5.3 Combined First and Second Law Expressions
The first law, written in a form that is always true:
For reversible processes only, work or heat may be rewritten
as
Substitution leads to other forms of the first law true for
reversible processes only:
(If the substance has other work modes, e.g., stress, strain,
where
is a pressurelike quantity, and
is a volumelike
quantity.)
Substituting for both
and
in terms of state variables,
The above is always true because it is a relation between properties
and is now independent of process.
In terms of specific quantities:
The combined first and second law expressions are often more
usefully written in terms of the enthalpy, or specific enthalpy,
,
Or, since
,
In terms of enthalpy (rather than specific enthalpy) the relation is
Douglas Quattrochi
20060806
