Heat and Work relation

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Chew 2H
Posts: 34
Joined: Fri Sep 29, 2017 7:05 am

Heat and Work relation

Postby Chew 2H » Mon Jan 22, 2018 3:25 pm

Based on the deltaU=q+w equation, if a system is endothermic and q is positive, would the work always be negative? If not, when does it become positive?

Jakob von Morgenland 2C
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Re: Heat and Work relation

Postby Jakob von Morgenland 2C » Mon Jan 22, 2018 4:16 pm

I don't think the work needs to be negative if the given q is positive; I believe it is not a causal relationship ("if q is positive, then w must be negative", etc.). If you take a look at 8.13, you can see that this problem gives a negative deltaU value and a negative (but not as large) deltaH value, so therefore a large negative w value is needed.

Sarah 2F
Posts: 33
Joined: Fri Sep 29, 2017 7:06 am

Re: Heat and Work relation

Postby Sarah 2F » Thu Jan 25, 2018 9:55 am

In that equation q and w are separate so they don't directly affect each other since they are calculated separately.

Sophie 1I
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Re: Heat and Work relation

Postby Sophie 1I » Thu Jan 25, 2018 9:59 pm

Not necessarily for this equation.

Jimmy Zhang Dis 1K
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Joined: Fri Sep 29, 2017 7:05 am

Re: Heat and Work relation

Postby Jimmy Zhang Dis 1K » Fri Jan 26, 2018 10:42 am

Normally when solving delta U = q(heat) + w(work) q and w are calculated separately. The work could have a negative or positive value depending on the delta V in the equation.

Guangyu Li 2J
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Re: Heat and Work relation

Postby Guangyu Li 2J » Fri Jan 26, 2018 11:02 am

The value of work is not necessarily negative.

According to what the textbook states in CH8: First Law of thermodynamics, △U=q+w
1. the q and w is determined by the realistic circumstance
2. the positive and negative sign is assigned to represent that if the work is done to the system and the system does work to the surroundings (or if the heat is transferred from the system to the surroundings or from the surroundings to the system)
3. the values of q and w are actually really random

Hope this helps!

Warda Sahib 2J
Posts: 29
Joined: Thu Jul 13, 2017 3:00 am

Re: Heat and Work relation

Postby Warda Sahib 2J » Sun Jan 28, 2018 2:53 pm

No, the work doesn't necessarily have to be negative in this case.


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