What is energy?
Energy is the ability to do work or cause change. You need energy to move, heat things up, make light or sound, and keep your body alive.
Energy is measured in joules (J). Lifting an apple 1 metre takes about 1 J. Food labels often use kilojoules (kJ): 1 kJ = 1000 J.
Forms of energy
| Form | What it is | Example |
|---|---|---|
| Kinetic | Energy of a moving object | A rolling ball, wind |
| Gravitational potential | Stored because of height | A book on a high shelf |
| Elastic potential | Stored in a stretched or squashed object | A drawn bow, a trampoline |
| Chemical potential | Stored in the bonds of chemicals | Food, fuel, batteries |
| Thermal (heat) | Energy of particles moving and vibrating | A hot cup of tea |
| Electrical | Carried by moving charges | Current in a wire |
| Light (radiant) | Carried by light waves | Sunlight, a torch |
| Sound | Carried by vibrations through a material | Music, a drum |
| Nuclear | Stored in the nucleus of atoms | The Sun, nuclear reactors |
The law of conservation of energy
Energy cannot be created or destroyed. It can only be transformed from one form to another, or transferred from one object to another.
The total amount of energy always stays the same.
Energy transformations
We show changes with an arrow diagram.
- Torch: chemical (battery) → electrical → light + heat
- Car engine: chemical (petrol) → kinetic + heat + sound
- Solar panel: light → electrical
- Kicking a ball: chemical (in muscles) → kinetic
- Bouncing ball falling: gravitational potential → kinetic
- Hydroelectric dam: gravitational potential (water) → kinetic → electrical
- Plants: light → chemical (photosynthesis)
Sankey diagrams
A Sankey diagram shows energy flowing through a device. The width of each arrow shows how much energy goes that way. The useful output goes straight on. Wasted energy bends off to the side.
- Kinetic
- Thermal
- Sound
- Light
- Electrical
- Gravitational
- Elastic
- Chemical
- Nuclear
- Not created
- Not destroyed
- Only transformed
- Arrow chains
- Sankey diagrams
Quick summary
- Energy is the ability to cause change. Unit: joule (J).
- Main forms: kinetic, gravitational, elastic, chemical, thermal, electrical, light, sound, nuclear.
- Conservation of energy: energy is never created or destroyed, only changed.
- Show changes with arrow chains: chemical → electrical → light + heat.
- Sankey diagrams use arrow width to show amounts of energy.
- "Wasted" energy usually ends up as heat spread into the surroundings.
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