A Note on the Correct View of Adiabatic Process

Abstract Note: Please see pdf for full abstract with equations An adiabatic process is a thermodynamic process in which there is no exchange of heat energy or particles between the system and its surroundings. However the surroundings can alter the mechanical energy or potential energy of the system. This fact is not borne out well in any of the standard textbooks. Here we explicitly discuss the case where we alter the potential energy component and its consequences. There is a misconception that the reduced internal energy performs an external work during this process and that the net energy change of the system is zero. We are going to bust both these myths. Both the internal energy and stored mechanical energy either increase or decrease together in tandem depending on the external forcing. They do not feed each other in the absence of externally induced changes. The usual relationships we use, that is PV γ = Constant and P 1-γ T γ = Constant are valid only when the changes in external forcing is very minute. When the imposed external pressure is very small or very large compared to the system pressure then we get large errors in the calculations. Here we derive accurate equations which are valid over the entire range of operation. We also note that the textbook expressions can not satisfy both the energy eqn and ideal gas law simultaneously. Further the textbook eqns indicate that it must be a reversible process. But the accurate eqns show that this process is inherently irreversible.