6.5.1Chemical Reactions
Energy Transfers in Appliances — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions
5 min read 5 parts Lesson 14 of 21 Unit quiz
Section 1 of 5Energy Stores
Kinetic
- Energy stored in moving objects, such as the rotating drum of a washing machine or the blades of a fan.
Thermal
- Energy stored in the particles of a substance, which rises when an appliance heats its surroundings.
Gravitational potential
- Energy stored when an object is lifted through a height, such as water raised by a pump.
Sound
- Energy transferred by vibrations of the air, such as the sound from a speaker or a bell.
Section 2 of 5Energy Transfers in a Motor
1
Electrical energy enters the motor from the supply through the wires.
2
The motor transfers most of the electrical energy to the kinetic store of the rotating blades or shaft.
3
Some energy is transferred to the thermal store of the surroundings by friction and by resistance in the windings.
4
A small amount of energy is transferred as sound by the vibrating casing and moving parts.
Section 3 of 5Efficiency
Efficiency
Efficiency
- The proportion of the energy supplied that is transferred to useful energy stores, expressed as a percentage.
Equation
- Efficiency (%) = useful energy output ÷ total energy input × 100.
Wasted energy
- Energy that is not useful, usually transferred to the thermal store of the surroundings, and which cannot be fully avoided.
Higher efficiency
- Appliances such as LED lamps transfer a larger proportion of energy usefully and waste less than filament lamps.
| Appliance | Useful energy | Wasted energy |
|---|---|---|
| Electric kettle | Thermal (water) | Thermal (surroundings) |
| Electric motor | Kinetic | Thermal and sound |
| Filament lamp | Light | Mainly thermal |
| LED lamp | Light | Much less thermal |
Section 4 of 5Worked Example
Question
- An electric motor is supplied with 200 J of electrical energy.
- It transfers 150 J to the kinetic store of a load and the rest is wasted.
- Calculate its efficiency.
- Wasted energy = 200 J − 150 J = 50 J.
- Efficiency = useful output ÷ total input × 100%.
- Efficiency = 150 J ÷ 200 J × 100% = 75%.
Answer The motor has an efficiency of 75%, so 25% of the energy is wasted.
Section 5 of 5Exam Checklist & Tips
6.5.1 – Key Points for the Exam
- Energy store A place in which energy is held, such as kinetic, thermal or gravitational potential.
- Energy transfer The movement of energy from one store to another.
- Useful energy Energy transferred to the store that the appliance is designed to produce.
- Wasted energy Energy transferred to stores that are not useful for the purpose of the appliance.
- Efficiency The ratio of useful energy output to total energy input, as a percentage.
Common Mistakes
- Saying energy is used up or lost from the universe; it is transferred to less useful stores.
- Calculating efficiency with the wasted energy in the numerator.
- Giving an efficiency above 100%, which is impossible.
- Forgetting to multiply by 100 to express efficiency as a percentage.
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