from the surface of a substance.Metals are used for electron emission
because a metal is made up of atoms bound in crystal lattices,of
electrons bound to the atoms,and of many free electrons which are not
bound to any particular location in the metal. The free electrons are
always in motion and travel more or less freely throughout the body of
the metal.However,these electrons are free only to the extent that
they may transfer from one atom to another within the metal but they
cannot leave the metal surface.If the kinetic energy of a electron is
increased by increasing the external energy then the electron will
cross over the surface carrier to escape from the metal.This amount of
kinetic energy required at absolute zero temperature is known as work
function of that metal.It is denoted by Ew and expressed in
electron-volt(eV).One eV is the amount me energy acquired by an fleas
when is is accelerated through a potential difference of one volt. 13:14
