Energy stores and transfers
Key idea: Energy moves between stores by mechanical or electrical transfer, heating or waves. It is never destroyed: energy dissipated to the surroundings is still there, just spread out.
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Guided lessons · coming soon
Guided lesson
Guided steps
- Read where energy is stored and how it moves.
- Correct the rubber-band description and explain what sound waves do.
- Follow what changes when you choose a different system.
- Choose two systems and describe where their energy goes.
- Follow each energy account from the starting store to the final store.
- Explain how the battery’s energy reaches the lamp and walls.
- Read how to account for every joule.
- Explain how braking increases internal energy stores.
- Follow the useful and dissipated transfers and check their total.
- Explain where the energy goes as the bounce heights fall.
- Find the energy dissipated by the lamp.
- Explain where the dissipated energy and absorbed light end up.
- Find the increase in the motor’s stored energy.
- Explain the missing energy and correct the assumption.
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ContinueWhen a battery runs a fan, petrol drives a car or the Sun warms a roof, energy moves from one place to another but never disappears. Describing it precisely means saying where the energy is held, how it moves and where it ends up, and keeping track of every joule.
Energy stores and transfer pathways
Every energy description answers two separate questions. Where is the energy? That is a store. How does it move? That is a transfer pathway. Keeping the two apart is what makes an energy explanation precise.
Energy stores
| Store | Where the energy is held | Example |
|---|---|---|
| kinetic | in a moving object | a rolling bicycle, flowing water |
| gravitational potential | in an object raised in a gravitational field (strictly, the object and the Earth) | a book on a high shelf, water behind a dam |
| chemical potential | in fuels, food and batteries, released by chemical reactions | petrol, bread, a charged battery |
| elastic potential | in an object that is stretched, squashed or bent | a stretched catapult band, a compressed spring |
| nuclear | in the nuclei of atoms, released in nuclear reactions | uranium fuel, the core of the Sun |
| internal | in the random motion and positions of the particles of a substance | hot water: its particles move faster than in cold water |
When an object warms up, its internal store increases.
Transfer pathways
Energy moves from one store to another by four pathways.
| Pathway | What causes the transfer | Example |
|---|---|---|
| mechanically | a force acting over a distance | a hand pushing a trolley; a crane lifting a load |
| electrically | an electric current | a battery driving a current through a lamp |
| by heating | a temperature difference | a hot drink warming your hands |
| by waves | electromagnetic or mechanical waves travelling | sunlight reaching the Earth; sound from a loudspeaker |
Heat, light, sound and electricity are not stores. “The battery stores electricity” should be “the battery has a chemical store; energy is transferred electrically when a current flows”.
Check your understanding 1
A student says: “A stretched rubber band has sound energy.” Correct the statement.
Show answer
The stretched band has an elastic potential store. Sound is a pathway: if the band is plucked, sound waves transfer energy from it.
Choose the system first
A system is the object or group of objects you choose to study. Its boundary decides which transfers you describe. Energy that crosses the boundary enters or leaves the system; transfers between objects inside the boundary only move energy between the system’s own stores.
Think of a battery-powered immersion heater warming water. If the system is the water alone, energy enters it by heating from the hotter heating element. If the system is the battery, heater and water together, the electrical transfer from the battery to the element happens inside the boundary: the battery’s chemical store decreases while the internal stores of the element and water increase. Energy that reaches the room crosses this larger boundary and leaves the system.
Check your understanding 2
A cup of hot tea is left on a table. Choose a system that makes the tea lose energy, and a system in which the total energy stays constant.
Show answer
System losing energy: the tea alone. It transfers energy by heating to the cooler cup, table and air, so its internal store decreases. Constant total: the tea, the cup, the table and the air of a closed room (ignoring transfers out of the room). The tea’s internal store decreases by as much as the internal store of everything else increases.
