Energy, but bitesize.

Part 1What is a fuel?

A fuel is anything that releases energy when it's burned or used. Wood, petrol, gas, coal — all fuels. When you burn wood in a fire, the wood doesn't just disappear: the energy it was holding is released and heats whatever's around it.

In science, we say the wood has a chemical energy store. Burning the wood decreases that store. The bigger the piece of wood, the more energy is in its chemical store to start with.

Part 2Batteries are chemical stores too

Here's a surprise: batteries are also chemical energy stores. They don't burn, but the chemicals inside them release energy when you use the battery — and you can refill the store by charging the battery up again.

So you can increase the chemical store by:

· charging a phone (refills the battery)
· cutting more wood (more wood = more store)
· making a bigger candle

And you decrease the chemical store by:

· using the phone (drains the battery)
· burning the wood
· lighting the candle

Part 1The joule (J)

Saying a candle has "a lot" or "a bit" of energy isn't scientific. We need numbers. The unit we use to measure energy is the joule. The symbol is a capital J (always capital — small j means something different).

It's exactly like using metres for distance or seconds for time. If something has 200 J in its chemical store, that's a precise amount we can write down and compare with other things.

Part 2When joules get big: kJ

Joules are tiny. A bag of crisps holds about 500,000 J of energy. Writing big numbers gets annoying, so we use kilojoules instead. The symbol is kJ, and:

1 kJ = 1000 J

So that bag of crisps is 500 kJ — much easier to read.

Part 1The thermal energy store

Hot things have more energy than cold things. We say a hot object has lots of energy in its thermal energy store. Cold objects have less.

We measure how hot or cold something is using a thermometer, and the unit of temperature is degrees Celsius (°C). A higher temperature means more energy in the thermal store.

Part 2Energy flows hot → cold

If you put a hot mug on a cold table, the mug cools down and the table warms up. Energy flows from hot to cold, never the other way around. We call this kind of energy movement a transfer by heating.

So in the mug-on-table example, the thermal store of the mug decreases and the thermal store of the table increases.

Part 1Kinetic energy store — anything moving

Any object that's moving has energy in its kinetic energy store. The faster it moves, the more energy in that store. A stationary object has zero kinetic energy.

Direction doesn't matter. A car going 30 mph north has the same kinetic energy as the same car going 30 mph south. What matters is how fast.

Part 2Gravitational energy store — anything lifted up

If you lift something up off the ground, it gains energy in its gravitational energy store. The higher you lift it, the more energy goes into the store. A book on a high shelf has more gravitational energy than the same book on a low shelf.

Where does that energy come from? You. Lifting takes effort because you're transferring energy from your chemical store (the food you ate) into the book's gravitational store.

Part 3Falling: gravitational → kinetic

When you let something fall, watch what happens to the two stores. As the object drops, it gets lower (gravitational store decreases) and faster (kinetic store increases). The energy is transferring from one store to the other.

Try the interactive below — drag the cart to different heights on the roller coaster track, and watch the two stores swap energy. Turn the friction toggle on to see what happens when some energy gets transferred to the thermal store of the surroundings as well.

Part 1The elastic energy store

Stretch a rubber band. Squash a spring. Twist a ruler. Each one stores energy as it changes shape — that energy is in the elastic energy store. The more you stretch, squash, or twist it, the more energy in the store.

Let go, and the energy gets released — the rubber band fires across the room, the spring jumps back, the ruler springs straight. The elastic store decreases, and energy transfers to the kinetic store of whatever's moving.

Part 2Transfer by a force

When you push or pull something — or when one object pushes/pulls another — the way energy moves between them is called a transfer by a force. The simple test: is there a push or a pull involved? If yes, the transfer is "by a force".

Examples:

· You push a trolley to make it move. Chemical store of you → kinetic store of trolley. Transfer by a force.

· A bike's brakes slow it down. Kinetic store of bike → thermal store of brakes. Transfer by a force (the brakes push on the wheel).

· You stretch a rubber band. Chemical store of you → elastic store of band. Transfer by a force (you're pulling the band).

Part 1Transfer by electric current

Whenever you see wires and a circuit, the energy is moving by electric current. A battery powers a torch: the chemical store of the battery decreases, energy travels through the wires by electric current, and ends up in the bulb.

Charging a phone works the other way: energy travels through the charging wire (by electric current) and ends up increasing the chemical store of the battery.

Part 2Transfer by waves — light and sound

Light and sound are both types of wave. They radiate out from a source — light from a bulb, sound from a speaker — and carry energy with them. We call this a transfer by waves.

A torch in action: the battery's chemical store decreases, energy is transferred by electric current to the bulb, and from the bulb it transfers by waves (light) into the room.