Question detail
A machine has a useful power output of 1500 W and a total power input of 2000 W. What is its efficiency?
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. 0.75
- B. 0.60
- C. 0.85
- D. 0.50
Answer
0.75
Explanation
The correct option is "0.75". 0.75 is right because it answers the Efficiency objective: Calculate efficiency using useful power output divided by total power input.. The distractors are incorrect because they either use the wrong Physics quantity, confuse an energy store with a transfer, or ignore the wording of this question.
Common mistake
Confusing power with energy in efficiency calculations
Students often use total energy input instead of total power input when calculating efficiency for a device that operates over a period of time, leading to incorrect efficiency values.
Remind students that efficiency = useful power output ÷ total power input. Use power (W) values, not energy (J), and ensure both numerator and denominator refer to the same time interval.
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