Write an energy account
An energy account follows one change from start to finish:
store that decreases → pathway → store that increases
Name the object each store belongs to. Where energy passes through a device, write each step.
Worked accounts: one for each pathway.
- Mechanically. A stretched catapult launches a stone. System: the catapult band and the stone. The band’s elastic potential store decreases → the band’s force acts on the stone over a distance (mechanically) → the stone’s kinetic store increases.
- Electrically. A battery-powered fan is switched on. System: battery, motor, blades and the room’s air. The battery’s chemical store decreases → a current transfers energy to the motor (electrically) → the motor turns the blades, which push the air (mechanically) → the kinetic store of the blades and air increases. The warm motor also transfers some energy by heating to the internal store of the surrounding air.
- By heating. A hot iron block is dropped into a beaker of cold water. System: the block and the water. The block’s internal store decreases → by heating, because the block is hotter than the water → the water’s internal store increases. The net transfer by heating between them stops when both reach the same temperature.
- By waves. Sunlight warms the water in a solar water heater on a roof. The Sun’s nuclear store decreases → electromagnetic waves (light and infrared) travel to the panel → the water’s internal store increases. A loudspeaker is the mechanical-wave case: sound waves carry energy from the vibrating cone through the air.
Check your understanding 3
A battery torch lights a dark room. The lamp gets warm, and the light is absorbed by the walls. Choose a boundary that lets you account for the battery’s decrease, then name the starting store, the pathway to the lamp and the pathway from the lamp to the walls. Where does the absorbed energy go?
Show answer
Include the battery, torch and room, and ignore transfers out of the room during the time considered. The battery’s chemical store decreases. Energy is transferred electrically to the lamp and by waves (light) to the walls. The light absorbed by the walls increases their internal store. The warm lamp also transfers energy by heating to the cooler air. Choosing only the battery as the system would instead show energy leaving the system.
Conservation of energy
Principle of conservation of energy: energy cannot be created or destroyed. It can only be transferred from one store to another, so the total energy of an isolated system stays constant.
An isolated system is one that no energy enters or leaves. For any other system, the energy that enters or leaves must appear in the account. Either way, every joule is somewhere.
Useful and dissipated energy
Energy that produces the intended effect is useful: light from a lamp, for example, or an increase in a lifted load’s gravitational potential store. Energy that spreads into the surroundings, mostly into internal stores, is dissipated. It becomes harder to use for the intended task, but it is not destroyed.
For a device with no overall build-up of stored energy during the interval considered,
energy input = useful energy transferred + energy dissipated
Friction and air resistance dissipate energy: a force acting over a distance (mechanically) warms the surfaces, increasing their internal store. The warm surfaces then transfer energy by heating to the cooler surroundings.
Check your understanding 4
Write an energy account for a car braking to a stop on a level road.
Show answer
The car’s kinetic store decreases → the friction forces in the brakes act over a distance (mechanically) → the internal store of the brakes increases. The hot brakes then transfer energy by heating to the internal store of the surrounding air.
Worked example 1
A motor lifting a load
Problem
An electric motor receives 5000 J electrically. It raises a load, whose gravitational potential store increases by 3200 J. The motor gets warm and hums. Over the interval there is no net build-up of energy in the motor’s own stores. Account for all the energy.
Show full solution
Choose the system
Method
Include everything that gains energy.Reason
Track the electrical energy entering this system and all stores that gain energy; transfers between its objects stay inside its boundary.Working
Answer
The motor, the load, the Earth and the surrounding air
Find the useful transfer
Method
Identify the store the motor is meant to fill.Reason
Raising the load is the motor’s job.Working
Answer
3200 J into the load’s gravitational potential store, transferred mechanically by the cable’s force
Use conservation
Method
Subtract the useful energy from the energy input.Reason
Energy is not created or destroyed.Working
Answer
Dissipated: 5000 - 3200 = 1800 J
Trace the dissipated energy
Working
Answer
The 1800 J is transferred by heating from the warm motor, and by sound waves from the humming, into the internal store of the surroundings.
| Energy | Amount (J) |
|---|---|
| transferred to the motor electrically | 5000 |
| useful: into the load’s gravitational potential store | 3200 |
| dissipated: into the internal store of the surroundings | 1800 |
| useful + dissipated | 5000 |
The totals match. If they did not, a transfer would be missing from the account.
