In: Physics
drive the change in entropy for an ideal gas
For an ideal gas, . Thus
Using the equation of state for an ideal gas ( ), we can write the entropy
change as an expression with only exact differentials:
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(5..2) |
We can think of Equation (5.2) as relating the fractional change
in temperature to the fractional change of volume, with scale
factors and
; if the volume increases
without a proportionate decrease in temperature (as in the case of
an adiabatic free expansion), then
increases. Integrating Equation
(5.2) between two states ``1'' and ``2'':
For a perfect gas with constant specific heats
In non-dimensional form (using )
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(5..3) |
Equation 5.3 is in terms of specific quantities. For moles of gas,
This expression gives entropy change in terms of temperature and volume. We can develop an alternative form in terms of pressure and volume, which allows us to examine an assumption we have used. The ideal gas equation of state can be written as
Taking differentials of both sides yields
Using the above equation in Eq. (5.2), and making use of the
relations ;
, we find
or
Integrating between two states 1 and 2
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(5..4) |
Using both sides of (5.4) as exponents we obtain
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(5..5) |
Equation (5.5) describes a general process. For the specific
situation in which , i.e., the entropy is constant,
we recover the expression
. It was stated that this
expression applied to a reversible, adiabatic process. We now see,
through use of the second law, a deeper meaning to the expression,
and to the concept of a reversible adiabatic process, in that both
are characteristics of a constant entropy, or
isentropic, process.