Download on Fast Moving Electrons

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?
1. THE WORK FUNCTION SHOULD BE LOW
2. 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.
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
(i) ELECTRON GUN
(ii) THE DEFLECTING SYSTEM
(iii) 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:
(i) TO INVESTIGATE THE WAVE FORM
(ii) IN TELEVISION AS A PICTURE TUBE
(iii) 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.