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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 m 3 . 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 c...