Discover the many forms energy takes, how it transfers between objects, and why the total amount of energy in a closed system never changes.
Energy is the ability to do work — to cause a change in an object or system. It is not a physical substance you can hold, but it is very real: it makes things move, heat up, glow, make sound, and grow. The SI unit of energy is the joule (J), named after James Prescott Joule. Larger amounts are expressed in kilojoules (kJ) or megajoules (MJ).
Energy exists in many forms. All forms can be placed into two broad categories: kinetic energy (energy of movement) and potential energy (stored energy).
| Form | Description | Example |
|---|---|---|
| Kinetic energy | Energy of a moving object. Depends on mass and speed. | A rolling ball, wind, flowing water |
| Gravitational potential energy | Energy stored because of an object's position above the ground. | A book on a shelf, water in a dam |
| Elastic potential energy | Energy stored in a stretched or compressed object. | A stretched rubber band, a compressed spring |
| Thermal (heat) energy | Energy due to the random motion of particles inside a substance. | A hot cup of tea, a fire |
| Chemical energy | Energy stored in the bonds between atoms in substances. | Food, fuel, batteries |
| Electrical energy | Energy carried by moving electric charges. | Lightning, a current in a wire |
| Light (radiant) energy | Energy carried by electromagnetic waves including visible light. | Sunlight, a torch beam |
| Sound energy | Energy carried by vibrations moving through a medium. | A speaking voice, a loudspeaker |
At Grade 8 level you should recognise the key relationships:
Where m = mass (kg), v = speed (m/s), g = 10 N/kg (gravitational field strength near Earth's surface), h = height above ground (m). Both give an answer in joules (J).
Energy can be transferred (moved from one object to another) or transformed (changed from one form to another).
One of the most important laws in all of science:
This means that whenever energy seems to "disappear," it has simply been transformed into a less useful form — usually thermal energy (heat) — which spreads out into the surroundings. We say energy is dissipated, not destroyed.
No machine can produce more energy than is put into it. A machine that produces 100% useful energy output from its input is called a perfectly efficient machine — in reality, no such machine exists because some energy is always dissipated as heat or sound.
Watch kinetic and potential energy exchange as the pendulum swings. Notice how total energy stays constant — that is conservation of energy in action.
| Height released, h (m) | GPE at top (J) | Speed at bottom, v (m/s) | KE at bottom (J) |
|---|---|---|---|
| 0.20 | ? | 1.9 | ? |
| 0.40 | ? | 2.7 | ? |
| 0.60 | ? | 3.3 | ? |