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An
equation of state describes the locus of equilibrium states of a system
in terms of thermodynamic parameters. For simple systems, such as an ideal
gas, the thermodynamic parameters (p, v, T) may specify the state of the
system.
For
the ideal gas the state equation:
pv = RT is
sufficient. Non-ideal gasses require more complex equations of state, such
as the van der Waals equation: p
= {RT/(v - b)} - {a/v2},
to account for intermolecular
interactions.
For
condensed phases, the (p, v, T) surface tends to be approximately parallel
to the p-v plane and the state equation may be written as V = V(p, T).
For a small change in v:
dv
= (dv/dT)pdT + (dv/dp)Tdp,
so that dv = vbdT
+ vkdp
, where b
is the volume expansion coefficient
and k
is the isothermal compressibility of the material. |
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