AQA Physics 6.1.2.2
Efficiency
Every device wastes some of the energy you put in. Efficiency is the share that ends up useful. Draw it, calculate it, and work out the waste.
Triple Science · Foundation tier
Efficiency from energies
Tap the quantity you want to find.
As on the equation sheet
efficiency = useful output energy transfer ÷ total input energy transfer
Efficiency from powers
Tap the quantity you want to find.
As on the equation sheet
efficiency = useful power output ÷ total power input
Start here
Two bulbs, the same brightness.
An old filament bulb and an LED bulb light a room equally well. After an hour, the filament bulb is too hot to touch. The LED bulb is only warm.
If you had to guess, which bulb wastes more of the energy it is supplied with?
Sankey diagram · drawn to scale
Split the input into useful and wasted.
The width of each arrow is proportional to the energy it carries. What goes in comes out: useful output plus wasted output equals the total input.
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.
Where the waste goes
Wasted energy is not destroyed.
Wasted energy is dissipated: it spreads out into the surroundings and is stored in less useful ways, mostly as a slight warming of the surroundings.
Lubrication
Oil between moving parts reduces friction, so less energy is transferred to the thermal store of the parts and dissipated.
Thermal insulation
Insulating a hot device or a building reduces the energy transferred to the surroundings.
Think again · which number goes underneath
“Efficiency is the useful energy divided by the wasted energy.”
Your call
CalculateA motor is supplied with 200 J. It transfers 150 J usefully and wastes 50 J. What is its efficiency?
Equations · efficiency
Same triangle for energies or powers. Tap the quantity you want to find. A percentage is not a triangle: it is a conversion line, × 100 to give one, ÷ 100 before you use one.
Equation
On the sheetEfficiency from energies
Side by side: multiply.
One over the other: divide.
Tap the quantity you want to find.
As on the equation sheet
efficiency = useful output energy transfer ÷ total input energy transfer
- useful
- useful output energy transfer
- J
- eff
- efficiency
- no unit
- total
- total input energy transfer
- J
Equation
On the sheetEfficiency from powers
Side by side: multiply.
One over the other: divide.
Tap the quantity you want to find.
As on the equation sheet
efficiency = useful power output ÷ total power input
- useful
- useful power output
- W
- eff
- efficiency
- no unit
- total
- total power input
- W
Command words in this lesson
- Calculate
- Same units top and bottom. Decimal first, then × 100 if a percentage is asked for.
- Compare
- Work out the value for both, then say which is higher and by how much.
- Explain
- Say what is reduced, where the energy would have gone, and the effect on efficiency.
Key fact
Efficiency = useful output ÷ total input. It has no unit and is never more than 1. Wasted energy = total input − useful output.
Key note · AQA 6.1.2.2
Efficiency
- Efficiency = useful output energy transfer ÷ total input energy transfer.
- Efficiency = useful power output ÷ total power input. Both are on the equation sheet.
- No unit; between 0 and 1. × 100 for a percentage; ÷ 100 before using a percentage.
- Wasted energy = total input − useful output. It is dissipated, stored in less useful ways.
- Sankey diagram: arrow width is proportional to energy; input = useful + wasted.
- Lubrication and thermal insulation reduce unwanted energy transfers.
Photograph this card. Tomorrow, cover it and say each line first.
Previous and next
Connects to
Ask MrBadmus
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.