Equation apply to reversible only or not?? My thinking must be faulty somewhere, but I can't work out whats gonig on... The equation Tds = du + pdv and du = dq + dw are supposed to be valid for all processes right? yet dw = -pdv only for reversible cases, yes? So how do you prove that du = Tds + fdx given that fdx = dw (for a stretched wire) if you don't know if its a reversible process? As fdx = dw can only be equated with the -pdv if its reversible. [or similarly, ydA in place of fdw for the surface tension of a droplet. Are you just supposed to know these are reversible?] Secondly, is Cp, 'heat capacity at constant pressure' independent of pressure? Anybody know the answers??