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October 10, 2025, 06:51:10 pm

Author Topic: Thermal energy  (Read 1961 times)  Share 

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Studyinghard

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Thermal energy
« on: April 11, 2011, 09:12:51 pm »
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Jamie is stacking shelves in the local supermarket, and does physical work of 12 kJ in 5 minutes. Jamie's efficiency for this activity is 12%. What is the rate at which thermal energy is generated? (in watt).

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schnappy

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Re: Thermal energy
« Reply #1 on: April 11, 2011, 09:16:19 pm »
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Power=12kJ/5min = 2.4 kJ/min.

Useful work is 12% of this, so the unuseful (presuming it is all heat) is (100-12)%=88%.

2.4*0.88=2.112 kJ/min

Then just convert to J/s (Watt)
(2.112*1000)*60=126.72 W

Studyinghard

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Re: Thermal energy
« Reply #2 on: April 11, 2011, 09:18:38 pm »
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Shouldnt it be 2.4/0.12
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Studyinghard

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Re: Thermal energy
« Reply #3 on: April 11, 2011, 09:31:31 pm »
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So does this mean that the answer to this

At what rate is heat energy lost due to sweating, if m = 0.1 kg of sweat is lost in 40 minutes? Use 2400 kJ as the total heat loss per kg of sweat or water evaporated. Answer in watt.

is 360
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Chromeo33

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Re: Thermal energy
« Reply #4 on: April 11, 2011, 11:41:42 pm »
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Whoa which area of study is this? Motion?

schnappy

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Re: Thermal energy
« Reply #5 on: April 12, 2011, 01:53:03 am »
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Yeah it's odd, but last year we got a question regarding efficiency. It's not motion but thermal energy as an energy loss in systems and the concepts of power, efficiency and energy are on the course.

Shouldnt it be 2.4/0.12
I don't see how.

So does this mean that the answer to this

At what rate is heat energy lost due to sweating, if m = 0.1 kg of sweat is lost in 40 minutes? Use 2400 kJ as the total heat loss per kg of sweat or water evaporated. Answer in watt.

is 360

240kJ/40min=6kJ/min=6000J/(40*60)s=2.5W.

Working looks right to me, maybe I'm doing something wrong?