TABLE OF CONTENTS What goes up must come down.
It's all done with mirrors.
19th century aphorisms
We have several times arrived at useful results by using the concept of equilibrium. It is a pretty basic tenet of science and like a similar idea, conservation of energy, it is always coming in handy. When we say that the electrons in a solid have a Fermi-Dirac distribution of energies, we are really saying two things: first, that the system is in equilibrium; second, that it has a particular temperature. Temperature is a statistical concept and is bound up with the idea of equilibrium. On the one hand, we cannot meaningfully speak of the temperature of a single particle; on the other, if we have a system of particles that is perturbed from equilibrium, say by accelerating some of them, then for a transient period the temperature cannot be specified, since there is no value of T that will make the Fermi function describe the actual distribution. Of course, for electrons in a solid, or atoms in the gaseous state, the effect of collisions rapidly flattens out the perturbation, the whole system returns to its equilibrium state, and the idea of temperature becomes valid again, although its actual value may have changed.
We have on one or two occasions considered perturbed equilibrium. We saw, for example, in Chapter 1 that large currents may flow in a conductor with a very slight change in the energy distribution. Thus, we could describe...
