Forms of U Equations


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Kimberly 1H
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Joined: Fri Sep 28, 2018 12:17 am
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Forms of U Equations

Postby Kimberly 1H » Wed Jan 30, 2019 9:53 pm

How can you tell when to use U=w vs U=w+q vs U=q?

Ava Kjos 1D
Posts: 65
Joined: Fri Sep 28, 2018 12:19 am

Re: Forms of U Equations

Postby Ava Kjos 1D » Wed Jan 30, 2019 10:19 pm

It depends on the what's happening in the problem. U=w+q is always true and can always be used. It just means that the change in the energy of the system is going to depend on the work done by or on the system and the heat released/absorbed. However, in some problems there is no work being done. If there's no volume change then w=0 and so you can use the equation U=q. Likewise, in other problems, there is no heat being released or absorbed, in which case q=0. For these problems U=w.

Athena L 1B
Posts: 59
Joined: Fri Sep 28, 2018 12:18 am

Re: Forms of U Equations

Postby Athena L 1B » Sat Feb 02, 2019 12:39 pm

If we have the equations U = q + w, q = H, and w = PV, why does U = H - PV, instead of U = H + PV?

Chloe Qiao 4C
Posts: 65
Joined: Fri Sep 28, 2018 12:27 am

Re: Forms of U Equations

Postby Chloe Qiao 4C » Sat Feb 02, 2019 2:08 pm

Athena L 1B wrote:If we have the equations U = q + w, q = H, and w = PV, why does U = H - PV, instead of U = H + PV?

Actually w=-PdeltaV(the negative charge refers to the direction that energy acts on), and this is why we get U = H - PV.


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