**Thermodynamic force:**- We introduced before
*conjugated variables:*Let us set zero at the equilibrium value: . This is called*thermodynamic force.* **Examples:**- If
*x*is volume, the force is*X*=-(*P*-*P*), if_{0}*x*is the number of particles, , etc.

In equilibrium (*x*=0) we have:

(9) |

Two ways to create a fluctuation:

- 1.
- Apply an external force
*X* - 2.
- Wait long enough, and it will happen
^{}

Suppose we have *many* variables *x*_{i}. We have many
thermodynamic forces *X*_{i}. Equations for relaxation:

**Onsager Law:**- The coefficients are symmetric:

© 1997

Tue Oct 28 22:13:33 EST 1997