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Re: Isobaric expansion



Glenn, to your question
Why does equilibrium thermodynamics work very well in one
situation and very poorly in another?

I would answer that equilibrium thermodynamics works well in both
cases you describe. In both cases it does what it claims to do; give
a complete description of the initial and final equilibrium states of
the systems.

It won't tell you anything of any non-equilibrium intermediate
process. In an irreversible process, such as a free expansion, many
thermodynamic parameters of the system such as pressure and
temperature are undefined and indefinable. In an (artificial)
quasi-static process, all intermediate states would be equilibrium
states, so I guess thermodynamics will tell you all about those states
but that wouldn't be much use, would it? Nevertheless such artificial
processes are the strength of thermodynamics since they allow you to
tread a path through natural irreversible changes from a real
equilibrium initial state to a real initial equilibrium final state.


Brian McInnes