CHAP. 10: TRANSPORT PROCESSES [CONTENTS] 332
Example
In the preceding example, the flux was the amount of heat, related to unit cross-section of the
rod, which passed through a given location per unit time. Heat was the intensive quantityY,
the change of temperature with location, or more accuratelyddTz, was the driving force.10.1.3 Basic equations of transport processes
It is postulated that the flux is proportional to the driving force
Jz=−CFz, (10.3)where the proportionality constantC depends on temperature, pressure, and in mixtures also
on composition. This postulate is usually fulfilled well. Deviations from proportionality may
be observed only in some materials or in the states very far from equilibrium.
Relation (10.3) is used when the flux occurs in only one direction (e.g. through a rod), or in
isotropic situations, i.e. when all directions are equivalent (heat spreading from a point source
in a thermally homogeneous environment may serve as an example).