Efficiency — Science definition

Defined in 2 courses:

Useful output energy divided by input energy, often expressed as a percentage. (Physical Science: Matter, Forces & Energy)
The fraction of input energy that comes out as useful output: efficiency = (useful output ÷ total input) × 100%. (Physics & Environmental Science)

Where it is taught

Efficiency & Improving a Design — Work & Machines, Physical Science: Matter, Forces & Energy, Grade 8
Conservation of Energy — Work, Energy & Momentum, Physics & Environmental Science, Grade 12

In use

Efficiency describes the fraction of input energy transferred to a useful output: efficiency = useful output energy ÷ input energy × 100%.
Efficiency & Improving a Design

Efficiency is useful output divided by input for a specified service and boundary.
Conservation of Energy

Questions using efficiency

Problem 1

Input energy is 80 J; useful output is 60 J. Find efficiency.

Answer and explanation

75 %

60 ÷ 80 × 100 = 75.

Problem 2

Why does low efficiency not violate conservation of energy?

Answer and explanation

All input energy is accounted for, including thermal energy and other outputs that are not useful for the task.

Problem 3

A lift takes in 200 J and gives the load 150 J. Find efficiency and account for the remaining energy. Explain the reasoning in your own words.

Answer and explanation

75% efficient; 50 J went to other parts of the system and surroundings.

Efficiency=(150/200)×100=75%.

Remaining energy=200−150=50 J in other transfers such as thermal energy and sound.

Response criteria

Use the stated conditions.
Show the evidence or calculation.
Explain the conclusion and any stated limitation.

Problem 4

A device receives 400 J and delivers 300 J useful output. Find efficiency. Show the reasoning, not just the conclusion.

Hints

Divide the useful 300 J by the supplied 400 J, then multiply by 100%. Account for the remaining 100 J separately.

Answer and explanation

300/400×100%=75%.

Taught alongside

Elsewhere

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