You already know energy is the get-up-and-go behind every change. At GCSE we get sharper about where the energy is and how it moves. The modern picture has just two ideas, and every energy problem you meet is built from them:
Nothing "is" kinetic energy the way a coin is a coin. Rather, a moving object has energy in its kinetic store; lift it and you shift some of that energy into its gravitational store. Keep that money-in-accounts picture in your head and the whole topic clicks into place.
A store is a way an object or system holds energy. There are eight you are expected to recognise. Learn them by the situation that fills each one:
Notice the pattern: the "potential" stores (gravitational, elastic, electrostatic, magnetic) are all about something being held against a force, ready to spring back. Kinetic, thermal, chemical and nuclear are the rest.
Older textbooks listed "light energy", "sound energy", "electrical energy" as if they were things you owned. The exam boards deliberately switched to stores and pathways because the old list muddled two different questions: where the energy is sitting versus how it is being carried. Light and sound aren't stores at all — they are energy on the move, in transit between stores. Splitting the two ideas apart makes tricky chains far easier to trace, which is exactly why we start there.
A pathway is a way energy moves from one store to another. There are just four, and this time you name them by the mechanism, not the situation:
Keep stores and pathways in separate mental boxes. "Kinetic" is a store — a place energy sits. "By heating" is a pathway — a way energy travels. Muddling the two is the single most common slip in this topic, so whenever you name something, ask yourself: is this a place, or a route?
The real skill is writing an energy chain: name the store you start in, the pathway, and the store you end in. Do it slowly, step by step.
Chain 1 — a ball thrown straight up. As it leaves your hand it is fast, so its energy is in the kinetic store. As it climbs, gravity does work on it (a mechanical pathway) and the energy shifts into the gravitational store, until at the very top it is momentarily still — all gravitational, no kinetic. Then it falls and the whole transfer runs backwards.
Chain 2 — a battery torch. The battery holds a chemical store. Switch on and a current carries the energy electrically to the bulb, where it leaves as light (radiation) and — annoyingly — a lot of heat.
Chain 3 — a car braking. The moving car's energy is in its kinetic store. The brakes rub the wheels — friction, a force doing work, so a mechanical pathway — and the energy ends up in the thermal store of the hot brake discs and the air around them.
In the 1840s James Joule hung weights from a string that turned a paddle wheel inside a sealed barrel of water. As the weights fell, their gravitational store emptied mechanically into the paddles, which stirred the water and nudged its temperature up — a tiny rise into the water's thermal store. By measuring exactly how far the weights fell and exactly how much the water warmed, Joule showed that a fixed amount of movement always produces the same fixed amount of heating. That is why the unit of energy is now called the joule (J): he was the first to nail down that stores swap one-for-one, with nothing lost in the exchange — the very idea in the next card.
Add up the energy in every store of a closed system — one nothing can get into or out of — and the total is fixed forever. Energy is only ever moved between stores, never conjured up and never wiped out. This is one of the deepest laws in all of physics.
This gives you a free calculation trick: whatever leaves one store must reappear, joule for joule, somewhere else.
Worked example. A
Step 1 — energy in the gravitational store at the top.
Step 2 — conservation. With no air resistance, all
Step 3 — solve for the speed.
We never needed to know the messy details of the fall — conservation handed us the answer directly.
If energy is never destroyed, why does a phone battery "run down" and a bouncing ball eventually stop? Because energy gets dissipated: spread out thinly into the thermal store of the surroundings, where it is too spread out to be useful again. It isn't gone — it's just scattered.
Every real device transfers some energy usefully and wastes the rest, almost always as heat. We measure how good a device is with its efficiency:
Worked example. An old filament torch bulb is supplied
These are the traps that cost the most marks in this topic:
Pick a system, then drag the process slider to run it forwards. Watch the coloured store bars empty and fill as energy is transferred along the way — and keep your eye on the grey Total bar on the right. However the energy is shuffled between stores, the total never moves: that is conservation of energy, live.