Energy makes things happen

Anything that changes — a car speeding up, a kettle boiling, a plant growing — needs energy. Energy is measured in joules (J). Big amounts are measured in kilojoules (kJ), where 1 kJ = 1000 J.

• Energy is never made or destroyed — we shift it around.
• We say energy is held in stores and moved by transfers.
• The joule (J) is the same unit whatever kind of energy you mean.

Watch out: people say energy gets "used up", but that is not quite true. Energy is only ever transferred from one store to another — the total amount stays the same.

Energy is held in stores

An energy store is a place where energy is waiting to be moved. These are the eight KS3 stores you should know:

Watch out: "heat", "electrical", "light" and "sound" are not stores — they are ways energy is transferred between stores. We'll meet those next.

Which store?

Four ways energy moves

Energy is moved between stores by one of four transfer pathways:

• Mechanically — a force does work (pushing, pulling, stretching, lifting). Sound waves carry energy this way too.
• Electrically — a current flows through a wire or appliance.
• By heating — energy passes from a hotter object to a cooler one.
• By radiation — carried away as light or infrared.

Name the transfer

Tap the pathway that does the main energy transfer in each case.

Store meets example

Tap a store on the left, then tap the example on the right that fills it.

Doing work moves energy

When a force moves an object, we say work is done — and doing work transfers energy from one store to another.

• Push a trolley → energy goes into its kinetic store (it speeds up).
• Lift a box onto a shelf → energy goes into its gravitational store.
• Stretch a rubber band → energy goes into its elastic store.

Where does the energy go?

Levers: bigger force, smaller movement

A lever is a simple machine: a bar that turns on a pivot. Push the long end down and the short end lifts with a bigger force, but it moves a smaller distance.

For a perfect lever, force × distance is the same at both ends. So a lever never makes extra energy: the work done on it equals the work it does.

Watch out: it works the other way round too. A big force over a small distance can give a small force over a big distance.

How far do you push?

What really happens when you “use” energy?

Kettle or television: which costs more to run?

Comparing food and fuels

Food and fuels both hold energy in a chemical store. We compare them by their energy value — how many kilojoules (kJ) you get from each gram.

Domestic idea: just like food labels show energy in kJ, the fuels we burn at home (like gas) are chosen partly for how much energy each gram or unit gives.

Reading the energy values

Your turn — energy in food

Where does your home's energy come from?

An energy resource is something we get useful energy from. Homes burn a fuel (like natural gas in a boiler) or use electricity from power stations, wind turbines and solar panels.

• Fossil fuels (coal, oil, natural gas) formed from the remains of living things over millions of years. They are non-renewable: we use them much faster than they form, so they will run out. Burning them releases carbon dioxide.
• Renewable resources (wind, sunlight, flowing water, wood from replanted trees) are replaced naturally.

Watch out: electricity is not an energy resource. It transfers energy electrically from a resource (a fuel, the wind, the Sun) to your home.

Power is how fast energy moves

Power tells you how quickly energy is transferred. It is measured in watts (W).

Every appliance has a power rating printed on it. A higher rating means it transfers energy faster:

Your turn — power

Comparing power ratings

Paying for energy at home

Energy companies don't charge in joules — that number would be huge! Instead they use the kilowatt-hour (kWh), the energy a 1 kW appliance uses in 1 hour.

Watch out: put the power in kilowatts (2000 W = 2 kW) and the time in hours before you multiply.

Your turn — energy used

Your turn — the cost

Kettle or television: which costs more to run?

If energy is never lost, why does a swing stop?

Energy is never lost

The law of conservation of energy: energy can be transferred and stored, but it cannot be created or destroyed. The total amount always stays the same.

On a swing, energy sloshes between the gravitational store (at the top) and the kinetic store (at the bottom):

The total energy never changes — it just moves between stores. Gravitational store lost = kinetic store gained (if we ignore friction).

Reading the swing

Energy spreads to the surroundings

If energy is never lost, why do things slow down and cool down? Because energy is dissipated — it spreads out to the surroundings, usually by heating, where it is too spread out to be useful.

Watch out: "wasted" energy has not disappeared and it has not been destroyed — it is still there, just too spread out to be useful. Energy is still conserved.

Where does wasted energy go?

Hot to cold, until they match

When two objects are at different temperatures, energy is transferred from the hotter one to the cooler one, in two ways:

• Conduction: through contact, where they touch.
• Radiation: by infrared, with no contact needed (the radiation pathway). Overall, energy goes from hotter to cooler.

The hotter object cools and the cooler one warms, so the temperature difference gets smaller. When they reach the same temperature, there is no overall transfer: they are in thermal equilibrium.

Insulators (like air, wool and foam) are poor conductors, so they slow the transfer down.

Watch out: "cold" does not flow in. A cold drink warms up because energy moves into it from the warmer air.

Two blocks touching

Saving energy at home

We can't stop energy dissipating completely, but we can slow it down — which cuts fuel bills and helps the planet:

• Insulation (loft, cavity walls, double glazing) keeps energy in the thermal store for longer.
• Draught-proofing reduces the warm air escaping.
• Lubrication (oil on moving parts) cuts friction, so less energy is dissipated by heating.
• Choose more efficient appliances (e.g. LED bulbs) — they waste less energy.

Keeping the heat in

If energy is never lost, why does a swing stop?

The big ideas to know

You've covered KS3 Physics — Energy: stores and transfers, levers, food, fuels and energy resources, power ratings and kWh, conservation, dissipation and heating. Press Finish to see your score.