3.2.4Conservation of Energy
Improving Efficiency — Edexcel GCSE Combined Science 1SC0 - Physics Revision Notes & Topical Questions
5 min read 8 parts Lesson 9 of 10
Section 1 of 8The idea behind efficiency (HT only)
Definition
- Definition Efficiency = useful output energy ÷ total input energy.
- It can be given as a decimal or as a percentage.
- Multiply the decimal by 100 to get a percentage, so 0.35 is 35%.
Raising efficiency
- Raising efficiency Improve a device by increasing the proportion of energy that ends up in useful stores, which means reducing wasteful transfers.
Typical waste
- Typical waste Machines lose energy to friction, air resistance, electrical resistance and sound.
- All of these end up as energy dissipated to the thermal store of the surroundings.
Section 2 of 8Four sources of waste and the fixes (HT only)
| Source | What happens | How to reduce it |
|---|---|---|
| Friction | Moving parts rub, heating the machine and surroundings | Add bearings; lubricate with oil or grease |
| Electrical resistance | Charge flowing through wires and components heats them | Use components or wires with lower resistance; reduce the current needed |
| Air resistance | Moving through the air transfers energy to the air by heating | Streamline the shape |
| Sound | Vibrating parts make noise that carries energy away | Tighten loose parts; lubricate |
Section 3 of 8Calculating efficiency (HT only)
| Device | Useful output | Total input | Efficiency |
|---|---|---|---|
| Motor lifting a load | 60 J | 80 J | 60 ÷ 80 = 0.75 (75%) |
| Lamp | 12 J as light | 60 J | 12 ÷ 60 = 0.20 (20%) |
| Kettle | 270 kJ to the water | 300 kJ | 270 ÷ 300 = 0.90 (90%) |
| Machine after oiling | Wasted energy falls from 40 J to 10 J out of 100 J input | 100 J | Useful rises from 60 J to 90 J, so 60% becomes 90% |
Section 4 of 8Reducing air resistance: cycling (HT only)
1
Identify what causes drag: the cyclist, clothing and bicycle all push air out of the way.
2
Adopt a lower, more streamlined posture.
3
Wear tight clothing and a smooth aerodynamic helmet.
4
Use a frame and wheels designed to cut through the air, so that less energy is transferred to the air and more goes into kinetic energy.
Section 5 of 8Evaluating an improvement (HT only)
1Find the waste
- Find the waste Work out the useful and wasted transfers.
2Pick a fix
- Pick a fix Choose a change that targets the largest loss.
3Check the result
- Check the result Less energy goes to the thermal store of the surroundings.
4Weigh the cost
- Weigh the cost Consider whether the extra cost of better materials or maintenance is worthwhile.
Section 6 of 8Efficiency in the real world (HT only)
Improving a device
Transport
- Transport Cars use streamlined bodies, low-friction bearings and light materials so that more fuel energy becomes kinetic energy.
Homes
- Homes Insulation, double glazing and LED lighting reduce the energy needed for the same heating and lighting.
Electrical systems
- Electrical systems Thicker, shorter wires have lower resistance and so transfer less energy to the thermal store of the cable.
Trade-offs
- Trade-offs A more efficient design may cost more to buy, so the energy saved over its lifetime must be compared with the extra price.
Section 7 of 8Key terms
| Term | Meaning |
|---|---|
| Efficiency | The fraction (or percentage) of the input energy that is transferred usefully. |
| Lubricant | A substance such as oil that reduces friction between surfaces. |
| Streamlined | Shaped to let air or water flow smoothly past, reducing resistance. |
| Electrical resistance | Opposition to the flow of current, which causes heating. |
| Bearing | A part that lets surfaces move without rubbing directly on each other. |
| Air resistance | A frictional force from the air that opposes the motion of an object moving through it. |
Section 8 of 8Exam technique
1
Saying efficiency can be over 100%. Useful output can never exceed total input.
2
Suggesting a device can be 100% efficient in practice. Some energy is always dissipated.
3
Listing the fix without linking it to less energy wasted by heating.
4
Forgetting that sound is also a wasted energy transfer from machines.
5
Dividing wasted energy by the input instead of useful energy by the input.
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