First Law of Thermodynamics - First Law of Thermodynamics For Open Systems

First Law of Thermodynamics For Open Systems

For the first law of thermodynamics, there is no trivial passage of physical conception from the closed system view to an open system view. For closed systems, the concepts of an adiabatic enclosure and of an adiabatic wall are fundamental. Matter and internal energy cannot permeate or penetrate such a wall. For an open system, there is a wall that allows penetration by matter. In general, matter in motion will carry with it some internal energy, and some potential energy changes will accompany the motion. An open system is not adiabatically enclosed. By definition therefore, adiabatic work cannot be done on an open system. In contrast to the case of closed systems, for open systems, in the presence of diffusion, there is no unconstrained and unconditional physical distinction between convective transfer of internal energy by bulk flow of matter, the transfer of internal energy without transfer of matter (usually called heat conduction), and change of various potential energies. The older traditional way and the Carathéodory way agree that there is no physically unique definition of heat and work transfer processes between open systems. The ideas of heat and work transfer for closed systems are superseded for open systems by the ideas of transfer of kinetic energy of bulk flow, of bulk potential and of internal energies, and of entropy.

An example is evaporation. One may consider an open system consisting of a collection of vapour in a controlled volume, enclosed except where it is allowed to receive more vapour from or to condense into its parent liquid, which may be considered as another open system in open contact with the vapour system. The process might be a mechanical increase in the controlled volume of the vapour. Some work will be done by the vapour in the mechanical part of the process, but also some of the parent liquid will evaporate and enter the vapour collection which is the system. Some internal energy will accompany the vapour that enters the system, but it will not make sense to try to uniquely identify part of that internal energy as heat and part of it as work. Consequently, the energy transfer of the process as a whole, though having a component of mechanical work, cannot be uniquely split into heat and work transfers to or from the open system. The component of total energy transfer that accompanies the transfer of vapour into the system is customarily called 'latent heat of evaporation', but this is a quirk of historical language usage, not in strict compliance with the thermodynamic definition of transfer of energy as heat. In this example, kinetic energy of bulk flow and potential energy with respect to external long range forces such as gravity are both considered to be zero. The first law of thermodynamics refers to the change of internal energy of the open system.