CHAP. 10: TRANSPORT PROCESSES [CONTENTS] 343
10.5 Kinetic theory of transport processes in dilute gases
Kinetic theory views transport processes as a consequence of processes going on at the molecular
level.
10.5.1 Molecular interpretation of transport processes.
Thermal conductivity is interpreted as the result of the transport of energy which molecules pass
to one another in mutual collisions. Faster molecules, which travel from places with a higher
temperature, pass part of their kinetic energy on collision with slower molecules at places with
a lower temperature and thus provide for energy transport in the system. The result of these
mutual collisions is the equalization of temperatures, i.e. attainment of thermal equilibrium.
Viscosity is the result of the chaotic movement of molecules during which the particles
travelling from areas with a higher flow rate of the fluid pass part of their momentum to
the slower molecules on collision. This leads to the equalization of the flow rates, which is
macroscopically manifested as internal friction in the flowing fluid.
Diffusion is also the result of the chaotic movement of molecules. By random moves the
molecules travel from higher-concentration areas to lower-concentration areas. This causes
spontaneous equalization of concentrations in the system.
Accuracy of the molecular description of transport processes depends primarily on the ac-
curacy of the model used for the description of forces acting between the molecules. We will
present only the two simplest models: an ideal gas and the hard spheres model.
10.5.2 Molecular models
Anideal gasis the simplest model of intermolecular forces. It assumes that molecules are
mass points which do not interact at all.
This model is not very successful in relation to transport properties—no energy or mo-
mentum transport by way of collisions between molecules occurs because the probability of a
collision of two mass points is zero. In this case the heat flow is caused solely by molecules
collisions with the warmer or colder walls of the containing vessel. The heat flow mechanism is
thus radiation rather than conduction.