Energy

The kinetic store

HintIt fills as something speeds up and empties again as it slows down.

The whyAnything moving has this store, and it grows sharply with speed — doubling how fast a car travels quadruples it, which is why speed limits matter so much.

The chemical store in the battery

HintWhatever is being run down inside the cell you will one day have to replace.

The whyThe torch makes no energy of its own — it moves it. An electrical pathway carries energy from the chemical store to the bulb, where it ends up as light and warmth.

The bath

HintOne is a speck and the other is enormous — quantity counts as well as how hot.

The whyTemperature tells you how much energy each particle has; the total also depends on how many particles there are. A sparkler is fiercely hot but has almost no mass, so it barely warms your skin.

It is replaced as fast as it is used

HintTomorrow there will be just as much of it as there is today.

The whyWind, solar, tidal and hydroelectric all top themselves up. Coal, oil and gas took millions of years to form and are being burned far more quickly than the Earth can make more.

Conduction

HintParticles jiggle harder and nudge their neighbours along.

The whyIn a solid the particles cannot travel anywhere, so they pass energy on by vibrating against each other. Metals do it fastest because their free electrons carry energy through as well.

It expands and becomes less dense

HintThe same amount of stuff now takes up more room, so it floats upwards.

The whyHeating a gas pushes its particles further apart, so a given volume of it weighs less than the cool air around it. Cooler air sinks to take its place, setting up a convection current that heats the whole room.

By radiation

HintThe one kind of transfer that needs no particles at all.

The whyConduction and convection both need matter, so neither works in a vacuum. This transfer travels as electromagnetic waves, which is why you feel the Sun the instant it appears from behind a cloud.

How fast it transfers energy

HintTwo kettles can boil the same water, but one gets there sooner.

The whyPower is the energy moved each second, measured in watts. A 3000 W kettle shifts energy three times as quickly as a 1000 W one, so it boils in a third of the time.

The kilowatt-hour

HintRun a one-bar heater for sixty minutes and you have bought exactly one.

The whyIt is the energy a 1 kW appliance uses in an hour — about 3.6 million joules. Bills use it because a joule is far too small a unit to count a household in.

It has spread out as heat and sound

HintNothing has vanished — feel the ball and listen to each bounce.

The whyThe total is always conserved: what goes in equals what comes out. What changes is how useful it is, because once energy has scattered into the warmth of the floor and the air we cannot gather it back.

The amount of energy

HintA fat arrow and a thin arrow are not carrying the same quantity.

The whySankey diagrams are drawn to scale, so an old filament bulb shows a narrow useful branch for light and a fat wasted branch for heat. The widths going in must always total the widths coming out.

distance

HintPush hard against a wall that does not budge and no work is done on it.

The whyWork done is measured in joules, the same unit as energy, because work is simply energy transferred by a force. Push a 20 N crate 3 m and you have transferred 60 J.