Key Stage 3 science · Physics – Energy

KS3 conservation of energy: energy stores and changes

The most important idea in physics at Key Stage 3 is that energy is never used up or created: the total is the same before and after any change. Children learn to treat energy as a quantity they can measure and add up, to compare the start and end of a process, and to describe which kinds of energy have increased and which have decreased. They also learn that to explain how a change happens, you use forces and other physical processes, not energy alone.

Energy in systems and conservation, explained

Energy is not a substance you can see or hold. It is a quantity, measured in joules, that scientists calculate to keep track of what happens in a change, rather like keeping accounts. Children describe a system, which is just the object or group of objects being studied, and compare its energy at the start and the end. The key rule is conservation of energy: the total amount of energy after a change is exactly the same as before. Energy can be moved from one place to another or shared out differently, but it cannot be created or destroyed.

To describe what has changed, pupils look at how different kinds of energy go up and down. Many schools use the language of energy stores: the energy of movement (kinetic), the energy of height above the ground in a gravitational field, the energy in a stretched or squashed spring or elastic band (elastic), the energy in a hot object (thermal) and the energy held in fuels, food and batteries (chemical). In a swinging pendulum, for instance, the gravitational store goes down and the kinetic store goes up as the bob swings down, then the reverse as it rises.

A common worry is that energy seems to run out: a bouncing ball stops, a car coasts to a halt, a battery goes flat. The explanation is that the energy has not vanished but has spread out into the surroundings, mostly as a small increase in the thermal energy of the air, the ground and the objects involved. Energy spread out like this is still there, but it is very hard to use again. Children are taught to check an energy account by asking “where did the rest go?” until the totals match.

The third idea is easy to overlook. Energy tells you how much changes, but it does not explain how it happens. A ball speeds up as it falls because gravity pulls on it; it stops on the floor because the floor pushes back and the ball deforms; the surroundings warm because particles are shaken. Pupils are asked to use these physical processes and forces, not just the word “energy”, to explain each intermediate step. Mixing the two kinds of explanation, “energy pushes the ball”, is the most common slip.

Words your child will hear

System: The object or group of objects you choose to study when looking at a change.
Conservation of energy: The rule that the total energy is the same before and after any change.
Kinetic energy: The energy an object has because it is moving.
Gravitational potential energy: The energy an object has because of its height above the ground.
Elastic energy: The energy stored in something that has been stretched or squashed, such as a spring.
Thermal energy: The energy associated with the temperature of an object, from its particles’ random motion.
Dissipate: To spread out into the surroundings, usually as thermal energy, so it is hard to use again.

What the National Curriculum says

The programme of study for Key Stage 3 says:

Pupils should be taught about energy as a quantity that can be quantified and calculated; the total energy has the same value before and after a change
Pupils should be taught about comparing the starting with the final conditions of a system and describing increases and decreases in the amounts of energy associated with movements, temperatures, changes in positions in a field, in elastic distortions and in chemical compositions
Pupils should be taught about using physical processes and mechanisms, rather than energy, to explain the intermediate steps that bring about such changes

Objectives from the National Curriculum in England, © Crown copyright, used under the Open Government Licence v3.0.

A worked example

A ball is held above the floor with 12 J of gravitational energy. It is dropped, bounces, and rises to a height where it has 9 J of gravitational energy. Describe the energy changes, say where the missing energy has gone, and explain the steps using forces.

Start: the ball is still, so it has no kinetic energy. The 12 J is all in the gravitational store.
Falling: gravity pulls the ball down, so it speeds up and its kinetic store grows as its gravitational store falls. Just before it hits the floor, almost all of the 12 J is kinetic. A small amount of energy has gone to the air, because air resistance pushes against the ball.
Bouncing: the floor pushes up on the ball, squashing it for a moment. The ball briefly stores energy in its squashed shape (elastic), then springs back and moves upward again. Some energy goes to the thermal store of the ball and floor, and some is carried away as sound.
Rising: gravity now slows the ball, so its kinetic store falls and its gravitational store rises again, but only up to 9 J. Do the energy accounting for the whole event.
```
start:      12 J
end (ball):  9 J
missing: 12 - 9 = 3 J
```
The 3 J has not been destroyed. It has gone to the thermal energy of the ball, the floor and the air, and to sound. Check the total: 9 J + 3 J = 12 J, the same as the start.

12 J in the gravitational store becomes kinetic as the ball falls, some is stored briefly in the squashed ball and then returned, and 9 J ends up back in the gravitational store. The other 3 J is shared with the surroundings as thermal energy and sound. The total is still 12 J, and the steps are caused by gravity, the floor’s push and air resistance.

Common mistakes and how to help

Believing energy gets used up

In daily speech we say we “use up” energy, and moving things do stop, so it looks as if energy disappears.

What helps: Ask where the energy went rather than whether it has gone. Rub your hands together and notice that they are warm: the movement has become thermal energy.

Thinking energy is a force or a fluid that gets pushed around

Phrases such as “the energy pushes the car” or “the energy flows into the bulb” treat energy as a thing that acts.

What helps: Keep roles separate. Forces push and pull; energy is a quantity that is measured. Use the question “what is pushing or pulling here?” to explain how something happens.

Thinking a stationary object has no energy

Children link energy with movement or heat.

What helps: Show a book on a shelf, a stretched elastic band or a tin of food. Each can release energy when something changes, so each has energy in a store, even though nothing is moving.

Believing that energy lost as heat is destroyed or can be reused for free

Energy that spreads out into the surroundings seems to vanish, or children assume it can be gathered back easily.

What helps: The energy is still there, but spread among enormous numbers of particles, in a very slightly warmer environment. Use the idea that spread-out thermal energy is hard to use again, so efficient devices try to reduce it.

How to help at home

Bounce and check. Drop a ball from a measured height and record how high it bounces, using a wall or a door frame marked in tape. Does it come back to the same height? Ask where the missing energy could have gone.
Swing watch. At a playground, ask your child to describe the swing in energy terms: where is it fastest, where is it highest, and where is it stopped for a moment? Then ask why it needs a push to keep going.
Rub it warm. Rub your hands together hard for twenty seconds, then touch your cheek. Talk about what kind of energy you started with, in your muscles, what happened to the movement, and where the warmth came from.
Draw an energy account. For a toy car on a ramp or a rubber-band-powered toy, ask your child to draw bars for the energy at the start and the end, then work out where the difference went. The totals should be equal.
Explain with forces. Choose a simple event, such as a ball rolling to a stop on a carpet. Ask your child to explain it without using the word “energy” at all, using only pushes and pulls such as friction. Then add the energy language back in.

Leads on to

Key Stage 4: Energy stores, transfers and power
Key Stage 4: Efficiency and energy resources