For a simple system, internal energy (u) is a function of two independant variables, thus we assume it to be a function of temperature T and specific volume v, hence:
Substituting equation (2) in the energy equation (1) and simplifying, we obtain:
Now for a constant volume process (dv = 0):
That is, the specific constant volume heat capacity of a system is a function only of its internal energy and temperature. Now in his classic experiment of 1843 Joule showed that the internal energy of an ideal gas is a function of temperature only, and not of pressure or specific volume. Thus for an ideal gas the partial derivatives can be replaced by ordinary derivatives, and the change in internal energy can be expressed as:
Consider now the enthalpy. By definition h = u + P v, thus differentiating we obtain:
Again for a simple system, enthalpy (h) is a function of two independant variables, thus we assume it to be a function of temperature T and pressure P, hence:
Substituting equation (6) in the energy equation (5), and simplifying:
Hence for a constant pressure process, since dP = 0:
That is, the specific constant pressure heat capacity of a system is a function only of its enthalpy and temperature. Now by definition
Now since for an ideal gas Joule showed that internal energy is a function of temperature only, it follows from the above equation that enthalpy is a function of temperature only. Thus for an ideal gas the partial derivatives can be replaced by ordinary derivatives, and the differential changes in enthalpy can be expressed as
Finally, from the definition of enthalpy for an ideal gas we have:
Values of R, CP, Cv and k for ideal gases are presented (at 300K) in the table on Properties of Various Ideal Gases. Note that the values of CP, Cv and k are constant with temperature only for mon-atomic gases such as helium and argon. For all other gases their temperature dependence can be considerable and needs to be considered. We find it convenient to express this dependence in tabular form and have provided a table of Specific Heat Capacities of Air.
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Engineering Thermodynamics by
Israel Urieli is licensed under a
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