Thermal emission , photoelectric effect , Cathode ray tube & Photoelectric cell
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 ( E – gun ) , 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 = ½ me v² = e V
The energy ( Joule ) = Energy in electron Volt × Charge 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 .
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
- The photoelectric cell consists of a metallic surface which is called cathode , This wire is called anode to do not hide the light falls on the cathode .
- When light falls on the metallic surface , some electrons take enough energy to escape .
- These electrons are picked by anode which causes current in the external circuit .
- The emission of the electrons ( called photoelectrons ) depends on the frequency of the incident light not the intensity of the light , Such that these electrons will not be emitted until the frequency of the incident light becomes greater than or equal to a certain value which is called the critical value ( νc ) however is the intensity .
- If the frequency of the incident light wave equal or greater than νc , so , the intensity of the photoelectric current ( number of electrons ) increases by increasing the intensity of the incident light to increase the number of incident photons on unit area of a surface , so , the number of electrons liberate from the surface increases .
- The velocity and the kinetic energy of the emitted electrons depend on the frequency of the incident light not its intensity .
- The emission of the electrons occur instantly as long as ν > νc , The electrons do not need time to collect energy even if the light intensity is low .
Classical theory failed to explain photoelectric phenomenon because based on classical postulates
- The emission of electrons ( photoelectrons ) depends on the intensity of incident wave regardless of its frequency .
- The kinetic energy ( or velocity ) of the emitted electrons increases with increasing the intensity of the incident radiation .
- Even in the case of low light intensity , giving sufficient time should give some electrons enough energy to be liberated , regardless of the frequency of the incident light .
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 ν ) 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 ν ) 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 ( νc ) .
Ew = h νc = h c / λc
If the falling photon has energy ( h ν ) which is greater than the work function ( Ew ) of the metallic surface , the electron will set free and the energy difference ( h ν − Ew ) is carried by the electrons as kinetic energy and it moves faster .
The photoelectric electrons are the emitted electrons from a metallic surface due to light falling with suitable frequency , Work function ( Ew ) is the minimum energy needed to free an electron from a metallic surface without gaining kinetic energy .
When the work function of zinc metal = 6.89 × 10−19 J , It means that the minimum energy needed to liberate an electron from the zinc metal surface without gaining kinetic energy = 6.89 × 10−19 J .
The critical frequency of a metal ( νc ) is the minimum frequency of light falling on a metallic surface to liberate the electron without gaining kinetic energy .
When the critical frequency of a surface = 4.8 × 1014 Hz , It means that the minimum frequency of light falling on the metallic surface to liberate the electron without gaining kinetic energy = 4.8 × 1014 Hz .
Photoelectric phenomenon is the emission of electrons from a metallic surface due to falling of light with suitable frequency .
The factors affecting the work function ( Ew ) of a metallic surface :
- The type of the material only .
- It does not depend on the light intensity , exposure time or potential difference between the anode and the cathode .
Relation between photocurrent and light intensity
If the frequency of the incident photon is less than the critical frequency of a surface , the photocurrent will not be emitted even if the light intensity or the time of falling of light increased , ν < νc .
If the photon frequency of incident light is greater than the critical frequency of a surface , the photocurrent intensity increases by increasing the light intensity ( increasing number of photons ) , ν > νc .
The relation between the kinetic energy of the emitted electrons from a metal surface and the frequency of the incident light :
The energy of the incident photon = The work function of a metallic surface + The kinetic energy of the emitted electrons .
E = Ew + K E
h ν = h νc + ½ mv² , This is Einstein’s equation for photoelectric phenomenon .