Thermodynamics - irreversible
Channel: bhadeshia123
Date: 16/10/2020

Thermodynamics as a subject is limited to the equilibrium state. Properties such as entropy and free energy are, on an appropriate scale, static and time-invariant during equilibrium.  There is an extension of the subject to systems that are close to  equilibrium so that they can be divided into subsystems where the rules of equilibrium can be applied locally {Denbigh:1955}. Parameters  not relevant to the discussion of equilibrium, such as thermal conductivity,  diffusivity and viscosity, then enter the picture   because they can describe a second kind of time independence, that of the steady state. For example, the  concentration profile does not change during steady-state diffusion, even though energy is being dissipated during diffusion. 


The thermodynamics of irreversible processes  deals with systems that are not at equilibrium but are nevertheless {stationary}.  The theory in effect uses thermodynamics to deal with {kinetic} phenomena. There is nevertheless, a  distinction  between the thermodynamics of irreversible processes and kinetics. The former applies strictly to the steady-state, whereas there is no such restriction on kinetic theory.




H. K. D. H. Bhadeshia
University of Cambridge
https://www.phase-trans.msm.cam.ac.uk/teaching.html