PRODUCING AND USING STREAMS OF ELECTRONS

THIS CHAPTER WILL HELP YOU TO UNDERSTAND WHAT HAPPENS WHEN ELECTRONS ARE HEATED WHAT ARE THE CHARACTERISTICS OF THERMIONS EMISSION OF THERMIONS DEPEND ON DIFFERENT FACTORS WHAT ARE THE TYPES OF THERMION EMITTER APPLICATION OF THESE CONCEPTS IN CATHODE RAY OSCILLOSCOPE

WHAT DO WE KNOW ABOUT ELECTRONS? THE ELECTRONS REVOLVE AROUND THE NUCLEUS THE ELECTRONS WHOSE ORBITS ARE CLOSE TO THE NUCLEUS ARE CALLED BOUND ELECTRONS BECAUSE THEY ARE TIGHTLY BOUND TO THE NUCLEUS BY THE ATTRACTIVE FORCE THE ELECTRONS IN OUTER ORBITS ARE WEAKLY ATTRACTED TO THE NUCLEUS AND ARE LOOSELY BOUND. WHEN SUFFICIENT ENERGY IS GIVEN THEY CAN GET EMITTED. SUCH ELECTRONS ARE CALLED FREE ELECTRONS.

WHEN THE ELECTRONS ARE EMITTED FROM THE METAL SURFACE DUE TO HEAT , THE EMISSIONS ARE CALLED THERMIONIC EMISSIONS. THE ELECTRONS EMITTED ARE CALLED THERMIONS. EDISON WAS THE FIRST PERSON TO FIND THAT HOT FILAMENT EMITS ELECTRONS. HOWEVER Sir J.J.THOMSON IN 1897 DISCOVERED THE ELECTRON WHICH LED TO MORE UNDERSTANDING OF EDISON’S EXPT IN 1883.

WHAT IS AN ELECTRON EMITTER? ALL METALS CONTAIN A LARGE NUMBER OF FREE ELECTRONS. SO ANY METALS WHEN HEATED WILL EMIT ELECTRONS OR THERMIONS WHAT IS THE CHARACTERISTIC OF A GOOD THERMION EMITTER? THE WORK FUNCTION SHOULD BE LOW THE MELTING POINT OF THE SUBSTANCE SHOULD BE VERY HIGH NOW WHAT IS MEANT BY WORK FUNCTION? THE MINIMUM ENERGY REQUIRED TO EMIT AN ELECTRON FROM THE METAL SURFACE IS CALLED THE WORK FUNCTION. LOWER THE WORK FUNCTION, BETTER THE METAL IS AS A THERMION EMITTER.

LET US UNDERSTAND THE WORKING OF A CATHODE RAY TUBE

PRINCIPLE: THERE ARE THREE PROCESSES (i) THERMIONIC EMISSION (ii) DEFLECTION OF BEAM BY ELECTRIC AND MAGNETIC FIELDS (iii) FLUORESCENCE PRODUCED BY THE ELECTRON BEAM ON A FLUORESCENT SCREEN

THERE ARE THREE MAIN COMPONENTS ELECTRON GUN THE DEFLECTING SYSTEM A FLUORESCENT SCREEN

ELECTRON GUN: PURPOSE : TO SEND OUT A FINE BEAM OF ELECTRONS IT CONSISTS OF A CATHODE WHICH IS INDIRECTLY HEATED BY A FILAMENT CONNECTED TO A LOW TENSION BATTERY. ( 6V) THE CATHODE ON HEATING EMITS ELECTRONS. (PRESSURE IS 0.001 MM OF MERCURY) ANODES: THE ELECTRONS ARE ACCELERATED BY THE TWO CYLINDRICAL ANODES THEY NOT ONLY ACCELERATE, BUT THEY ALSO FOCUS THEM INTO A FINE BEAM. THERE IS A GRID BETWEEN THE CATHODE AND ANODE WHICH HELPS TO CHANGE THE NUMBER OF ELECTRONS. ( KEPT AT NEGATIVE POTENTIAL -20V TO 0 VOLT)

DEFLECTING SYSTEM: IT CONSISTS OF TWO PAIRS OF PLATES. ONE PAIR OF PLATES YY IS KEPT HORIZONTAL AND XX IS ANOTHER PAIR KEPT VERTICAL. WHEN A PD IS APPLIED ACROSS ANY PAIR OF PLATES, AN ELECTRIC FIELD IS SET UP WHICH DEFLECTS THE ELECTRON BEAM. DEFLECTION OF BEAM BY ELECTRIC FIELD: THE ELECTRON BEAM GETS DEFLECTED TOWARDS THE POSITIVE PLATE. MORE THE INTENSITY OF THE ELECTRIC FIELD, MORE IS THE DEFLECTION OF THE BEAM. DEFLECTION OF BEAM BY MAGNETIC FIELD: THE ELECTRON BEAM GETS DEFLECTED IN A DIRECTION GIVEN BY FLEMING’S LH RULE. MORE THE INTENSITY OF MAGNETIC FIELD, MORE IS THE DEFLECTION OF BEAM.

THE FLUORESCENT SCREEN: HERE A VISUAL PATTERN IS FORMED WHEN THE ELECTRON BEAM STRIKES AT IT.THE SCREEN IS COATED WITH BARIUM PLATINO CYANIDE OR ZINC SULPHIDE. THE ELECTRIC SIGNAL APPLIED ON THE DEFLECTING PLATES CHANGES INTO THE VISUAL PATTERN ON THE SCREEN. USES: TO INVESTIGATE THE WAVE FORM IN TELEVISION AS A PICTURE TUBE TO CHECK THE WAVE FORM OF A VARYING ELECTRICAL SIGNAL

WHAT HAPPENS WHEN THE FILAMENT CURRENT IS INCREASED? THE RATE OF EMISSION OF ELECTRONS FROM THE CATHODE WILL INCREASE. WHAT HAPPENS WHEN THE PD IS INCREASED? THE ENERGY OF THE ACCELERATING ELECTRONS WILL INCREASE WHAT HAPPENS WHENTHE POTENTIAL ON THE GRID IS VARIED? THE BRIGHTNESS OF THE PATTERN WILL VARY.