Think in stores, not "types"

OCR wants you to describe a system using energy stores and the transfers between them. We never "use energy up" — we shift it from one store to another.

Watch out: "electrical", "heating" and "radiation" name how energy is transferred — they are pathways, not stores.

Four ways energy is transferred

Whenever a system changes, the energy moves along one of four pathways:

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

Which store fills?

Filling which store?

For each change, tap the store that is being filled (gaining energy).

Energy is conserved

The law of conservation of energy: energy can be transferred, stored or dissipated, but it is never created or destroyed.

In a closed system (no energy in or out) the total energy stays the same — there is no net change. Energy just moves between stores.

Misconception: energy is not "used up". Even wasted energy still exists — it has just spread out to the surroundings where it is no longer useful.

Falling: gravity store → kinetic store

When an object falls, energy shifts from its gravitational store into its kinetic store. On a swinging pendulum it sloshes back and forth between the two:

If we ignore air resistance, the energy leaving the gravitational store equals the energy gained by the kinetic store, so m g h = ½ m v². The mass cancels, so a heavy and a light object reach the same speed after the same drop.

Reading the swing

Kinetic energy of a moving body

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

Because speed is squared, trebling the speed gives nine times the kinetic energy. This is investigated with light gates and trolleys.

Your turn — kinetic energy

Energy of an object raised up

Lift an object and you fill its gravitational store. The work done lifting it equals the energy stored:

Your turn — gravitational store

Your turn — falling object

Energy in a stretched spring

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

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

Your turn — elastic store

Work done by a force

When a force makes an object move, the force does work — and the work done equals the energy transferred mechanically:

One joule is the work done when a force of 1 N moves an object 1 m (1 J = 1 N·m). Dragging a heavier crate, or dragging it further, does more work.

Your turn — work done

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). The power rating of an appliance tells you how much energy it transfers each second when in use.

Misconception: a higher power does not mean more total energy — it means the same energy can be transferred in less time. A 2 kW kettle boils the same water as a 1 kW one, but in half the time.

Your turn — power

Useful vs dissipated energy

In every transfer, some energy is dissipated — stored in less useful ways (usually spread out by heating). A Sankey diagram shows this: the wider the arrow, the more energy.

That motor's efficiency = 375 ÷ 500 = 0.75 = 75%. Efficiency is always less than 1 (under 100%) and has no units.

Your turn — efficiency

Reducing unwanted transfers

• Lubrication — oil between moving parts cuts friction, so less energy is dissipated by heating.
• Thermal insulation — keeps energy in the thermal store for longer.
• A building cools more slowly when its walls are thicker and made of a material with a lower thermal conductivity (qualitative only).

Keeping the heat in

Increasing efficiency (HT only · P7.2e)

The P7 equations to know

You've covered both parts of OCR Gateway P7 — P7.1 Work done (stores, transfers, conservation in a closed system, the energy calculations) and P7.2 Power and efficiency (dissipation, efficiency, reducing unwanted transfers). Press Finish to see your score.