6.3.1Generating and Distributing Electricity and Household Use
Energy Transfers in Appliances — Oxford AQA International GCSE Combined Science - Physics Revision Notes & Topical Questions
5 min read 6 parts Lesson 6 of 8
Part 1 of 4Where the energy comes from
Cells and batteries
- Cells and batteries Devices such as remote controls take energy from the chemical store of a battery, which direct current delivers to the components.
Mains supply
- Mains supply Most household appliances use the alternating-current mains: 230 V and 50 Hz in the UK.
How charge carries energy
- How charge carries energy The power supply transfers energy to the charges;
- as they pass through each component they transfer energy to it.
Part 2 of 4Two common outcomes
| Appliance | Main useful transfer | Typical wasted transfer |
|---|---|---|
| Kettle | Electrical work to the thermal store of the water | Heating the kettle casing, the air, and evaporation from the water |
| Washing machine | Electrical work to the kinetic store of the motor and drum | Heating of the motor and wiring, and sound |
| Vacuum cleaner | Kinetic energy of a motor drawing in air | Heating and sound |
| Filament lamp | Light from a hot filament | Most energy heats the surroundings |
| Toaster | Thermal store of the hot element, passed on by radiation to the bread | Heating of the casing and air |
Part 3 of 4Following a transfer: electric heater
1Mains supply
- Mains supply Electrical work is done on the heating element.
2Element
- Element Energy goes to the thermal store of the element.
3Useful
- Useful Air in the room is warmed, raising the thermal store of the surroundings.
4Wasted
- Wasted Some energy is radiated or dissipated in other ways, ending up spread out in the surroundings.
Part 4 of 4Power ratings and energy use
How much energy is transferred
Power rating
- Power rating The label shows the potential difference needed, the supply frequency and the power in watts.
- A 2000 W kettle transfers 2000 J every second.
Time switched on
- Time switched on Total energy = power × time, so a 1 kW iron for 1 hour uses the same energy as a 2 kW iron for 30 minutes.
Higher rating
- Higher rating The higher the power rating, the faster the appliance transfers energy between stores.
Correct supply
- Correct supply Plugging a device into a supply with a much higher voltage than its label states can burn it out or cause a fire.
Key terms
| Term | Meaning |
|---|---|
| Power rating | The energy an appliance transfers each second when working normally, in watts. |
| Useful energy | Energy transferred to the store where it is wanted. |
| Wasted energy | Energy transferred to stores where it is not wanted, usually as heat to the surroundings. |
| Dissipated | Spread out in the surroundings so that it is difficult to use again. |
| Mains supply | The 230 V, 50 Hz alternating current supplied to UK homes. |
Avoid these mistakes
1
Saying energy is 'used up' - it is transferred to other stores and is wasted when spread out.
2
Assuming a higher-powered appliance always uses more energy - it depends on how long it is on.
3
Calling heat 'wasted' in a heater - in that case the thermal energy in the room is the useful output.
4
Forgetting to name the energy stores at the start and end of a transfer.
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