What is mean by Electron Emission?

At Room Temperature,the Thermal Energy in the Conductor is adequate to break the bonds of the Valence Electrons and leave them unattached to any one
Nucleus.These Unbound Electrons moves at random within the Conductor and are known as Free Electrons.

If an Electric Field is applied across the Conductor,these Free Electrons move through the Conductor in an orderly manner,thus constituting Electric Current.

Many Electronic Devices depend for their operation on the movement of Electrons in an evacuated space.For this purpose,the Free Electrons must be
ejected from the surface of Metallic Conductor by supplying sufficient Energy from some external source.This known as Electron Emission.

Electron Emission:-

Metals are used for Electron Emission,as they have many Free Electrons.If a piece of Metal is investigated at Room Temperature,the random motion of Free
Electrons is within surface of the metal.

These Electrons are free only to the extent that they may transfer from one Atom to another within the Metal Surface.But they could not leave the Metal
Surface to provide Electron Emission.

This is due to the Metallic Surface offers a barrier to Free Electrons and is known as Surface Barrier.

If we increase the Kinetic Energy of Free Electrons,by giving sufficient external Energy,the Electron will cross over the surface barrier to leave the Metal.

The amount of additional Energy required to emit an Electron from a Metallic Surface is known as Work Function of the Metal.

The Work Function of pure metal varies roughly from 2 to 6 eV.It depends upon the nature of the Metal,its purity and the conditions of its surface.It may be
noted that it is desirable that the metal used for Electron Emission should have Low Work Function,so that a small amount of Energy is required to cause
Emission of Electrons.

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