Electron Affinity

Author: E.E. Kimberly

In an electron tube the element from which it is intended to draw electrons is called the cathode. The element to which the electrons are drawn is called the anode. Electrons are reluctant to leave a cathode at ordinary room temperatures, and high voltage is required to release them. If the temperature of the cathode is raised, the electrons acquire added velocities which' if the temperature is high enough, enable them to leave the cathode; however, they tend to fall back again. In the state of separation from the cathode the restraining attraction of the electron toward the cathode is greatly decreased, and the electron may be attracted to the anode by a voltage much less than that which would be necessary with a cold cathode. When an electron is acted upon by a potential, as from battery B in Fig. 27-1, it leaves the cathode at a velocity that depends on the plate potential and the initial velocity attributable to temperature. The electron mass acted upon by the potential, called the anode or plate potential, is accelerated until it reaches the anode. The velocity v of an electron "falling through" a voltage E is

(27-1)

The velocity v is in centimeters per second, and is expressed in "electron volts" The initial velocity required for the electron to leave the cathode depends largely on the material of the cathode, and may vary between 1 and 6 electron volts. This voltage is called the electron affinity of the metal.

Every metal when heated will emit electrons, and the rate of emission increases with rise in temperature. Pure metals have a high affinity and must be heated to very high temperatures. Only tungsten and tantalum will withstand the necessary high temperature.