elastic store of the rubber.
Letting go: energy from the elastic store of the rubber is transferred mechanically to the kinetic store of the stone.
This is a chain: chemical, then elastic, then kinetic, with a mechanical transfer at each link.
Worked example 3: the car after it has stopped
Question: A car brakes and stops. The brakes are now hot. Describe what happens to the energy next.
The hot brakes are warmer than the air around them. Energy is transferred by heating from the thermal store of the brakes to the thermal store of the surroundings. The brakes cool down and the air warms up a little.
Always say whose store you mean. 'Kinetic store of the stone' is a better answer than 'kinetic'.
The units for this topic
You have used all of these units in earlier topics. This table shows where each one appears in the energy lessons that follow.
| Unit | Symbol | Measures | Where you will use it |
|---|---|---|---|
| joule | J | energy | Every store and every transfer, including work done |
| kilogram | kg | mass | Gravitational and kinetic store calculations |
| metre | m | distance or height | Work done, and the height an object is lifted |
| metre/second | m/s | speed | Kinetic store calculations |
| metre/second2 | m/s2 | acceleration | Objects speeding up as they fall |
| newton | N | force | Work done by a force |
| second | s | time | Power calculations |
| watt | W | power | How quickly energy is transferred |
The equations that use these units come later, in Work Done, Gravitational Potential Energy, Kinetic Energy and Power.
Common mistakes
- Writing 'light energy', 'sound energy' or 'electrical energy' as a store. Light and sound are transfers by radiation. Electricity is a transfer too. None of them is one of the eight stores.
- Writing 'heat energy'. The store is the thermal store. Heating is the transfer.
- Naming a store but no transfer. 'Chemical to kinetic' misses the mark for how the energy moved. Say mechanically, electrically, by heating or by radiation.
- Mixing up gravitational and kinetic. Height means gravitational. Speed means kinetic. A ball at the top of its flight has a large gravitational store even though it is hardly moving.
- Forgetting the last store. Light and sound do not vanish. The energy ends up in the thermal store of the surroundings.
Exam-style question
A wind-up torch has no battery. The user turns a handle, which winds up a spring inside the torch. When the torch is switched on, the spring unwinds and turns a small generator. The generator makes a current in a lamp.
(a) State the unit of energy. [1 mark]
(b) Name the energy store that increases as the spring is wound up. [1 mark]
(c) Describe how energy is transferred from the user to the spring. [2 marks]
(d) Describe the energy transfers from when the torch is switched on until the light reaches the walls of the room. [3 marks]
Model answer
(a) The joule, J (1).
(b) The elastic store of the spring (1).
(c) Energy from the chemical store of the user (their muscles) (1) is transferred mechanically, by the force turning the handle (1).
(d) Energy from the elastic store of the spring is transferred mechanically to the generator (1). It is then transferred electrically to the lamp (1). The lamp transfers energy by radiation (light) to the thermal store of the walls or surroundings (1).
Exam tip
In part (d), write one short sentence for each link in the chain and underline the transfer word in each one. Three marks means three links: mechanically, electrically, by radiation. Light is a transfer, not a store, so finish with the thermal store of the walls or surroundings.