Tuesday, February 26, 2019
What are free electrons?
1) Free electrons are presents in the outermost orbit of the atom.2) In an atom, the nuclear attraction on these electrons is minimum.
3) Free electrons can move freely inside the metal from atom to atom.
4) The energy of these electrons is maximum.
5) The property of all free electrons is the same.
6) In a metal, the number of free electrons per unit volume is about 1029 per m3.
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| free electrons |
Potential barrier or surface barrier
1) Potential barrier or surface barrier is a barrier at the surface experienced by the electron, which arises due to coulomb attractive force between the nucleus and the electrons of the atom of the material of that surface.2) At ordinary temperature, the energy of free electrons in a metal is less than the potential barrier energy hence they cannot leave the surface of metal.
3) When the kinetic energy of free electrons becomes more than the potential barrier energy, only then they can come out of the surface of metal.
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| Potential barrier or surface barrier |
Fermi energy
1) It is the maximum possible energy possessed by free electron of a metal at absolute zero temperature.2) The value of Fermi energy for different material is different.
3) Fermi level in semiconductor- It is that the energy level in energy-band-diagram for which the probability of occupancy becomes half.
4) The Fermi level may be defined as that energy level in energy band diagram which corresponds the centre of gravity of conduction electrons and holes weighted according to their energies.
5) The probability of finding the electrons of a conductor in Fermi level at zero Kelvin is 50%. It means, half of the electrons of the system are present in the Fermi energy level at that system.
6) The Fermi energy level is a reference energy level with respect to which the energies of other energy levels (like conduction energy level etc.) are measured.
Thermionic emission or Edition effect
1) It was discovered by Edison and hence known as Edition effect.2) It is a phenomenon of emission of electrons from the surface of metal when heated suitably. Here the energy required for the emission of electrons from the metal surface is being supplied by thermal energy.
3) The electrons so emitted are called thermal electrons or thermions.
4) The kinetic energy and velocity of these thermions are different.
5) Thermionic emission for different materials takes place at different temperature.
6) From the surface of less work function, at equal temperature more thermions are emitted per unit area. And from the surface of more work function, at same temperature, less electrons.