The problem of electron velocity distribution

The object of this tutorial paper is to clarify the basic ideas referring to the electron velocity spread associated with thermionic emission. In particular we discuss the use of suitable probability density functions, the definition of mean velooity and mean energy and the concept of momentum and energy flow. In all cases the physical significance of the relevant quatities is considered, their relation to thermionic power conversion being noted, where applicable. The discussion deals with conservative, time-independent systems, a plane, parallel diode being used as a simple model.