Electron Emission
Electron emission is actually the basic of tube working principal, it is defined as liberation of free electron from a surface of a substance caused by the externak energy transfered to the electrons.
Electron emission tends to occur on metal, because metal is a substance with much free electron in between its molecul. Nucleous attracting force does not strong enough to put the electron standstill.
Every time the free electrons move around from one molecul to another but it can't leave out from metal surface.
In order to emit from the metal surface these free electrons require additional external energy.
The amount of outside energy require by electron to emit from the metal surface is known as work fuction.
The work function ussually defined in elctron volt (eV) unit.
Types of electron emission
The electron emission from the surface of a metal is possible only if sufficient addition energy (equal to work function of the sources such as
heat energy, energy stored in electric field, light energy or kinetic energy of the electric charges bombarding the metal surface.
Accordingly there are following four principal method of obtaining electron emission from the surface of a metal:
Thermionic emission
In this method, the metal is heated to a sufficient temperature (about 2500oC) to enable the free electrons to leave the metal surface.
The number of electrons emitted depends upon the temperature. The higher the temperature, the greater is the emission of electrons. This type of emission is employed in vacuum tubes.
Field emission
In this method, a strong electric field is applied at the metal surface which pulls the free electrons out of the metal because of the attraction of positive field.
The strong the electric field, the greater is the electron emission.
Secondary emission
In this method, the energy of light falling upon the metal surface is transferred to the free electrons within the metal to enable them to leave the surface.
The greater the intensity of light beam falling on the metal surface, the greater is the photoelectric emission. Photoelectric emission is utilized in photo tubes which from the basis of television and sound films.
Photovoltaic emission
In this method, a high velocity beam of electrons or other out.
The intensity of secondary emission depends upon the emitter material, mass and energy of bombarding particles.
Work function
The minimum amount of energy required to be provided to an electron to pull it out of the metal surface is called the work function of the metal and the work function of a metal depends on the following:
The properties of metal,
The purity of the metal and nature of the metal surface