Section 1 of 8Useful or wasted?
| Type | Definition | Example (electric motor lifting a load) |
|---|---|---|
| Useful energy | Energy transferred to the store, or in the form, that we intended | Kinetic store of the motor to gravitational potential store of the load |
| Wasted energy | Energy that is not useful and is dissipated to the surroundings | Energy to the thermal store of the motor and the air through friction and sound |
Section 2 of 8Where does wasted energy end up?
Dissipation
- Dissipation Dissipated energy spreads out into the surroundings and is very hard to get back.
- The total energy is conserved, but it becomes less concentrated and so less useful.
Thermal stores
- Thermal stores Friction and electrical resistance cause heating.
- The particles vibrate more, so the temperature of the objects rises.
Sound
- Sound Sound waves make air particles vibrate and carry energy away;
- this ends up in the thermal store of the air.
Light
- Light Light is electromagnetic radiation, so unwanted light is energy transferred by radiation.
Section 3 of 8Analysing a device
1
State the purpose of the device (e.g. a motor lifting a weight).
2
Identify the useful transfer: which store loses energy and which store gains the energy we want.
3
Look for temperature rises, noise and light you did not intend.
4
Name the wasted transfer: energy moves by heating or radiation to the thermal store of the device and surroundings.
Section 4 of 8How waste happens in mechanical systems
1Moving parts rub
- Moving parts rub Friction acts between surfaces that slide over each other.
2Work done against friction
- Work done against friction Energy is transferred to the thermal store of both surfaces.
3Temperature rises
- Temperature rises The device and its surroundings warm up.
4Energy dissipated
- Energy dissipated The energy spreads out into the surroundings and can no longer be used for the job.
Section 5 of 8Wasted energy in common devices
| Device | Useful output | Main wasted output |
|---|---|---|
| Filament lamp | Light | Heating of the filament and the air around it (most of the input) |
| LED lamp | Light | A small amount of heating |
| Electric kettle | Thermal store of the water | Heating of the kettle body and the surrounding air |
| Car engine | Kinetic store of the car | Heating of the engine, exhaust gases and air; sound |
| Television | Light and sound | Heating of the circuitry and screen |
Section 6 of 8Reducing wasted energy
Making devices more efficient
Lubrication
- Lubrication Oil between moving parts reduces friction, so less energy is transferred to the thermal stores of the surfaces.
Thermal insulation
- Thermal insulation Lagging a hot water tank or fitting loft insulation slows the transfer of energy to the surroundings.
Better design
- Better design Low-energy LED bulbs and streamlined shapes transfer more input energy to useful forms and less to waste.
Limits
- Limits No device is perfect, because some energy is always dissipated, so the useful output is always less than the input.
Section 7 of 8Key terms
| Term | Meaning |
|---|---|
| Useful energy | Energy transferred for the intended purpose. |
| Wasted energy | Energy not used for the intended purpose; dissipated to the surroundings. |
| Dissipate | To spread out into the surroundings in a less useful way. |
| Open system | A system where energy can enter or leave, so some is transferred to the surroundings. |
| Friction | A force between surfaces in contact that opposes motion and causes heating. |
| Lubrication | Adding oil or grease between surfaces to reduce friction. |
Section 8 of 8Exam technique
1
Saying wasted energy is destroyed. It is still there, but spread out and not useful.
2
Describing sound as wasted without saying where it goes: the energy warms the air as the waves make particles vibrate.
3
Calling all heating wasted. For a kettle or heater, the thermal store of the water or room is the useful output.
4
Naming only the store that gains energy without saying where it came from.
5
Saying a device is 100% efficient. Some energy is always wasted.