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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 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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