Energy transfers · Classify
Energy stores
Roll a ball across a carpet and it stops. The energy it had is gone. Except that nothing in physics is ever gone — so where is it?
Start here
The ball stops. The energy does not.
A ball rolls across a carpet, slows, and stops. Before, it was moving and had energy. After, it is still. Nothing hit it and nothing took it away.
Commit to where the energy went.
Into the carpet and the ball, as a very small rise in temperature — a few thousandths of a degree, spread across a wide area, far too little to feel but exactly enough to account for every joule. A sensitive enough thermometer would find it. “Used up” is what it looks like; “moved somewhere hard to notice” is what happened.
Energy is not a substance and it is not a fuel. It is a number — a quantity you can calculate for a situation, which comes out the same before and after no matter what happened in between. To keep track of that number you need somewhere to write it down, and the places you write it down are called stores. This lesson is about learning the list, and about one distinction that trips up almost everyone.
The store audit · fill in the ledger
Which stores empty, and which stores fill?
0 of 5 ledgers balanced
Pick a scenario. Tick the stores that empty and the stores that fill. Then check.
A stone is loaded into a stretched catapult and released. It flies off horizontally.
A diver stands still on a high board, then steps off and falls. Judge the moment just before they hit the water.
A car travelling at 30 mph brakes hard and comes to a complete stop.
A kettle is switched on at the wall and boils a litre of water.
A ball is dropped, squashes flat against the floor for an instant, and bounces back up. Judge the drop, and the moment of maximum squash.
Emptied
Filled
Elastic before, kinetic after. Notice the count does not go up: the energy in the stretch is the energy in the flight. Whoever pulled the catapult back put it there, out of their own chemical store. A real catapult warms its band a little too — we are counting only the stores that change enough to matter.
Gravitational before, kinetic after. Height is emptied and speed is filled. The diver was not moving at the start and is not raised up at the end — the ledger swaps sides completely.
Kinetic before, thermal after. This is the hook, with a bigger ball: the energy is in the brake discs, the tyres and the road, and after a hard stop the discs are genuinely too hot to touch. Nothing was used up.
Chemical before, thermal after — with the chemical store sitting in a fuel somewhere in a power station, not in the kettle. The electric current is how it got here; it is a pathway, not a store, which is the next section of this lesson.
Gravitational before; elastic and thermal at full squash. The thermal one is the interesting tick — it is why the ball never returns to the height it was dropped from, and why a ball squashed and released over and over gets warm.
Key fact
Energy is never used up or created. It moves between stores, and the total is always the same before and after.
Think again
“A torch turns electrical energy into light energy and sound energy.”
Every word of that is in common use and three of them are not stores at all. Sort each one before you read on.
Kinetic
A store. Pause the film and a moving object still has it.
This one is a store — pause the film and the object is still moving, and you can still calculate the number.
Light
A pathway. Light carries energy from one store to another; it is not a place energy sits.
Not a store. Light is energy on its way somewhere — there is no object holding “light energy” once you pause.
Chemical
A store. Food, fuel and batteries hold it whether or not anything is happening.
This one is a store. A battery in a drawer still has it a year later.
Sound
A pathway. Sound is energy travelling through a material as a wave.
Not a store. Nothing sits holding “sound energy” — the sound is the transfer itself.
Elastic
A store. A stretched band holds it indefinitely, doing nothing.
This one is a store. Leave a bow drawn overnight and the energy is still in it.
Electrical
A pathway. A current is a way of moving energy along a wire, not somewhere it rests.
Not a store — this is the one almost everyone gets wrong. The battery has a chemical store; the current is the route out of it.
A store is somewhere energy sits and can be counted while nothing is happening. Pause the world and a stretched spring still has its energy; a hot cup still has its energy. A pathway is a way energy travels from one store to another. Pause the world and there is no light in transit to count — light is a journey, not a destination.
So the torch does not turn electrical energy into light energy. It empties a chemical store in the battery and fills a thermal store in the room, and the light and the electric current are the two routes by which it happens. Saying it properly takes longer, and it is the difference between describing what happened and just naming things.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Recall
A stretched elastic band holds energy in which store?
Rung 2 · The one that catches people
A cyclist freewheels down a hill and then brakes to a stop at the bottom. Which describes the energy correctly?
Rung 3 · Explain
A wind-up torch is cranked for thirty seconds and then shines for two minutes. Describe the energy stores at the start, in the middle and at the end — and say which parts of the description are pathways rather than stores.
Rung 4 · Take it somewhere new
A student says: “A phone battery holds electrical energy, and when it goes flat that energy has been used up.” Two things in that sentence are wrong. Correct both.
Key note
Energy is a number you can calculate for a situation. It is held in stores — kinetic, gravitational, elastic, thermal, chemical, magnetic, electrostatic and nuclear — and it moves between them along pathways. Light, sound and electric current are pathways, not stores.
Going further
Nobody has ever seen energy. There is no instrument that detects it and no sample of it in any laboratory in the world. Richard Feynman made the point to his own students as plainly as it can be made: physics has no idea what energy is. What it has is a rule for working out a number from a situation, and the discovery — the whole discovery — is that if you work that number out before something happens and again afterwards, you get the same answer. Every time. That is not a description of a substance; it is a fact about arithmetic that the universe turns out to obey. It is also why “where did the energy go?” always has an answer.
Before this lesson
- Nothing — this is where the unit starts.
At GCSE this becomes
- Energy stores and transfers quantified — kinetic energy as ½mv², gravitational potential energy as mgh, and efficiency as a calculated fraction.
Where to next
Ask Mr Badmus AI
Still not sure why electricity is a pathway and not a store?
Lesson content © MrBadmusAI.