Efficiency cannot exceed 100% because a device cannot give out more energy than it receives. Energy is transferred, never created, and some of it always goes to heat or sound.
This lesson is part of the electricity and energy cluster. It pairs with interpreting an energy-flow diagram, where the wasted part is drawn out.
What is efficiency?
Efficiency is the fraction of the input energy that becomes useful output, written as a percentage. Useful output means the form of energy you wanted, such as movement from a motor or light from a lamp.
The wasted part is also real, and it is usually heat. Input = useful output + wasted energy, so useful output can at most equal the input.
Worked example: an invented motor
An invented classroom motor takes in 200 J of electrical energy. It lifts a small load, giving 150 J of useful energy. The rest is heat and sound.
Step 1: find the wasted energy. 200 − 150 = 50 J.
Step 2: apply the efficiency formula. Efficiency = 150 ÷ 200 × 100% = 75%.
Step 3: check. Useful 150 J + wasted 50 J = 200 J, which equals the input. Nothing is missing and nothing is extra.
Now a second question: a student measures a different motor and writes “input 200 J, useful output 240 J, efficiency 120%”. The working is correct arithmetic, since 240 ÷ 200 × 100% = 120%, but the result is impossible.
The mistake that costs marks
The slip is to accept 120% because the division is right. A student writes “the motor is 120% efficient, so it is very good”.
The correct reasoning is that 240 J out of a 200 J input means 40 J appeared from nowhere, which breaks the conservation of energy. Say this directly: “Efficiency cannot exceed 100% because useful output cannot be greater than input”. Then suggest a cause, such as a measurement error or another energy source that was not counted.
Why does the title say “stated” input?
The input is the energy the question says was supplied. Extra energy could enter from another source, such as a flywheel that was already spinning, and then the stated input would be incomplete.
So an efficiency above 100% tells you to check what the input really includes. Once every source is counted, efficiency cannot go above 100%.
Check yourself
An invented kettle takes in 6 000 J and gives 4 800 J of useful heat to the water. A student writes that its efficiency is 125% because 6 000 ÷ 4 800 = 1.25. Find the error and the correct efficiency.
Answer
The student divided input by output. Efficiency is useful output ÷ input: 4 800 ÷ 6 000 × 100% = 80%.
The 125% result is impossible because it would mean more energy came out than went in, which shows the division was the wrong way round.
What to study next
Continue with comparing two classroom energy systems under the same assumptions, where efficiency is one of the criteria. Return to the cluster overview for the full order.
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