Question detail
A light bulb has a useful power output of 60 W and an efficiency of 0.15. What is the total power input?
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. 400 W
- B. 300 W
- C. 450 W
- D. 500 W
Answer
400 W
Explanation
The correct option is "400 W". 400 W is right because it directly matches Efficiency: Calculate efficiency using useful power output divided by total power input.. The other options are less suitable because they change the required Physics relationship, confuse the relevant energy store or use a distractor that does not answer the 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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