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Energy, Work, and Power
14 questions By Tony R. Kuphaldt
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Question 13 of 14
A DC electric motor transforms 1.75 kW of electrical power into mechanical form. If the motor’s operating voltage is 300 volts, how much current does it “draw” when operating at full load (full power output)?
Also calculate the motor’s output power in units of horsepower, assuming 100% efficiency.
Reveal answerMotor current = 5.833 amps. Motor power = 2.346 horsepower.
Notes:To answer both parts of this question, students will have to apply Ohm’s (Joule’s) Law to calculate power from figures of voltage and current, and they will also have to research the conversion factor between watts and horsepower.
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Question 14 of 14
Two construction workers, standing on the roof of a building, hoist identical buckets of tools to the top by pulling upward on long ropes. The weight of the tools and buckets are the same for each worker, as is the distance each bucket is hoisted. However, one worker hoists their bucket from the ground to the roof in one-third the time that it takes the other worker. Compare the efforts of the two workers in terms of energy and power.
Reveal answerBoth workers perform the same amount of work, and expend the same amount of energy in doing so. However, the worker who hoists the bucket up in only one-third the time performed that work with three times as much power as the slower worker.
Notes:It is a common misconception to equate energy with power. Energy, technically, is a measure that is irrespective of time. Power, on the other hand, implicitly references the time taken to do a certain amount of work.
Thank you so much for this reviewer