The thermodynamics of irreversible processes postulates that, when local entropy production is represented as a sum of products of fluxes and forces, both must be linearly related to each other [22]. This can be expressed in the form
In equilibrium the entropy S is a maximum and the are therefore
zero. Thus the
are a measure of the deviation from the equilibrium
state. If the
were to be zero for all
, each flux
would depend only on its own driving force
,
and the various processes could be considered as independent. Thus the quantities
are a measure of the interference of the
th process with the
th process [7].
Onsager's theorem states that
The structure of the highly depends on the system. If the medium
is anisotropic, the
are tensors, and the
become scalars
if the medium is isotropic.
Rodrigo Torres 2003-03-26