In the thermionic diode a beam of electrons
moves a cross the tube and hits the positive anode.
If a hole is made in the anode
the electrons will go through it and a stream of electrons will emerge on the other side. This
device is called an electron gun.
Usually the anode is made cylindrical as shown in
the diagram. A high voltage, often as high as 3000 V, is connected between the cathode and
the anode and this accelerates the electrons to a high speed – around 30 000 000 m/s or
about 1/10 of the speed of light!
The
beam of electrons is called a cathode ray because it starts from the
cathode.
Experiments with cathode rays show that they have the following
properties:
1. Cathode rays are beams of fast moving electrons
2. Increasing the
heater voltage heats the cathode more and so it gives out more electrons. This makes the
beam stronger (brighter)
3. Increasing the anode voltage makes the electrons move
faster by giving them more energy
4. Cathode rays do not pass through glass or
metal
5. Cathode rays travel in straight lines in a vacuum
6. Cathode rays travel in
circles in magnetic fields
7. Cathode rays travel in parabolas in electric fields
8.
Cathode rays have a limited range in gases. This range increases as the energy of the
electrons in the beam increases.
You can work out the energy and speed of the
electrons using the following formulae.
Since: Energy = charge x voltage
the electrical energy of one electron is: Energy = eV
Where e is the charge
on the electron and V is the accelerating voltage, the voltage difference between the cathode
and the anode. But this is converted in to the kinetic energy of the electron (½mv2) where m
is the mass of one electron and v is its speed.
Therefore: ½mv2 = eV and so the
speed of the electron can be found by using the formula: