Electronics by Shiza

Definition
Electronics is the branch of Physics that deals with emission and effects of electrons and with the use of electronic devices.

Thermionic emission
If we heat a negatively charged piece of metal, we find that some of the conduction electrons have sufficient kinetic energy to escape from the surface of the wire. It is quite easy to imagine this if we think about a metal wire as a lattice of ions in a sea of free electrons. In effect we are evaporating the electrons off. This effect is called thermionic emission.

Figure 1

Figure 1 shows the effect of heating a cathode.



Figure 2

 Figure 2 shows an experiment to demonstrate thermionic emission.

The glass tube contains two pieces of metals called electrodes. The small metal plate connected to the positive terminal of the battery is the anode, while the small tungsten filament connected to the negative terminal is the cathode. The cathode is heated by passing a small current from the filament supply. Air is absent from the tube so that electrons do not collide with air molecules and their flow is not impeded.

Despite there being a gap between the cathode and anode, a current still flows when a p.d is applied between the electrodes. This is because the electrons emitted from the cathode are attracted towards the positive anode.

If the current through the filament is switched off, thermionic emission ceases. If the connections of the cathode and anode are reversed, no current flows as the electrons are pushed away from the anode and none can reach it. '' The tube and the electrodes together make the thermionic diode.

The Maltese cross tube
The Maltese cross tube is used to demonstrate some of the properties of cathode rays. The cathode rays, or electron streams, emitted by the tube's electron gun are directed towards a fluorescent screen in front of which hangs a metal Maltese cross. Those electrons that hit the screen give up their kinetic energy and cause the coating to fluoresce. However, the sharply defined cross-shaped shadow cast on the screen shows that electrons are unable to pass through the Maltese cross. Cathode rays are thereby shown to travel in straight lines, and to cause fluorescence.

The following figure shows the inside of a Maltese Cross tube. It has the same basic parts as the thermionic diode, but the anode has a hole in it.

The video shows how the beam of electrons makes the screen fluoresce.



Why does it get brighter when the cross falls down?

What is happening is that the glass is slowly damaged by the impact of the high energy electrons. This radiation damage﻿ slowly degrades the fluorescence properties of the glass. Because the glass in the shadow of the cross receives less total dosage, it is in better shape. Thus, when you lower the cross, the glass that was shadowed glows brighter. This phenomena is essentially identical with the "burning" you see on CRT computer monitors.

The deflection tube
In the deflection tube, a narrow beam of electrons leaves the hole in the anode and passes between two horizontal metal plates. The fluorescent screen in between the plates shows the path of the beam if a high p.d is applied. The positive metal plate attracts, and the negative plate repels the beam so that it is deflected downwards. (See figure)



The higher the voltage, the more the deflection.