The Thermionic effect[]
Thermionic emission[]
A metal filament or cathode in a vaccum can eject charge carriers, such as electrons or ions from its surface by the induction of heat to it, the heat will overcome the electrostatic forces holding the electrons to the atom. The electrons emitted will form an electric field around the metal filament, they form a cloud around the filament as the negative electrons are attracted to the positive metal filament. Taken as a whole system the charge of the metal and the cloud of electrons is neutral, it is just that some of the negative charge is displaced from the metal to its surroundings, this is the process of thermion emission.
ejected electrons and positive metal filament
Uses of the effect[]
This process is used to generate electrons in many devices used today including Cathode-ray oscilloscopes, x-ray tubes, Cathode-ray televisions and other visual display units. The electrons can be focused using another electrode and directed.
Work function[]
To enable the ejection of the electrons the thermal energy supplied to the metal must overcome the work function of the electrons.The work function is the minimum amount of energy needed for the electron to be emitted from a material. Differant metals will have differant work functions, by coating metals with differant metalic oxides the work functions can be reduced, and therefore less heat is required to cause the thermionic effect. Free electrons present in the metal with varying velocities can also gain energy and leave the material.
Ways in which metal filiment is heated[]
Direct heating[]
This is when a current is passed through the metal filament or cathode and the resistant properties of the conductor cause energy to dissipate in it, this dissapated energy causing the conductor to heat up will cause thermionic emission.
Indirect heating[]
Indirect heating is another way in which heat is provided to the metal filament and allows electrons to be emitted. When a differant heated filament close by provides the heat for the metal to release electrons from its surface.