Equations of non-equilibrium thermodynamics
The so-called equation of the entropy balance (8.4) plays a central role in non-equilibrium thermodynamics. This equation expresses the fact that the entropy of a certain volume element varies with time for two reasons. First, it changes due to the presence of some entropy flow into a given volume element; secondly, due to the presence of some source of entropy, the existence of which is due to irreversible phenomena within the volume element. We always deal with a positive source of entropy, since entropy can only arise, but not be destroyed. With reversible processes, there are no sources of entropy. This is the local formulation of the Second Law of thermodynamics.