How would I show that during quastatic isothermal expansion of a monatomic ideal gas, the change in entropy is related to the heat input Q by the simple formula: delta S=Q/T Show that it is not valid for the free expansion process described. Answer: Putting heat into a system always increases its entrophy. deltaS=Nk lnVf/Vi (U,N fixed). Ireally don't know how to do this. Can someone explain?