HW 8.17 Work done in a phase change

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kshalbi
Posts: 68
Joined: Fri Sep 25, 2015 3:00 am

HW 8.17 Work done in a phase change

Postby kshalbi » Mon Jan 11, 2016 3:25 am

For image b, how do you know that work is done on the system?

Wendy_Xie
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Re: HW 8.17

Postby Wendy_Xie » Mon Jan 11, 2016 8:52 am

Because of the phase change.

Janae Yip 3G
Posts: 15
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Re: HW 8.17

Postby Janae Yip 3G » Mon Jan 11, 2016 10:22 pm

I am confused by this question as well. If the phase change goes from vapor to liquid, then heat should be released, signifying that the system is doing work. However, the answer says that work is done on the system. If that were the case, wouldn't the energy inside the system increase? Then there is no way vapor could change to liquid.

kshalbi
Posts: 68
Joined: Fri Sep 25, 2015 3:00 am

Re: HW 8.17

Postby kshalbi » Mon Jan 11, 2016 11:19 pm

Yeah, I still don't get it.

Chem_Mod
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Re: HW 8.17

Postby Chem_Mod » Wed Jan 13, 2016 12:19 am

Vapor to liquid is a reduction in volume which is doing work on the system (w<0). Also this phase change is exothermic (q>0). These signs being opposite allows U=q+w to be ZERO when the phase change occurs at a constant temp.


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