Question detail
A machine has a useful power output of 80 W and a total power input of 200 W. What is its efficiency expressed as a percentage?
Try the question, check the answer, then read the explanation to understand the curriculum point.
At a glance
MCQ
Type
practice
Style
Topic
Conservation and dissipation of energy
Question
- A. 40%
- B. 50%
- C. 60%
- D. 70%
Answer
40%
Explanation
Efficiency (%) = (useful power ÷ total power) × 100 = (80 W ÷ 200 W) × 100 = 40%.
Common mistake
Confusing Efficiency Equations
Students often confuse the two efficiency equations, using useful energy output for total power input instead of total energy input.
Remember that efficiency can be calculated using either useful energy output divided by total energy input or useful power output divided by total power input. Always check which type of input is required.
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