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Thermionic Emission

The emission of particles from a hot filament was first discovered by Thomas Edison in 1883 but the effect was left unexplained until the discovery of the electron by J.J. Thomson in 1897.

In Edison's experiment, he inserted a second filament in the evacuated bulb of a carbon filament lamp and found that a current could be made to flow between the two filaments provided the second filament is made positive with respect to the first. There was no current when the second filament was made negative with respect to the first or when the first filament was not hot.

The results of his experiments showed that:

  • There were charged particles moving across the empty space. These particles gave rise to a current.
  • The particles were attracted by a positive potential, indicating that their charge must be negative.
  • The charged particles were emitted from the hot filament.
Edison Apparatus

Edison's Apparatus

After Thomson's discovery of the electron, the mechanism behind thermionic emission was explained. By then it was known that all metals contain some electrons which are free to move about. The atomic forces on these electrons are not sufficient to bind them altogether. If energy is supplied to these electrons in the form of heat, they gain enough kinetic energy and the electrons from the surface of the metal escape by the process of thermionic emission.

The hot filament giving off the electrons is known as the cathode and the filaments with the positive potential is known as the anode. Electrons given out by the cathode are attracted by the anode and hence give rise to the current observed by Edison.

Thermionic emission is the phenomenon of emission of electrons from the surface of a metal, when it is heated to a high temperature. The flow of (thermions) electrons constitutes the thermionic current. Metals like tungsten, tantallum, thoriated tungsten and metals coated with oxides of barium, strontium etc. are commonly used.

 

Work Function

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The minimum amount of energy required to emit electrons from a metal surface is called the work function or threshold energy of that metal.

The number of electrons emitted per second i.e. the rate of emission depends upon
  • the material of the surface
  • temperature of the surface
  • area of the surface
To obtain a good emission of electron the material used must have the following properties:

1) low work function so that the electrons may be emitted at low temperature.

2) High melting point so that the metal does not melt on heating to a high temperature.

Tungsten is a suitable metal, has a melting point of 3655 K but work function is high about 4.52 eV (electron volt-energy unit). It starts emission at 2500 K.

If thoriated tungsten (coated with carbon and thorium) is used work function reduces to 2.6 eV and emission begins at 2000 K.

If tungsten is coated with barium and strontium oxide work function reduces to

1 eV and emission begins at 1000 K and a good supply of electrons is obtained.

A metal contains a large number of free electrons. They move at random within the metal with different velocities. But at ordinary temperatures, they cannot leave the surface due to attractive forces between the electrons and the positive ions. If the metal is heated, the kinetic energy of the electrons increases.

Above a certain temperature, a large number of electrons get energy greater than the work function of the metal and the thermionic emission begins. When a metal surface is coated with oxides of barium, strontium etc., the work function is reduced.

Work Function for Some metals

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MetalWork function in eV
Barium coated tungsten1.6
Caesium coated tungsten1.4
Chromium (Cr)4.37
Platinum (Pt)6.2
Potassium (K)2.26
Sodium (Na)2.3
Tungsten (W)4.52
Zinc (Zn)4.24


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