Take the tour ×  Could someone please give me a good definition
of the following electric terms? Despite what searching I have
done, I have not come across a definition that I have found
clear for me to understand:    • Free electron   • Mobile charge
share|improve this question add comment  1 Answer  up vote 3
down vote accepted  I assume that this is in the context of
solid state, rather than, say, free electron lasers.  The
heuristic picture is that when you have solids, the atomic
orbitals overlap between nearby atoms and it is possible that
some of these orbitals essentially change shape so much that one
get orbitals which extend over the entire solid. The electrons
in these extreme molecular orbitals are called "free", as
opposed to the still localised ones which are referred to as
"bound". These free electrons can then transport charge around
and you have a conductor.  In metals that's about it, and there
are lots and lots of electrons about. In a semiconductor
however, the density of free electrons is