Question detail
A light bulb has a useful power output of 40 W and a total power input of 100 W. Calculate the efficiency of the light bulb as a percentage.
Try the question, check the answer, then read the explanation to understand the curriculum point.
At a glance
Question
Type
exam_style
Style
Topic
Conservation and dissipation of energy
Question
A light bulb has a useful power output of 40 W and a total power input of 100 W. Calculate the efficiency of the light bulb as a percentage.
Answer
40%
Explanation
This question requires the calculation of efficiency expressed as a percentage, reinforcing the concept of energy transfer in devices. It tests the ability to apply the efficiency formula in a real-world scenario. This answer is linked to Efficiency because it applies the learning objective: Calculate efficiency using useful power output divided by total power input.. It shows the Physics relationship clearly and keeps energy, power, work done and efficiency terms distinct where they appear.
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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