Energy stores and systems

What you need to know

• A system is an object or group of objects; changes are described as energy shifting between named stores.
• The eight energy stores are: kinetic, gravitational potential, elastic potential, chemical, thermal, magnetic, electrostatic, and nuclear.
• A system can be changed by three mechanisms: heating, work done by forces, or work done when an electric current flows.
• Energy is always measured in joules (J) — this common unit lets you compare stores and see the full redistribution clearly.

The big picture

A system is any object or group of objects you choose to focus on. When something happens — a ball is thrown, a kettle heats water, a car brakes — energy shifts from one store to another within that system. The total energy in a closed system stays the same; energy is always conserved, never destroyed. Understanding which stores are involved, and how they are filled or emptied, is the foundation of all energy work at GCSE.

Energy store changes — five key AQA scenarios

For each scenario, follow which store empties, which fills, and what the transfer mechanism is.

1. 1

   Object projected upwards

   Work done by the throwing force transfers energy from the chemical store of the thrower's muscles to the kinetic store of the ball. As the ball rises, the gravitational force does work: the kinetic store empties and the gravitational potential store fills. At the highest point, kinetic store ≈ 0.

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2. 2

   Moving object hits an obstacle

   When the object decelerates to rest, forces at the collision do work against the motion. The kinetic store empties and the thermal stores of the object and obstacle fill. Sound transfers a small amount of energy to the thermal store of the surrounding air.

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3. 3

   Object accelerated by a constant force

   A force does work on the object over a distance. The source driving that force — such as a fuel's chemical store — empties; the object's kinetic store fills. The greater the distance the force acts over, the more the kinetic store fills.

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4. 4

   Vehicle slows down

   Brakes apply a friction force opposing the motion. Work done against friction empties the kinetic store. The thermal stores of the brakes, tyres, and surrounding air fill. This energy is dissipated — spread out into the surroundings and no longer useful.

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5. 5

   Water is heated

   The mechanism here is heating. Energy from the chemical store of the fuel, or from an electric current doing work, is transferred to the thermal store of the water. The water's particles move faster — its temperature rises.

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Stop and think

Commit to an answer before you reveal — this is where the real learning happens.

A rubber ball is dropped from a height, hits the floor, and bounces back up — but only to half the original height. Where has the energy from the missing height gone?

Tap any cell to reveal it. Tap a column header to read one property down every item.

Memory hook

Think of energy stores as buckets: every event tips energy from one bucket into another — no bucket ever gets a top-up from nowhere, and none ever drains to nothing.

Check yourself

Without looking — a compressed spring launches a ball upward. Name the store that empties, the store that fills first, and the store that fills at the top of the ball's flight.