Question

from where does a metal gets energy to free an electron ?

Answer 1

If you are speaking of the photoelectric effect, the energy comes from incident photons, where the minimum energy to release eject electrons is equal to teh work function of the metal.

if you are talking of Thermionic emission, electrons are emitted through the applied heat giving the electrons enough thermal energy to over come the potential barrier at the surface.

There is also Fowler-Nordenheim tunnelling (AKA Field electron emission), in which an applied electric field lowers the potential barrier enough for electrons to quantum mechanically tunnel out of the metal. In this case the energy comes from the electric field.

Finally the last one I can think of is the Auger effect, where the kinetic energy of an incoming electron can knock an electron from an atom in the metal.

Answer 2

Thank you