Key Stage 3 science · Physics – Energy

KS3 energy transfers: heating, insulation and simple machines

In Key Stage 3, children move from naming energy to tracking where it goes. They learn that energy always moves from a hotter object to a cooler one until their temperatures match, that this happens by conduction or radiation, and that insulators slow it down. They also meet simple machines such as levers and ramps, which trade a smaller force for a longer movement, and they describe energy transfer in everyday processes.

Energy changes and transfers, explained

Heating is the transfer of energy from a hotter object to a cooler one. Put a hot drink on a table and energy leaves the drink; the drink cools and its surroundings warm up very slightly. This carries on until both are at the same temperature, which is called thermal equilibrium. Children should be able to say that transfers tend to reduce the temperature difference, and that energy does not move from a cooler object to a hotter one by itself. A fridge can only do it because it is plugged in and uses electricity to pump the energy out.

Energy can be transferred by contact or by radiation. Conduction is the contact route: in a solid, the particles vibrate more strongly where it is hot and pass the vibrations to their neighbours, so energy spreads along a metal spoon in a hot mug. Metals are good conductors; wood, plastic and wool are poor ones. Radiation (infrared radiation in this case) needs no contact and no material at all, which is how the Sun’s energy crosses empty space and how you feel warmth from a fire on your face.

An insulator is a material that is a poor conductor, so energy passes through it slowly. Wool, foam and feathers trap small pockets of air, and air itself conducts badly, which is why a duvet or a loft layer slows heat loss. Insulators don’t make heat; they simply slow the transfer, which keeps warm things warm and cold things cold. That is why a vacuum flask can keep soup hot and juice cold, and why a wrapped ice lolly lasts longer in a bag.

The curriculum also asks about simple machines, such as levers, ramps, pulleys and gears. Their rule is that they give a bigger force in return for a smaller movement, or the other way round, so the force multiplied by the distance moved stays the same in the ideal case. A machine therefore makes a job easier to do, not smaller. Finally, children describe energy transfer in everyday processes: changing motion, dropping an object, completing a circuit, stretching a spring, the body breaking down food and burning a fuel.

Words your child will hear

Thermal equilibrium: The point where two objects reach the same temperature and there is no net transfer of energy between them.
Conduction: Energy passing through a solid, or between objects in contact, as vibrating particles pass it on.
Radiation: Energy transferred by waves such as infrared, which needs no contact and can cross empty space.
Insulator: A material that conducts energy poorly and so slows the transfer, such as wool or foam.
Conductor: A material that passes energy on easily, such as a metal.
Lever: A simple machine, such as a seesaw or crowbar, that turns about a fixed point to give a bigger force.
Metabolism: The chemical reactions in the body’s cells that release energy from food.

What the National Curriculum says

The programme of study for Key Stage 3 says:

Pupils should be taught that simple machines give bigger force but at the expense of smaller movement (and vice versa): the product of force and displacement is unchanged
Pupils should be taught about heating and thermal equilibrium: a temperature difference between two objects leads to energy transfer from the hotter to the cooler one, through contact (conduction) or radiation; such transfers tend to reduce the temperature difference; and the use of insulators
Pupils should be taught about other processes that involve energy transfer: changing motion, dropping an object, completing an electrical circuit, stretching a spring, metabolism of food, burning fuels

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

Try it: a ramp trades force for distance

A box weighing 200 N has to be raised 1 m onto a platform. You can lift it straight up, or push it up a smooth ramp that is 4 m long. Ignoring friction, what force is needed on the ramp, and what do you give up in return?

Find the product of force and distance for lifting straight up. The force needed is the box’s weight, 200 N, over a distance of 1 m.
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200 N × 1 m = 200 J
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A simple machine doesn’t change this product. The ramp is a longer route, so the force along the slope multiplied by the 4 m of ramp must also be 200 J.

Solve for the force along the ramp.

force × 4 m = 200 J
force = 200 ÷ 4
      = 50 N

Describe the trade: the force is four times smaller (50 N instead of 200 N), but the box has to move four times as far (4 m instead of 1 m).
Compare with real life. A real ramp has friction, which transfers some of the energy to the thermal store of the box and the ramp, so slightly more than 50 N would be needed. The ramp still helps, but it does not remove the effort.

About 50 N along the ramp, a quarter of the 200 N needed to lift the box directly. The cost is a movement four times as long, because force × distance stays the same.

Common mistakes and how to help

Thinking machines reduce the energy needed for a job

A lever or ramp feels easier, so children conclude that less is being done.

What helps: Look at both numbers. The force is smaller but the distance is greater, so the product is the same. Try lifting a book straight up and then sliding it up a long slope, and compare the push with the length of the journey.

Believing cold travels into warm things

A cold drink “makes” your hand cold, so it seems that something cold has entered it.

What helps: Always describe the energy: it moves out of your hand into the glass. Ask which way the energy goes, then check that the answer is from hotter to cooler.

Believing metal is naturally colder than wood or plastic

A metal tap feels colder than a wooden door, even in the same room.

What helps: Both are at room temperature. Metal conducts energy away from your warm hand faster, so it feels colder. Use a thermometer on both to show they match.

Thinking insulators produce warmth or that a coat gives you heat

Wrapping up makes people feel warmer, so the coat seems to be the source.

What helps: Wrap one warm bottle of water in a jumper and leave another bare. The bottle in the jumper stays warmer for longer because the jumper slows the energy transfer; it has no energy of its own to add.

How to help at home

Insulation race. Wrap ice cubes in different materials, such as foil, a woolly sock, kitchen roll and bubble wrap, and leave one bare. Predict the order they melt in, then time them. Keep the other variables the same: the same size of cube, the same place and the same start time.
Cooling mugs. Pour the same amount of warm (not hot) tap water into a metal mug, a ceramic mug and a plastic cup. An adult should do the pouring. Use a thermometer to record the temperature every five minutes and draw a graph of how the three cool towards the temperature of the room.
Door lever. Ask your child to close a door by pushing near the hinge, then at the handle. Which needs less force, and which moves further? This is a lever and it shows the force-for-distance trade in a way they can feel.
Slope versus lift. Tie a rubber band to a heavy book and lift it straight up to the height of a chair seat, noting how far the band stretches. Then drag it up a long, smooth board propped against the chair to the same height. The band stretches less on the slope, but the book travels a lot further. Friction means the saving is smaller than the ideal sums suggest.
Name the transfer. Pick an everyday event, such as a kettle boiling, a ball dropping or a child eating lunch, and ask where the energy starts and where it ends up. Sketch it as a simple chain of arrows with a short label for each step.

Leads on to

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