Hello All, I am interpreting a diagram from the following question. (I didnt have a chance to take thermo and I learned it by myself so I might have some problems) http://www.physicsgrad.com/pgre/9677-15 I understand that I can just do ΔW and I get the right answer, but upon doing it with the specific heats, I am a little confused. The isobaric process (B->C) specifically: I looked at this solution: http://grephysics.net/ans/9677/15 Why, in the calculation of U, is there C_v present? I thought the pressure was constant, and the volume changes, then why is U = C_v(ΔT)?? Any advise appreciated.