HW 9.1 Rate of entropy of generation


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Nisha Patel 3I
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Joined: Fri Sep 25, 2015 3:00 am

HW 9.1 Rate of entropy of generation

Postby Nisha Patel 3I » Tue Jan 19, 2016 10:40 am

#9.1: "A human body generates heat at the rate of about 100. W (1 W  1 Js1). (a) At what rate does your body heat generate entropy in your surroundings, taken to be at 20. C?"


In the solutions manual, they derived the following equation: Rate of entropy generation = )

Can someone explain why there is a negative in front of the qrev?

704578485
Posts: 62
Joined: Fri Sep 25, 2015 3:00 am

Re: HW 9.1 Rate of entropy of generation

Postby 704578485 » Tue Jan 19, 2016 1:54 pm

Since the value qrev in this case will be the heat you body is giving off, it will be a negative value. During this process, your body is losing heat. Therefore, the surroundings are gaining heat and the heat gained is -qrev, since the problem and solutions manual assigns the variable qrev to be the heat lost by your body. You could have also done the problem by assigning qrev as the heat gained by the surroundings. In this case +qrev would be the surrounding's heat, while -qrev would be your body's lost heat.


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