When a system changes, stores change

A system is just the object (or group of objects) you choose to study. When the system changes — something speeds up, gets lifted, gets warmer — energy moves between its stores.

• A ball is kicked → its kinetic store fills.
• A crate is dragged up a ramp → its gravitational store fills.
• Brakes grip a wheel → the thermal store of the brakes fills.

Describe the change, not just the labels. Edexcel wants you to say which store empties and which fills — e.g. "the chemical store of the fuel empties and the kinetic store of the car fills."

Describe the store change

Drawing energy transfers

An energy-transfer diagram shows stores as boxes and the transfer as a labelled arrow. Label the arrow with the pathway doing the shifting.

Closed system (8.3): if you draw a box around the whole set-up so nothing crosses the boundary, the total energy never changes — there is no net change. Energy only moves around inside.

How is the energy shifted?

Edexcel lists three ways a system's energy can be changed: work done by forces, electrical working, and heating. Tap the right one.

Work done by a force

When a force makes something move, the force does work — and the work done equals the energy transferred. They are measured in the same unit, the joule.

Misconception — no movement, no work. If the force does not move the object along its line of action, no work is done. Holding a heavy bag still does zero work on it (d = 0), even though your arm gets tired!

Doing the same work in less time means a greater power: the rate of doing work, measured in watts (W). You'll calculate it later in this lesson.

Your turn — work done

Rearranging E = F × d

Same work, so who is more powerful?

Which car holds more energy?

Work against gravity → gravitational store

Lift an object and your force does work against gravity. That work is stored in its gravitational store:

Δh is vertical height only. Carry a box 10 m along a flat corridor and its gravitational store doesn't change — Δh = 0. Only the rise counts.

Your turn — gravitational store

Kinetic store of a moving object

Work done to speed something up fills its kinetic store. It depends on mass and, much more strongly, on speed:

Speed is squared, so a van going twice as fast has four times the kinetic energy — and needs four times the work to stop it.

Your turn — kinetic store

Which car holds more energy?

Where does the bulb's extra energy go?

Energy dissipated to less useful stores

In every real change, some energy is dissipated — spread out into stores that are less useful to us, almost always by heating the surroundings.

Edexcel singles out mechanical processes: friction between moving parts causes a rise in temperature, so energy is dissipated by heating the surroundings and can't be recovered.

Where does it go?

Power — how fast energy is transferred

Power is defined as the rate at which energy is transferred, i.e. the rate of doing work. Same energy, less time → more power.

One watt is one joule per second (1 W = 1 J/s).

Misconception — power ≠ amount of energy. Power tells you how fast energy is transferred, not how much. Two machines can transfer the same energy; the one that does it in less time has the higher power.

Your turn — power (energy ÷ time)

Your turn — power (work ÷ time)

Useful vs total — efficiency

Because some energy is always dissipated, only part of the input ends up where you want it. Efficiency is the fraction that is useful:

That drill's efficiency = 300 ÷ 500 = 0.6 = 60%. Efficiency can never be more than 1 (100%) — that would mean creating energy.

Your turn — efficiency

Where does the bulb's extra energy go?

Equation ↔ what it finds

Tap an equation, then tap the quantity it calculates.

Power or energy?

Topic 8 in a nutshell

You've covered the key ideas in Edexcel Topic 8 — Energy: forces doing work. Press Finish to see your score.