CHAPTER 5:ELECTRONICS

Rainwater in a puddle was found disappear as the day warms through the evaporation process. The rainwater molecules are released out through the surface of the rainwater into vapor. 

A similar process occurs when the particles in a heated  metal and reach a certain temperature and the process known as thermionic emission..

A large number of electrons are free roam about inside a metal but an electron traveling outwards the surface is held back by the attractive forces of the atomic nuclei near the surface.

However, when the metal is heated , some of electrons have gained enough kinetic energy (thermal energy) to escape from its surface.  

Thermionic emission can be only produced with certain metals, because it occurs at temperatures similar to their melting point.

1)          Surface area of the metal

2)         Temperature of the metal surface

3)          Types of metal

4)          The coated material of the metal surface

The filament can be supplied with either d.c. or a.c. because both can produced heat.

The anode has a small hole in it to produce narrow beam electrons.

EHT is used to produce a stronger. electric field between anode and cathode caused cathode rays move with a higher speed.

The tube must be vacuum to avoid collisions between electrons and gas molecules  or gas molecules of the air, which would attenuate the beam because lost of energy.