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1 lgasl

Fig. 6.2 Modified Joule apparatus

If an equation of state appropriate to a nonideal gas is used in the bracketed term in Eq (6.30), the temperature change on expansion can be predicted. If the initial pressure is sufficiently high and the initial temperature low enough, the nonideal effect will be large enough to be measured.

Example: What is the change in temperature of nitrogen when the specific volume initially at 20oC and 10 MPa is doubled at constant internal energy?

Noting that CV = CV0 = 7/2R = 29.1 J/mole-K, and using Eq (2.3) for the equation of state in Eq (6.30) yields:

Integrating from v1 to 2v1 gives a temperature change of:

2CV0v1

2CV0RT1

It is clear from this equation that measurement of the gas temperature decrease in a Joule expansion provides a means of experimentally determining the coefficient a in the Van der Waals equation under conditions in which the gas exhibits nonideal behavior.

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Getting Started With Solar

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