The passage of electrons from a metal into a gas or vacuum is
called electron emission. The mechanism of electron emission is classified into the followings:
(1) Thermionic emission: When energy applied to
the conductor to emit electrons is in the form of heat, the process is called
thermionic emission.
(2) Photoelectric emission: When the energy
applied to the metal is in the form of light energy, the process of electron
emission is known as photoelectric emission.
(3) Secondary emission: If a fast moving electron
collides with the surface of the metal then the emission of electrons takes
place. This is known as secondary emission.
(4) Field emission: If the electrons are emitted
from the surface of metal by maintaining a certain amount of electrostatic
field between the two plats of vacuum tube then emission is known as field
emission.
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