The electron emission from the surface of a metal is possible only if sufficient additional energy is supplied from some external source. This external energy may come from a variety of sources such as heat energy, energy stored in electric field, light energy etc.,
there are four principal methods of obtaining electron emission from the surface of a metal
1) Thermionic emission:
In this method the metal is heated to sufficient temperature (about 2500 deg C) to enable the free electrons to the 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.
2)Field emission:
In this method, a strong electric field (high positive voltage) is applied at the metal surface which pulls the free electrons out of metal because of the attraction of positive field. The stronger the electric field, the greater is the electron emission.
3)Photo-electric 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 photo electric emission.
4)Secondary emission:
In this method, a high velocity beam of electrons strikes the metal surface and causes the free electrons of the metal to be knocked out from the surface.
there are four principal methods of obtaining electron emission from the surface of a metal
1) Thermionic emission:
In this method the metal is heated to sufficient temperature (about 2500 deg C) to enable the free electrons to the 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.
2)Field emission:
In this method, a strong electric field (high positive voltage) is applied at the metal surface which pulls the free electrons out of metal because of the attraction of positive field. The stronger the electric field, the greater is the electron emission.
3)Photo-electric 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 photo electric emission.
4)Secondary emission:
In this method, a high velocity beam of electrons strikes the metal surface and causes the free electrons of the metal to be knocked out from the surface.
