Does somebody understand well what are the fundamental reasons for
degradation of system's internal energy into heat (some books say
degradation of work into heat) during irreversible processes? Or the
reasons vary on a system to system basis? Or it's just a cyclic
definition: "irreversible processes are those in which portion of
system's internal energy transforms into heat" and "portion of
system's internal energy always transforms into heat in irreversible
processes"? The classic textbook example of a cylinder with a
frictionless piston and sand on the top does not explain fundamental
reasons, as for me.
Textbooks say that the systems in equilibrium cannot do work.
However, in the same books, reversible process is represented as a
chain of equilibrium states. The work done by this "chain" process is
maximum possible. Another contradiction I have trouble with.