THERMIONIC EMISSION
& RADIOACTIVITY
Bound electrons
Conduction electrons
Conduction electrons
Conduction electrons
Conduction electrons
• Kinetic energy to e- provided by heating the metal - e- escapes
Thermionic emission
Edison’s experiment
B A
Edison effect
Factors affecting rate of thermionic emission
1. Temperature of the surface: t , rate of emission
2. Nature of the metal surface: work function, rate of emission
3. Surface area of the metal: surface area , rate of emission
Requisites of a good e- emitter1. Work function: low
2. Melting point of the metal: high
Tungsten – melting point - 3655 K work function - 4.52 eV
Thoriated tungsten - work function – 2.6 eV
Tungsten coated with Ba/Cs/Sr - work function – 1 eV
Cathode Ray Tube
Cathode Ray Tube
Electron Gun
Electron Gun
Deflecting system
Deflecting system
Electrostatic deflection
Deflecting system
Electromagnetic deflection
Deflecting system
Electromagnetic deflection
Fluorescent screen
CRT in televisions
CRT in colour televisions