Thermionic emission:
Thermionic emission is the emission of electrons from a hot metal surface such as a heated filament.
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| Thermionic Diode |
A thermionic diode as shown in the above diagram is an apparatus used to demonstrate thermionic emission.
In the above experiment, the anode is connected to a d.c supply of (say) 400 V d.c and it ( the anode) is connected to the positive connection of the d.c supply whereas the cathode is heated, this other connection ( the negative connection) is linked to the cathode to complete the circuit of the 400V d.c. To detect if there is a current through the circuit, an milliammeter is parked within the circuit, between the d.c supply and the anode.
As a variaton to the above arrangement, the cathode can also be connected to a separate 6 V d.c supply used to heat the cathode (which is also the tungsten filament) by passing a small current through it as shown below.
A metal surface has many free electrons which are bound to the surface because they do not have sufficent kinetic energy to break free. Once metal (such as the filament) or the cathode in the above diagram, is heated, electrons in the atoms absorb heat energy. This will give some of them enough kinetic energy to overcome the attraction that binds them to the nucleus.
The moment they overcome the attraction, they are released from the atom. Since heat is what is causing this emission, this is known as thermionic emission. It can be analogous to the evaporation of boiling water although it is not exactly the same. Enough energy must be absorbed before electrons are emitted from the metal surface.The heat that brings in the boil gives enough energy to water vapors to escape into the atmosphere. Thus thermionic emission can in one way be called the boiling and escaping of electrons out of highly heated metals.
The negatively charged electrons travels in high velocity. The vacuum allows electrons to move freely without collision with air molecules.
The negatively charged electrons are repelled from the cathode (or tungsten filament) and attracted to the anode ( which is positively charged as it is connected to a positive of the d.c supply).
The electrons that arrived at the anode complete the supply 400V d.c supply circuit. As a result, the milliammeter registers a reading.
If the connection to the 400V d.c supply is reversed no current reading on milliammeter.
This is because the electrons emitted from the filament are repelled by the anode whenever the anode is negativey charged. No electrons will reach the anode. Therefore the circuit is incomplete. Hence no reading on the milliammeter.
Experiment shows that there is:
(a) No emission when the cathode is not heated or cold - it is basically an open circuit.
(b) No emission when the polarity of the anode and cathode are reversed
(c) No current when the tube is filled with a gas
(d) The anode current increased as the heater ( the cathode) is made hotter. The current reaches a peak no matter what voltage was applied to the anode for a given temperature of the cathode. This happens when all the electrons emitted by the cathode are being attracted to, and collected by, the anode.
A thermionic diode allows electrons to flow in only one direction.
When an electron leaves the cathode it leaves behind a positive charge, equal but opposite to that of the electron. In fact there are many millions of electrons leaving the cathode. As unlike charges attract, this means that there is a force pulling the electrons back to the cathode. Unless there are any further influences the electrons would stay in the vicinity of the cathode, leaving the cathode as a result of the energy given to them as a result of the temperature, but being pulled back by the positive charge on the cathode.
Factors affecting the rate of thermionic emission:
1. Surface area of metal - rate increases as size of surface increases, larger surface area allows more electrons to be emitted.
2. Temperature of metal - Rate increases as temperature of metal increases. Higher temperatures allows more electrons to gain sufficient kinetic energy to break free from the nucleus ( the metal surface).
3. Types of metal - Rate depends on type of metal. If temperature remains constant, different types of metals require different amounts of energy to eject electrons.
4. Nature of metal surface. - Rate increases when metal is coated with metal oxide. Metal oxide such as barium oxide and strontium oxide emit electrons at a lower temperature.
Hello Tuisyen, your blog is a good explanation about how vacuum tubes work. In my oblog I wrote a simple program to simulate the action. Ss I referred to your blog as a good introduction. Thank you.
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