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Thermionic Emission
•When a metal is heated up the electrons more kinetic energy.
•During heating electrons in the piece of metal leap out into its
surroundings.
•The metal, having lost an electron, gets a positive charge.
•The result is an electrostatic attraction between the (negatively charged)
electrons that have leapt out of the metal and the (positively charged)
metal they have left.
•This tends to pull them back.
Thermionic Emission
•The result of the above is to produce what is called a “space charge
effect”.
•The hot metal becomes surrounded by a “cloud” of electrons, that have
jumped out of the metal, but are then drawn back by the attraction
between the electron and the metal.
•Taken overall, the system is still electrically neutral since we have the
negative electrons and the positive metal.
•The usual practice with valves is to use the heated metal as a cathode
This allows us to pass a current though the cathode and heat it up.
When we heat up the metal we get a cloud of electrons that are ‘boiling’
off the metal surface and then falling back again.
This property of ‘boiling off’ electrons is called ‘Thermionic Emission’ as
the emission of electrons is produced by the heat.
Thermionic Emission
Emission of electrons or ions by substances that are highly
heated
The driving force of this process is the thermal energy
The charged particles are called thermions
The number of thermions emitted increases rapidly as the
temperature of the substance rises.
The heated material may be in the form of a metal filament
or of some compound that coats and is heated by the
filament.
If the heated body carries a positive or negative charge, the
thermions will be of the same charge.
Thermionic valves
Devices which allow electron flow in one
direction only & work using heat
Thermionic Diode
The anode and cathode are made of good conductors (e.g. metals),
but are separated by an insulating envelope and the vacuum inside.
•heat the cathode by applying a voltage between the two cathode
heater leads, H1 and H2.
•The anode develops a negative potential with respect to the cathode,
and some current will flow through the resistor
•The amount of current depends on the temperature of the cathode,
its surface area, the distance to the anode, etc
Diode Valves
•A diode consists of an evacuated glass tube
into which are sealed into two separate
electrodes.
•The cathode or filament is constructed so that
as current flows through it, a space charge of
electrons developed around it as result of
thermal effect of the current
•The anode or the plate is the other electrode
•When positive it attracts electrons across the
valve
•Electrons can pass only from cathode to anode
as there is no space charge around the cold
anode
•Thermionic valve allows electrons to flow in
one direction, but not in the reverse direction
Triode Valves
•Works on exactly the same principle as the
diode valve, but has a third electrode (grid)
placed between the cathode & the anode
•It is possible, using an external circuit, to
make the grid negative, positive or neutral
•If neutral, the grid will not affect electron flow
across the valve
•If positive, it will attract electrons away from
the cathode & thus amplify the electron flow
through the valve
•If negative, the grid will repel electrons &
reduce or stop the electron flow
•In the above case valve can act as a switch or a
regulator
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