CHAPTER 5:ELECTRONICS

CHAPTER 5:ELECTRONICS

5.1: ELECTRONS

LEARNING OBJECTIVES

• 5.1.1      Describe thermionic emission and cathode rays

• Describe effects of electric and magnetic fields on cathode rays

• Determine velocity of an electron in cathode ray tube

INTRODUCTION

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. 

cont...

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

Multiple Choice

The emission of electrons from the surface of a heated metal is called

• Vaporization

• Condensation

• Cathode emission

• Thermionic emission

The meaning of thermionic emission

• The liberation (emission) of electrons from a heated metal when it is  heated to particular temperature.

The mechanism of 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.  

Sources to produce the thermionic emission

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

Factors that affect the rate of  thermionic emission

1)          Surface area of the metal

2)         Temperature of the metal surface

3)          Types of metal

4)          The coated material of the metal surface

Cathode rays

A narrow beam of a fast electrons moving in a vacuum

The production of cathode rays by using vacuum tube

• A vacuum tube ,containing anode , cathode , a heating filament and high voltage power supply EHT.  

• The cathode is heated by a tungsten filament

• The heated cathode emits electrons through thermionic emission process

• They are  accelerated at a high speed between anode and cathode known as cathode rays.

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.