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Thermal emission , photoelectric effect , Cathode ray tube & Photoelectric cell

From Online Sciences - April 15, 2018

Any metal contains positive ions & free electrons which can move around inside the metal but can not leave it due to the attractive forces of the surface which may be represented by a surface potential barrier , Some of these electrons can escape if given enough energy in the form of heat and overcome the forces of attraction at the surface .

This is the idea behind the cathode ray ( CRT ) which is used in TV , computer monitors and photoelectric cell , Surface potential barrier is the attractive force which attracts the electrons from inside & prevents them from liberating from the metal surface ,

Cathode ray tube

Cathode ray tube is used in television and computer monitors , It depends on the emission of the electrons from surface of a metal at heating ( Thermoelectric effect ) .

Structure & working method

The tube consists of a metal surface called the cathode , The cathode is heated by a filament , electrons are emitted from the electron gun ( Egun ) , due to heat , some electrons may overcome the forces of attraction at the surface ( surface potential barrier ) .

These electrons are liberated from the metal and then picked up by the screen , which is connected to a positive pole called anode , thus causing current in the external circuit , When the electrons hit the screen , they emit light which varies in intensity from point to point on the fluorescent screen , depending on the intensity of the electrical signal transmitted .

The energy of electrons ( K E ) is determined from the relation : K E =me v = e V

The energy ( Joule ) = Energy in electron VoltCharge of the electron

Where : ( ( me ) is the electron mass , ( V ) is the average velocity of electron , ( e ) is electron charge , ( V ) is the potential difference between cathode and anode ) .

The intensity of the electron beam can be controlled by using grid in the way of the electrons as well as using magnetic and electric fields ( plates X , Y ) to direct the bundle of electrons to sweep the screen point by point generating the picture , so called raster until the frame is completed .

Photoelectric cell

It is used to convert the light energy into electric energy as in the calculators , opening and closing the doors and some instructions , It depends on the emission of electrons due to falling of light on a metallic surface ( photoelectric effect ) .

Photoelectric cell structure
Observation :
Classical theory failed to explain photoelectric phenomenon because based on classical postulates

Einstein explanation

Einstein put an interpretation for all of this , which led him to Nobel prize in Physics in 1921 , He explained the photoelectric phenomenon as follows : A certain amount of energy is needed to liberate the surface electrons called the work function ( Ew ) .

For every metal there is a work function( Ew ) , if the falling photon has energy ( h ) which is less than the work function( Ew ) of the metallic surface , the electron would not be emitted at all no matter how intense the light might be and the emission is instantaneous .

If the falling photon has energy ( h ) which is equal to the work function (Ew ) of the metallic surface , the electron will be barely set free without any energy in this case , the frequency of the photon is equal to the critical frequency ( c ) .

Ew= h c = h c / c

Relation between photocurrent and light intensity

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