Energy is held in stores

We never "use energy up" — we shift it from one store to another. AQA expects you to recognise these stores:

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

Which store?

Four ways energy moves

Energy is transferred between stores by one of four pathways:

• Mechanically — a force does work (pushing, pulling, stretching).
• Electrically — a charge moves through a potential difference.
• By heating — energy passes from a hotter to a cooler object.
• By radiation — e.g. light or sound carrying energy away.

Name the pathway

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

Kinetic energy store

Every moving object has energy in its kinetic store. It depends on mass and, much more strongly, on speed:

Because speed is squared, doubling the speed gives four times the kinetic energy — a key reason stopping distances grow so fast.

Your turn — kinetic energy

Gravitational potential store

Lift an object up and you fill its gravitational store:

Your turn — gravitational store

Elastic potential store

Stretch or compress a spring (within its limit of proportionality) and you fill its elastic potential store:

e is the extension (how much longer it gets), not the total length. Like kinetic energy, the extension is squared.

Your turn — elastic store

Energy is never lost

The principle of conservation of energy: energy can be transferred, stored or dissipated, but it cannot be created or destroyed.

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

In a perfect (closed) system the totals always balance: g.p.e. lost = k.e. gained. In real life a little is dissipated by air resistance and friction — heating the surroundings.

Reading the swing

Specific heat capacity

To warm something up you fill its thermal store. How much energy that takes depends on the material's specific heat capacity, c:

The specific heat capacity is the energy needed to raise 1 kg of a substance by 1 °C. Water's is high (4200 J/kg°C), which is why it is slow to heat and slow to cool.

Required practical: heat a known mass of a block with an electric heater, measuring energy supplied and temperature rise, then calculate c = ΔE ÷ (m Δθ).

Your turn — heating water

Power is the rate of transfer

Power tells you how quickly energy is transferred (or how quickly work is done):

One watt is one joule per second (1 W = 1 J/s). A 2000 W kettle and a 1000 W kettle do the same job — but the 2000 W one gets there in half the time.

Your turn — power

Useful vs wasted energy

In every transfer, some energy is dissipated — spread out to the surroundings (usually by heating) where it is no longer useful. A Sankey diagram shows this: the wider the arrow, the more energy.

That lamp's efficiency = 20 ÷ 100 = 0.2 = 20%. An LED reaching 90% wastes far less.

Your turn — efficiency

Wasting less energy

• Lubrication — oil between moving parts cuts friction, so less energy is dissipated by heating.
• Thermal insulation — traps energy in the thermal store for longer.
• A wall loses energy more slowly when it is thicker and made of a material with a lower thermal conductivity.

Keeping the heat in

Where our energy comes from

We draw energy from many resources. The big divide is renewable (won't run out on a human timescale) vs non-renewable (finite):

Trade-offs: fossil fuels are reliable but release CO₂ (and contribute to climate change). Renewables are cleaner but many (solar, wind, tidal) are less reliable — they depend on weather or time of day. Bio-fuel is renewable but still releases CO₂ when burned.

Renewable or not?

Tap a resource, then tap the box it belongs in.

Choosing a resource

The equations to know

You've covered all three parts of AQA 4.1 — energy changes in a system, conservation & dissipation, and energy resources. Press Finish to see your score.