I tried lots of research, but still couldn't wrap my head around the difference between free and mobile electrons.

Free electrons, from wikipediaLink(I mean free particle bullet point, because other bullet points are not really free electron - as an example valence electron is not free, but mobile) are electrons that don't experience any external force, hence it experiences zero "electric field" from others.

as for the mobile electrons, they are loosely bound to the atom (valence electrons). Though, even if these electrons are ejected from the atom, they still experience some electric field from other atoms and could jump into their orbitals.

If the above is correct, how can both of these cases exist? how can free electrons not experience any electric field, while mobile electrons do? I know that in sun's core, there're free electrons moving through material. Why don't they then experience the electric field of other atoms?

  • $\begingroup$ Does this help answer your question? physics.stackexchange.com/questions/272831/… $\endgroup$
    – Amit
    Commented May 8, 2023 at 22:31
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    $\begingroup$ Does this answer your question? Do free electrons exist? $\endgroup$
    – Farcher
    Commented May 8, 2023 at 22:43
  • $\begingroup$ @Amit This is a question on the difference between free and bound electrons in condensed matter. What is the direct relation with a question about the coupling between electrons and photons in QED as a gauge theory? $\endgroup$ Commented May 11, 2023 at 7:44
  • $\begingroup$ @GiorgioP-DoomsdayClockIsAt-90 I simply found that the answer addresses the question of the existence and meaning of what are free electrons as well. $\endgroup$
    – Amit
    Commented May 11, 2023 at 9:20
  • $\begingroup$ @Amit Yes, but the meaning of free in that question is different from the meaning in this question. One should not stick to a single word. The result is that this question is in the close queue as a duplicate. $\endgroup$ Commented May 11, 2023 at 11:50

1 Answer 1


Consider a piece of metal. We call the electrons in the metal "mobile" because they can move around in the metal. On the other hand, we don't consider them "free": they cannot be removed from the metal without some added energy.

An electron may be freed from the metal by thermionic or photoelectric processes. Of course, it's never completely beyond interaction with photons and matter, but it is no longer confined to the piece of metal.


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