4.2.7Guiding Spaceship Earth Towards a Sustainable Future
Energy Efficiency — AQA GCSE Combined Science Synergy 8465 - Physical Sciences Revision Notes & Topical Questions
5 min read 8 parts Lesson 13 of 15
Section 1 of 8Useful and wasted energy
Useful output
- Useful output The energy transferred to where it is wanted, in the form that is wanted, such as light from a lamp or kinetic energy from a motor.
Wasted output
- Wasted output Energy transferred to the surroundings in a way that is not wanted, usually as thermal energy and sound.
Conservation
- Conservation Total energy input = useful energy output + wasted energy output, so no device can be more than 100% efficient.
Sankey diagrams
- Sankey diagrams A Sankey diagram shows energy flow as arrows whose width is proportional to the energy;
- the useful arrow goes straight on while the wasted arrow bends away.
Section 2 of 8Calculating efficiency
| Version | Equation | Unit |
|---|---|---|
| Using energy | efficiency = useful energy output ÷ total energy input | No unit (a ratio) |
| Using power | efficiency = useful power output ÷ total power input | No unit (a ratio) |
| As a percentage | efficiency (%) = efficiency × 100 | % |
| Finding useful output | useful energy output = efficiency × total energy input | J |
| Finding wasted energy | wasted energy = total input − useful output | J |
Section 3 of 8Worked example
1
A motor takes in 500 J of electrical energy and gives out 350 J of useful kinetic energy.
2
Efficiency = 350 ÷ 500 = 0.70.
3
As a percentage, this is 0.70 × 100 = 70%.
4
The wasted energy is 500 − 350 = 150 J, mostly lost as thermal energy and sound.
5
Check the answer: useful 350 J + wasted 150 J = 500 J, which equals the input, so energy is conserved.
Section 4 of 8Worked example using power
| Quantity | Value | Working |
|---|---|---|
| Total power input | 60 W | Given for a filament lamp |
| Useful power output (light) | 3 W | Given |
| Efficiency | 0.05 | 3 ÷ 60 |
| Percentage efficiency | 5% | 0.05 × 100 |
| Wasted power | 57 W | 60 − 3, mostly as heat |
Section 5 of 8Improving efficiency
Reducing waste
Reduce friction
- Reduce friction Lubricate moving parts so less kinetic energy becomes thermal energy.
- Using ball bearings or wheels in place of sliding surfaces also lowers the friction losses.
Insulate
- Insulate Use materials of low thermal conductivity to cut heat loss.
Better design
- Better design LED lamps waste far less energy as heat than filament lamps, so they transfer more of their input as light.
Streamlining
- Streamlining A smooth shape lowers air resistance so less energy is wasted pushing air aside.
Section 6 of 8Efficiency in everyday devices
| Device | Useful output | Typical efficiency |
|---|---|---|
| Filament lamp | Light | About 5% |
| LED lamp | Light | About 40% or more |
| Electric motor | Kinetic energy | 70% to 90% |
| Petrol car engine | Kinetic energy | About 25% to 30% |
Section 7 of 8Key terms
| Term | Meaning |
|---|---|
| Efficiency | The proportion of the input energy that is transferred to a useful form. |
| Useful energy | Energy transferred in the form that is wanted. |
| Wasted energy | Energy transferred to unwanted stores, usually thermal. |
| Dissipated | Spread out into the surroundings and becoming less useful. |
| Power | The rate at which energy is transferred, measured in watts. |
| Sankey diagram | A diagram in which arrow width represents the amount of energy in each transfer. |
Section 8 of 8Exam technique
1
Giving an efficiency greater than 1 or 100%; this would break conservation of energy.
2
Dividing the wrong way round; it is useful divided by total.
3
Including a unit such as joules with the final efficiency.
4
Saying that wasted energy is destroyed instead of dissipated.
5
Using useful output and wasted output as if they were the total input.
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