Modern Thermodynamics
- Chapter 1
(a) (b)
Fig. 1.4 van der Waals considered molecular interaction and molecular size to improve the ideal gas
equation. (a) The pressure of a real gas is less than the ideal gas pressure because intermolecular
attraction decreases the speed of the molecules approaching the wall. Therefore p = pideal δp (b)
Since the molecules of a gas have a non-zero size, the volume available to molecules is less than the
volume of the container. Each molecule has a volume around it that is not accessible to other molecules
because the distance between the centers of the molecules cannot be less than the sum of the molecular
radii. As a result, the volume of the gas cannot decrease below this "excluded volume". Thus V in the
ideal gas equation is replaced with (V– bN) so that as p→∞, V→ bN.
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