Hello PF! I have some questions regarding these concepts. First of all, are the following expressions valid for any case? i.e. any kind of process, like isochoric, isobaric. [tex]\Delta U = \int C_v \ dT[/tex] [tex]\Delta H = \int C_p \ dT[/tex] Or is the ΔU expression only valid when dV = 0, and ΔH when dP = 0? Also, when we are having an irreversible process, we have a gas with pressure P and it's going to expand against an opposing pressure p. When do we use the specific heat at constant pressure Cp, when the gas pressure P is constant, or when the opposing pressure p is constant? I'm also having the same doubt with enthalpy, which pressure are we using in the function of enthalpy? Is it H = U + pV or H = U + PV? Thanks in advance!