Work Function - Thermionic Work Function

Thermionic Work Function

The work function is also important in the theory of thermionic emission. Here the electron gains its energy from heat rather than photons. According to the Richardson-Dushman equation the emitted electron current density, J (A/m2), is related to the absolute temperature T by the equation:

where W is the work function of the metal, k is the Boltzmann constant and the proportionality constant A, known as Richardson's constant, is given by

where m and -e are the mass and charge of an electron, and h is the Planck constant.

Thermionic emission—electrons escaping from the heated negatively-charged filament (hot cathode)—is important in the operation of vacuum tubes. Tungsten, the common choice for vacuum tube filaments, has a work function of approximately 4.5 eV. Various oxide coatings can substantially reduce this.

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