When we introduce the definition of α into the equation for (∂U/∂T)p, we obtain
p
=απTV + CV (2.44)
This equation is entirely general (provided the system is closed and its composition is
constant). It expresses the dependence of the internal energy on the temperature at
constant pressure in terms of CV, which can be measured in one experiment, in terms
ofα, which can be measured in another, and in terms of the quantity πT. For a perfect
gas, πT = 0, so then
p
= CV (2.45)°
That is, although the constant-volume heat capacity of a perfect gas is defined as the
slope of a plot of internal energy against temperature at constant volume, for a perfect
gas CV is also the slope at constant pressure.
Equation 2.45 provides an easy way to derive the relation between Cp and CV for a
perfect gas. Thus, we can use it to express both heat capacities in terms of derivatives
at constant pressure:
Cp − CV =
p
−
V
=
p
−
p
(2.46)°
Then we introduce H = U + pV = U + nRT into the first term, which results in
Cp − CV =
p
+ nR −
p
= nR (2.47)°
which is eqn 2.26. We show in Further information 2.2 that in general
Cp − CV = (2.48)
When we introduce the definition of α into the equation for (∂U/∂T)p, we obtainp=απTV + CV (2.44)This equation is entirely general (provided the system is closed and its composition isconstant). It expresses the dependence of the internal energy on the temperature atconstant pressure in terms of CV, which can be measured in one experiment, in termsofα, which can be measured in another, and in terms of the quantity πT. For a perfectgas, πT = 0, so thenp= CV (2.45)°That is, although the constant-volume heat capacity of a perfect gas is defined as theslope of a plot of internal energy against temperature at constant volume, for a perfectgas CV is also the slope at constant pressure.Equation 2.45 provides an easy way to derive the relation between Cp and CV for aperfect gas. Thus, we can use it to express both heat capacities in terms of derivativesat constant pressure:Cp − CV =p−V=p−p(2.46)°Then we introduce H = U + pV = U + nRT into the first term, which results inCp − CV =p+ nR −p= nR (2.47)°which is eqn 2.26. We show in Further information 2.2 that in generalCp − CV = (2.48)
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