Efficiency as a decimal and a percentage, Sankey diagrams, how lubrication and insulation cut wasted energy, and the thermal insulation practical.
What you will learn
- Calculate efficiency with energies or powers, and rearrange it
- Read a Sankey diagram
- Explain how lubrication, streamlining and insulation reduce wasted energy
- Analyse results from the thermal insulation practical (Physics only)
Useful and wasted
No machine turns all its input energy into what you want.
The rest is wasted: it is dissipated, usually into the thermal store of the surroundings, where it spreads out and is no longer useful.
efficiency = useful output energy transfer ÷ total input energy transfer
efficiency = useful power output ÷ total power input
Efficiency has no unit. It is a decimal from 0 to 1; multiply by 100 for a percentage. Recall both forms: they are not on the sheet for AQA, Edexcel or OCR.
- A motor is supplied with 800 J and transfers 600 J as kinetic energy.
- efficiency = 600 ÷ 800 = 0.75
- as a percentage: 0.75 × 100 = 75%
- wasted = 800 − 600 = 200 J
To find the useful output, multiply: useful output = efficiency × total input, with the efficiency as a decimal (a 40% efficient lamp given 50 J gives 0.4 × 50 = 20 J of light).
To find the input, divide by the efficiency: a 60% efficient pump giving 0.3 kW useful needs 0.3 ÷ 0.6 = 0.5 kW. If your input is smaller than your output, you multiplied.
The rest of this lesson and its 10 practice questions are locked.
In this lesson
- Useful and wasted
- Sankey diagrams
- Wasting less
- The thermal insulation practical (Physics only)
- Try it: Your first efficiency
10 practice questions, marked as you go; about 25 minutes in all.
Key terms
Lubrication, Streamlining, Thermal insulation, Low thermal conductivity, Fair test, Directly proportional
In this unit: Energy stores, transfers and efficiency
Energy stores and how energy moves between them, kinetic and potential energy, specific heat capacity, power, efficiency and the energy resources that make our electricity.