PHYSICS · Stage 9 · Chapter 12: Energy Transfer
Physics 12.2 Conservation of energy — Law of Conservation of Energy and Efficiency
5 min read 5 parts Lesson 2 of 7
Lesson summary
Physics 12.2 Conservation of energy — Law of Conservation of Energy and Efficiency — Alhussan International Academy — Physics (Stages 7–9) notes: State the law of conservation of energy and apply ideas about conservation, dissipation and efficiency.Law — Energy cannot be created …
- Energy and coins
- Importance
- Missing energy
- Useful energy
- Wasted energy
- Dissipated
Section 1 of 5The law of conservation of energy
- Law
- Energy cannot be created or destroyed, only transferred or stored.
- Energy and coins
- Like coins in a pocket, if you know how much energy you have at the start of a process, you have the same amount at the end.
- Importance
- It is one of the most important laws in science.
- Missing energy
- If a process seems to have less energy at the end, another type of energy must be involved.
Section 2 of 5Useful and wasted energy
- Useful energy
- Energy transferred in the way we want, such as the light from a bulb.
- Wasted energy
- Energy transferred in ways we do not want, usually heating the surroundings.
- Not lost
- Wasted energy is not lost; it is just not useful.
- Cooking
- Heating is not always waste, but the heating of the air around a pan is not useful.
Section 3 of 5Dissipation of energy
- Friction
- In machines with moving parts, friction makes the parts warm.
- Dissipated
- The energy is transferred to the surroundings, which heat up, and spreads out so it cannot easily be used again.
- All processes
- In all processes some energy is wasted, usually heating the surroundings.
- Conserved overall
- Energy is conserved if you consider the surroundings together with the machine.
Section 4 of 5Efficiency
- Efficiency = the fraction of the energy put in that is transferred as useful energy.
- Equation
- Efficiency = × 100%.
- (useful energy output ÷ total energy input)
- Efficiency = × 100%.
- Energy-efficient devices
- They waste less, for example an energy-efficient bulb that does not get as hot as a normal bulb.
- Never 100%
- No device is 100% efficient, because there is always friction, resistance or sound.
- Cost
- More efficient electrical devices waste less energy, so they cost less to run.
Section 5 of 5Energy transfer diagrams
- Diagram
- The input arrow splits into a useful output arrow and a wasted output arrow, with widths proportional to the energy.
- Improving efficiency
- Lubricate moving parts to reduce friction, use insulation to reduce thermal losses and choose efficient devices such as LEDs.
| Device | Useful output | Typical waste |
|---|---|---|
| Filament lamp | Light | Thermal energy |
| Electric motor | Kinetic energy | Thermal energy and sound |
| Kettle | Thermal energy in the water | Heating the kettle and surroundings |
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