The macroscopic description of nonequilibrium states of fluid systems
requires independent variables to specify the extent to which the system
deviates from equilibrium and dependent variables to express the rates of
processes.
The three principal transport processes are heat conduction, diffusion,
and viscous flow.
Each transport process is described macroscopically by an empirical
linear law.
Molecular theories of transport processes in dilute gases are based on gas
kinetic theory.
Transport processes in liquids are visualized as the motion of molecules
from one “cage” to another with the cages being made up of neighboring
molecules.
The electrical conductivity of solutions of ions can be understood on the
basis of ionic motion in an electric field.
