KS3 · Physics

Transferring energy between stores

A football rolls across the grass and stops. Everyone says it 'ran out of energy'. It didn't, and you're about to find out where its energy went.

Follow the stores: a falling ball

1Held at the top23450 blocksBlocks moved from the gravitational store into the kinetic store

Stage 1 of 5: Held at the top. Blocks moved from the gravitational store into the kinetic store: 0 blocks. paused

Held at the top · 1/5Drag along the fall, or press play. Notice what changed, and what didn't.

Held still, high up. Gravitational store: 10 blocks. Kinetic store: 0 blocks. Total: 10 blocks.

Scrub through the fall and watch two stores at once. The numbers are pretend, there to show the total never changes. We are ignoring air resistance.

Physics · Energy transfers

A catapult: a chain of two transfers

Put the stores in the order the energy reaches them, as a person stretches a catapult and then lets go.

1 · Energy is here first3 · Energy ends up here

  1. The ball's kinetic store

  2. The person's chemical store

  3. The catapult's elastic store

Physics · Energy transfers

The chemical store: up or down, and why?

Pick an event, then pick the box where it belongs.

Does the chemical store go down or up?

Still to sort

Chemical store goes DOWN (0)

Energy moves out of the chemical store.

Chemical store goes UP (0)

Energy moves into the chemical store.

4 of 4 still to sort.

Four everyday events. Sort them first by what happens to the chemical store, then use the rule switch to sort the very same events by what caused the transfer.

Watch out: Easy trap: eating and charging feel like 'using' something, but they fill a chemical store. Check the direction before you decide.

Predict, then check

Commit to an answer before you reveal it. Think about where the energy goes, not just what the device is for.

A laptop is switched on to play a film. Playing the film is its job. Apart from that, which store of energy is also increasing?

Physics · Energy transfers

Back to the football

A football rolls across the grass, slows down and stops.

The football has stopped. Which idea is closest to what you think?
How sure are you?

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

It only moves between stores

What you need to know

  • An energy transfer happens when the energy in some stores decreases and the energy in other stores increases.
  • Energy cannot be created or destroyed, or used up. It can only be transferred between stores. This is the law of conservation of energy.
  • For any event, ask three questions: which store goes down, which store goes up, and what caused the transfer?
  • Always check the surroundings. Very often the surroundings' thermal store is one of the stores that goes up.

The big picture

In an energy transfer, the energy in some stores goes down while the energy in other stores goes up by the same total amount. Energy is never created, destroyed or used up. It is transferred, and very often it ends up in the surroundings.

Key points

1A store of energy is one of the ways energy can be stored: gravitational, thermal, elastic, chemical or kinetic. The surroundings are everything around an object.
2The kinetic store increases when an object speeds up and decreases when it slows down. The gravitational store increases when an object is raised higher and decreases when it is lowered.
3When a store decreases, other stores must increase by the same total amount. For a falling ball, ignoring air resistance, the kinetic store increases by the same amount that the gravitational store decreases.
4Transfers can be caused by forces (gravitational forces, friction, drag and pulling forces), an electric current, a difference in temperature, a chemical reaction, and light and sound waves interacting with matter.
5Energy often goes to the surroundings' thermal store as well as doing its job, because of friction, electrical devices warming up, hot parts of machines, the movement of living things, and objects emitting light or sound.

Worked example

Problem

A hot cup of tea is left on a table and cools down. Which store decreases, which store increases, and what caused the transfer?

⚠ Watch out

Saying energy has been 'used up' or 'lost' when something stops moving or cools down. Energy cannot be used up. It has been transferred to another store, often the surroundings' thermal store.

🧠

Memory hook

Down, up, why? Which store goes DOWN, which store goes UP, and WHY did it move? Then peek at the surroundings.

✓

Check yourself

Pick something that moves, warms up or glows, like a lamp or a kettle. Which store goes down, which goes up, and what caused it? Did you check the surroundings?

Flashcards

(13)
What is an energy transfer?
Energy moving out of one store and into another: some stores decrease while others increase.
What happens to the kinetic store when an object speeds up? And when it slows down?
It increases when the object speeds up and decreases when it slows down.
What happens to the gravitational store when an object is raised higher? And when it is lowered?
It increases when the object is raised higher and decreases when it is lowered.
A ball is dropped. Which store goes down and which goes up as it falls?
Its gravitational store decreases and its kinetic store increases, by the same amount if we ignore air resistance.
When one store of energy decreases, what must happen?
Other stores must increase by the same total amount.
State the law of conservation of energy.
Energy cannot be created or destroyed (or used up). It can only be transferred between stores.
What do we mean by an object's 'surroundings'?
Everything that is around it.
A person stretches a spring. Which stores change?
The person's chemical store decreases and the spring's elastic store increases.
A stretched catapult is released. Which stores change?
The catapult's elastic store decreases and the ball's kinetic store increases.
Name three kinds of event that change a chemical store.
Any three of: burning fuels, taking in food, a plant growing, a human body moving, using or recharging a battery.
Does a candle burning increase or decrease a chemical store? What about a phone charging?
A candle burning decreases a chemical store. A phone charging increases one.
Name three causes of energy transfer.
Any three of: a force, an electric current, a difference in temperature, a chemical reaction, light and sound waves interacting with matter.
Why does energy often go to the surroundings' thermal store as well as doing a job?
Friction between surfaces, electrical devices warming up, hot parts of machines, the movement of living things, and objects giving out light or sound all transfer energy to the surroundings.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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