Metals have free electrons (valence electrons). To be freed from the metal, they need some minimum energy, called work function of the metal. Its unit is electron volt (eV). The work function of a metal depends upon the type of the metal and its temperature. To free them, energy may be supplied by any one of the following methods.
( 1 ) Thermionic Emission:
When current is passed through a filament so that it gets heated sufficiently, free electrons from the metal of the filament get emitted. Such emission occurs in diode, triode and TV tube (Cathode ray tube).
( 2 ) Field Emission:
When a metal is subjected to strong electric fields of the order of 108 V / m, electrons get emitted from the metal.
( 3 ) Photoelectric Emission:
When electromagnetic radiation of sufficiently high frequency is incident on a clean metal surface, free electrons are emitted from the surface. This method is called photoelectric emission and the electrons so emitted are called the photoelectrons.
No comments:
Post a Comment