Energy transformation and conservation
Energy keeps changing from one form to another, but the total stays the same. It can be neither created nor destroyed: the law of conservation of energy.
Energy changes form
Energy rarely stays in one form. A bulb turns electrical energy into light. Our bodies turn the chemical energy of food into muscular energy. This change is called energy transformation. In every change, the total energy stays the same. This is the law of conservation of energy: energy can neither be created nor destroyed, only changed from one form to another.
Common energy transformations
| Device or process | Energy in | Energy out |
|---|---|---|
| Electric bulb or tube light | Electrical | Light (and some heat) |
| Electric iron, heater or oven | Electrical | Heat |
| Fan, motor or pump | Electrical | Mechanical (kinetic) |
| Dry cell or battery | Chemical | Electrical |
| Solar cell | Light | Electrical |
| Microphone | Sound | Electrical |
| Loudspeaker | Electrical | Sound |
| Photosynthesis in leaves | Light (solar) | Chemical (food) |
| Fireworks | Chemical | Heat, light and sound |
| Rubbing your hands | Mechanical (work) | Heat |
A falling ball: 0.5 kg dropped from 10 m (g = 9.8 m s⁻², air ignored)
| Position | Height | PE = mgh | KE | PE + KE |
|---|---|---|---|---|
| Top, at rest | 10 m | 49 J | 0 J | 49 J |
| Halfway down | 5 m | 24.5 J | 24.5 J | 49 J |
| Just above the ground | 0 m | 0 J | 49 J | 49 J |
Worked example: speed of the falling ball
A 0.5 kg ball is dropped from a height of 10 m. Use conservation of energy to find its speed just before it hits the ground. (g = 9.8 m s⁻²; ignore air resistance)
- PE at the top = mgh = 0.5 × 9.8 × 10 = 49 J; KE there = 0.
- At the ground, all of this PE has become KE: ½mv² = 49 J.
- v² = 2 × 49 ÷ 0.5 = 196, so v = √196 = 14 m s⁻¹.
14 m s⁻¹. The mass cancels, so v = √(2gh) for any mass
Energy changes in a hydroelectric power station
In a thermal power plant the chain starts differently: chemical energy of coal, then heat, then steam turning a turbine (mechanical), then electrical energy.
Where does the energy 'go'?
- A ball that bounces lower each time has not lost energy. At each bounce, part of it turns into heat and sound.
- Brakes stop a cycle by turning its KE into heat in the brake pads and the rim.
- No machine turns all its input energy into useful work; some always becomes heat. But the total energy is still conserved.
Hydroelectric stations on Himalayan rivers turn the potential energy of stored water into electricity for homes. When you charge a phone, its battery stores the electrical energy as chemical energy.
2 marks: “State the law of conservation of energy.” Energy can neither be created nor destroyed. It only changes from one form to another, and the total energy stays constant.
3 marks: “A ball is dropped from a height. Explain how its energy changes as it falls, and why its total mechanical energy stays the same.” At the top it has only PE (mgh) and no KE. As it falls, h decreases, so PE falls, while its speed rises, so KE grows by the same amount. Just before landing it has only KE. PE + KE is the same at every point, ignoring air.
1 marks: “Name the energy change in a solar cell.” Light energy into electrical energy.
Energy changes its clothes but never disappears.