AQA Combined Science (8464) 6.1.2.2 · Quantitative
Efficiency
Combined Science · Foundation tier
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An old filament bulb and an LED light a room equally well.
If you had to guess, which one wastes more energy?
Key words
- efficiency
- useful energy
- wasted energy
- dissipated
Key knowledge
Efficiency is how much of the energy you put in is used for the job you want.
Wasted energy is not destroyed.
Most wasted energy is dissipated to the surroundings as heat, warming them slightly. It is then stored in less useful ways.
The width of each arrow shows the amount of energy.
Energy in = useful energy out + wasted energy out.
USEFUL
10 J
WASTED
90 J
efficiency = 10 ÷ 100 = 0.10 = 10%
Efficiency has no unit. As a decimal it is between 0 and 1. Multiply by 100 for a percentage.
efficiency = useful output energy transfer ÷ total input energy transfer
efficiency = useful power output ÷ total power input
Efficiency has no unit. It is always less than 1. Multiply by 100 to get a percentage.
Lubrication reduces friction, so less energy is dissipated.
Thermal insulation reduces the energy lost to the surroundings.
Wrong: efficiency = useful ÷ wasted
Right: Always divide by the total energy in.
Wrong: An efficiency of 120%
Right: Nothing gives out more useful energy than goes in.
Wrong: Wasted energy disappears
Right: It is dissipated to the surroundings.
Key fact
Efficiency compares the useful energy out with the total energy in. Wasted energy is dissipated to the surroundings, mostly as heat, so no device is 100% efficient.
Find the useful energy first. Then subtract it from the total.
- Calculate: work out a number and show each line.
- Describe: say what happens.
- Explain: say what happens and why.
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Not sure which number goes on the bottom?
The Sankey diagram is drawn to scale for an input of 100 J. Device values are typical round figures for teaching, not measurements.