Every machine wastes some of the energy it is given, usually as heat to the surroundings. Efficiency is the simple fraction that tells you how much of the input actually ends up doing what you wanted, and how much leaked away.
Efficiency = useful energy output ÷ total energy input (× 100%). Equivalently, efficiency = useful power output ÷ total power input (× 100%). On a Sankey diagram, it is the useful branch as a fraction of the whole.
Efficiency is the useful energy output divided by the total energy input, or equally the useful power output divided by the total power input.
Efficiency has no units and is never more than 100%.
Adjust the input energy and how much is wasted. The Sankey diagram splits the input into a useful branch and a wasted branch, and the efficiency is the useful share of the total.
Four quick checks on the definition, its limits, and the power form. Each correct answer earns XP and lights this skill on your star map.
Efficiency is calculated as...
The efficiency of a real device can never be greater than...
Efficiency has...
Efficiency can also be calculated from...
The defining equation, the power form, and the rearrangement for useful output:
A machine is supplied with 800 J of energy and produces 200 J of useful output. Find its efficiency.
Keep both terms the same kind. Divide energy by energy, or power by power, never a mixture of the two. Put the useful amount on top and the total input on the bottom, and remember to multiply by 100 if a percentage is asked for. A bare ratio above 1 means you have flipped the fraction.
Unlocks once the four checks above are done. Worth more XP, written in the style of Paper 2.
A motor is supplied with 500 J and does 300 J of useful work. Its efficiency is...
A lamp is 20% efficient and is supplied with 50 J of energy. The energy wasted is...
A pump has a useful power output of 120 W from an input power of 400 W. Its efficiency is...
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