Like other models, mixture theory requires constitutive relations to close the system of equations.

For real-world materials, the constitutive relations are not linear, except approximately.

In general, the constitutive relations can usually still be written:

To more exactly describe the passive elements, their constitutive relation is used instead of simple proportionality.

From any two of the circuit variables there are six constitutive relations that can be formed.

In some special cases the constitutive relation simplifies to a function of one variable.

There are, however, even more important considerations to be made regarding the general constitutive relation.

Frame indifference requires that the constitutive relation should not change when the location of the observer changes.

Material behavior is represented in physical theories by constitutive relations.

See Maxwell's equations and constitutive relation for more details.