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Heat and Work relation

Posted: Mon Jan 22, 2018 3:25 pm
by Chew 2H
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?

Re: Heat and Work relation

Posted: Mon Jan 22, 2018 4:16 pm
by Jakob von Morgenland 2C
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.

Re: Heat and Work relation

Posted: Thu Jan 25, 2018 9:55 am
by Sarah 2F
In that equation q and w are separate so they don't directly affect each other since they are calculated separately.

Re: Heat and Work relation

Posted: Thu Jan 25, 2018 9:59 pm
by Sophie 1I
Not necessarily for this equation.

Re: Heat and Work relation

Posted: Fri Jan 26, 2018 10:42 am
by Jimmy Zhang Dis 1K
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.

Re: Heat and Work relation

Posted: Fri Jan 26, 2018 11:02 am
by Guangyu Li 2J
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!

Re: Heat and Work relation

Posted: Sun Jan 28, 2018 2:53 pm
by Warda Sahib 2J
No, the work doesn't necessarily have to be negative in this case.