Which are the Important Methods for obtaining Electron Emission?

From my previous post,you come to know about "Emission of Electrons".This Electron Emission can be classified in four different types.The details about that
four types are as below.

The External Energy which is used for Emission of Electrons,may come from variety of sources such as Heat Energy,Energy stored in Electric Field,Light
Energy or Kinetric Energy of the Electric Charges Bombarding the Metal Surface.

Accordingly,there are four principle methods of obtaining Electron Emission from the Surface of Metal.

01)Secondary Emission:-

When High-Speed Electrons suddenly strike a Metal Surface,they may give some or all of their Kinetic Energy to the Free Electrons in the Metal.

If the Energy of the striking Electrons is sufficient,it may cause Free Electrons to leave from the Metal Surface.This phenomenon is known as Secondary
Emission.

The Electrons that strike the Metal are called Primary Electrons while the Emitted Electrons are known as Secondary Electrons.

02)Thermionic Emission:-

In this method,the Metal is Heated about 2500 Degree Centigrate to enable the Free Electrons to leave the Metal Surface.The number of Electrons emitted
depends upon the Temperature.Greater the Emission of Electrons,with Higher the Temperature.This type of Emission is employed in Vaccum Tubes.

03)Field Emission:-

A strong Electric Field is applied at the Metal Surface which pulls the Free Electrons out of Metal because of the attraction of Positive Field.Very intense
Electric Field is required to produce Field Emission.

Usually,a Voltage of the order of million volts per centimeter distance between the Emitting Surface and the Positive Conductor is necessary to cause
Field Emission.Field Emission can obtained at Room Temperature,that's why,some time it is called as Cold Cathode Emission or Auto Electronic Emission.

04)Photo-Electric Emission:-

In this method,the Energy of Light falling upon te Metal Surface is Tranferred 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, greater is the Photo-Electic Emission.Photo-Electric Emission is utilised in Photo Tubes
which form the basic of Television and Sound Films.

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