THERMIONIC EMISSION & RADIOACTIVITY. Bound electrons.

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