What is energy?
Energy is what is needed to make something change: move, heat up, light up, stretch or react. We measure it in joules (J). Lifting an apple (about 1 N) by 1 metre needs about 1 J.
Four big properties of energy:
- It can be stored.
- It can be transferred (moved) from one store to another or from one object to another.
- It is conserved: the total is never made or destroyed.
- It tends to spread out (dissipate), mostly as heat, and then it is less useful.
Energy stores (forms of energy)
Older books say forms of energy; many courses now say stores. They mean the same idea.
| Store | Where you find it |
|---|---|
| Chemical | food, fuel, batteries |
| Kinetic | anything moving |
| Gravitational potential | anything raised up |
| Elastic potential | stretched or squashed springs, bows, rubber bands |
| Thermal (internal) | hot objects; the faster the particles jiggle, the more |
| Magnetic | two magnets held apart or together |
| Electrostatic | charges near each other, e.g. a thundercloud |
| Nuclear | inside atoms: the Sun, nuclear power stations |
Light, sound and electricity are not stores. They are ways energy travels.
Transfers and transformations
Energy moves between stores by four pathways:
- Mechanically: a force moves something (work done). Example: a kick.
- Electrically: a current flows. Example: a battery runs a fan.
- By heating: from a hotter to a colder object.
- By radiation: light, infrared or sound waves. Example: sunlight warms a road.
Chain example, a torch: chemical store of the battery → electrically → bulb → by light (useful) and heating (wasted) → thermal store of the room.
A Sankey diagram draws this as a thick arrow that splits: width = amount of energy.
Efficiency = useful energy out ÷ total energy in × 100%. An LED bulb is about 40–50% efficient; an old filament bulb only about 5%.
Heating: conduction, convection, radiation and insulators
Energy flows by heating only from hot to cold. It stops when both are at the same temperature: thermal equilibrium.
- Conduction: in solids, particles pass on vibrations to their neighbours. Metals are good conductors (a steel spoon in hot tea gets hot).
- Convection: in liquids and gases, warm fluid rises and cool fluid sinks, making a current (sea breeze, boiling water).
- Radiation: infrared waves; needs no material, so it crosses space from the Sun. Dark, dull surfaces absorb and give out more; shiny ones reflect.
Insulators (air, wool, plastic, foam, wood) slow down heating. A flask has a vacuum (stops conduction and convection) and shiny walls (stop radiation). Thick walls, double glazing and loft insulation keep homes warm in winter and cool in summer.
Simple machines and energy at home
A simple machine (lever, pulley, ramp, gear) does not create energy. It lets a smaller force act over a longer distance: work = force × distance stays the same (or a bit more, because of friction). A ramp twice as long needs about half the force to push a box up.
At home energy runs cooking, heating, cooling, lighting and devices. Industry uses it for machines, furnaces and transport. Energy you pay for is often measured in kilowatt-hours: 1 kWh = 3.6 million J. Saving tips: LED bulbs, switching off standby, good insulation, and pressure cookers.
Try it at home
Drop a ball from 1 m onto a hard floor and mark how high it bounces. It never comes back to 1 m. Where did the rest go? Touch the ball after 20 hard bounces: it is slightly warm (thermal store) and you heard sounds (radiation pathway). Then wrap one cup of hot water in a woolly sock and leave another bare; after 15 minutes, which is warmer?
Key formulas and definitions
- Unit of energy: joule (J); 1 kJ = 1000 J; 1 kWh = 3 600 000 J
- Kinetic energy: KE = ½ m v²
- Gravitational potential energy: GPE = m g h (g ≈ 9.8 N/kg)
- Work done (mechanical transfer) = force × distance
- Efficiency = useful energy out ÷ total energy in × 100%
- Energy in = useful energy out + wasted energy
Worked examples
1. Name the store and pathway when a phone battery runs the screen.
Chemical store of the battery → electrical pathway → light (radiation) from the screen, plus some heating of the phone (thermal store).
2. A kettle uses 2000 J of electrical energy and 1600 J heats the water. Find the efficiency.
Efficiency = 1600 ÷ 2000 × 100% = 80%. The other 400 J heats the kettle body and air.
3. A 2 kg bag is lifted 1.5 m. How much energy goes into its gravitational store? (g = 10 N/kg)
GPE = m g h = 2 × 10 × 1.5 = 30 J.
4. A 0.5 kg ball moves at 4 m/s. Find its kinetic energy.
KE = ½ × 0.5 × 4² = ½ × 0.5 × 16 = 4 J.
5. A motor is 75% efficient and gets 400 J. How much is wasted?
Useful = 0.75 × 400 = 300 J. Wasted = 400 − 300 = 100 J (mostly heat and sound).
6. Why does a steel spoon in hot chai get hot but a wooden spoon does not?
Steel is a good conductor: its particles and free electrons pass energy along quickly. Wood is an insulator, so energy moves through it very slowly.
Common mistakes
- Saying energy is "used up". It is not destroyed; it is transferred, often to the thermal store of the surroundings.
- Calling light, sound or electricity a store. They are pathways that carry energy.
- Thinking cold flows into a warm room. Energy flows from hot to cold only.
- Thinking a lever or ramp gives you free energy. It lowers the force but increases the distance.