A battery that has “run out” has not destroyed any energy. It can no longer drive a useful current, and the energy it has already transferred is in other stores, mostly the internal stores of the device and its surroundings.
Check your understanding 5
A ball is dropped and bounces, rising a little less each time until it stops. Energy is conserved, so where has the energy gone? Start with the store that decreases when the ball first falls.
Show answer
As the ball falls, its gravitational potential store decreases and its kinetic store increases. On impact the ball squashes, so energy goes into its elastic potential store, and most of it returns to the kinetic store as the ball rebounds. At every bounce some energy is dissipated: the ball and floor warm up (internal store) and the bounce makes a sound (waves). Each rise is lower because less energy returns to the gravitational potential store. The gravitational potential energy released by the fall has increased internal stores of the ball, floor and air. Sound also increases internal stores when it is absorbed.
Check your understanding 6
An LED lamp receives 800 J electrically. It transfers 480 J by light. Assume there is no net build-up of energy in the lamp and that the room absorbs all the light. Calculate the energy dissipated during the interval.
Show answer
800 - 480 = 320 J is dissipated: it is transferred by heating from the warm lamp to the internal store of the surroundings. The light is absorbed by the walls and furniture, so those 480 J also end in the internal store of the room. In the end all 800 J is in internal stores.
Check your understanding 7 Where does the lamp’s energy end up?
The room absorbs all the light from the lamp above. Explain where the energy transferred by light and the dissipated energy end up. You may explain aloud or sketch the account on paper.
Show answer
The dissipated 320 J reaches the internal store of the surroundings by heating. The walls and furniture absorb the 480 J of light, increasing their internal stores. Your account should name both transfers and their destinations; all 800 J eventually reaches internal stores in the room.
Check your understanding 8 Include energy still inside the device
During a short lifting interval, a motor receives 1000 J electrically. The load’s gravitational potential store increases by 650 J, and 200 J reaches the surroundings. There are no other transfers. Find the increase in the motor’s own stored energy.
Show the steps Optional working
Show answer
The motor retains 1000 − 650 − 200 = 150 J. Check the full account: 650 + 200 + 150 = 1000 J.
Check your understanding 9 A warm motor does not destroy missing joules
For the motor above, a learner concludes that the remaining 150 J has been destroyed because input does not equal useful output plus energy reaching the surroundings. Correct the account and state which assumption from the earlier device equation does not apply.
Show answer
The remaining 1000 − 650 − 200 = 150 J stays in the motor’s own stores, for example its internal store as it warms. The account is input = gain in the load’s store + energy reaching the surroundings + gain in the motor’s stores. The simplified input = useful transfer + dissipated transfer equation assumed no overall build-up in the device. That assumption fails here; conservation of energy still holds.
Common mistakes
- Calling heat, light and sound energy stores. They describe how energy moves; say “transferred by heating” or “by waves”, then name the store that gains.
- Saying energy is used up. It cannot be destroyed; it is transferred, often dissipated into the internal store of the surroundings.
- Saying a battery stores electricity. It has a chemical store, and energy is transferred electrically when a current flows.
- Saying friction destroys kinetic energy. Friction transfers it mechanically to the internal store of the surfaces.
- Saying cold flows from the ice into the drink. Energy is transferred by heating from the warmer drink to the colder ice.
- Saying energy was lost because a system’s total went down. The energy left the system and increased a store outside it.
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Syllabus and review details
- SEC G3 Combined Science Physics component 2027 · 2027
Physics section, PDF pages 8–24
Last reviewed: