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In my lecture notes it is said:

In normal circumstances every atom, of any substance (conducting or not), has an equal amount of positively and negatively charged particles, hence it is neutral. Also, the body made from these particles, is neutral.

Also, free charge is defined as charge that doesn't belong to any atom or molecule, it's free to move and conducting substance is one that has many of these.

How can a substance have free charge, have every one of its atoms in neutral state and not be charged?

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  • $\begingroup$ A surface can only have free charge, when excess electrons are supplied to it or removed from it. On the other hand, conductorz have many free electrons in their valence bands, which are easily tramsferred to the conduction band on application of an emf. $\endgroup$
    – Lelouch
    Commented Oct 24, 2016 at 16:20
  • $\begingroup$ @Lelouch Or, when we apply an external field? $\endgroup$
    – Desperado
    Commented Oct 24, 2016 at 16:23
  • $\begingroup$ When we apply an external field, the free electrons flow in an orderly fashion. $\endgroup$
    – Lelouch
    Commented Oct 24, 2016 at 16:27
  • $\begingroup$ In a metal, the conduction electrons don't belong to any particular atom. But the total number of electrons is equal to the total number of protons. The metal has free charges, but is also electrically neutral. $\endgroup$
    – garyp
    Commented Oct 24, 2016 at 16:35
  • $\begingroup$ @garyp Would that mean that not all atoms are neutral, that is we have positive ions(without any field)? $\endgroup$
    – Desperado
    Commented Oct 24, 2016 at 17:53

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It depends on the context or to what extent. In metal, outermost electrons are ionized and form electron sea. The metal contains metal cations and free electrons. When applying a voltage on the metal, electrons move and form electric current. If some electrons are removed or added on the metal so that there's net charge on the metal, electrostatic field is produced.

On the other hand, when a photon has sufficient energy (i.e. the wave frequency is high enough to overcome the work function) hitting on the metal surface, electrons may leave the metal, this is called the photoelectric effect. In this case, the photoelectric current can be regarded as a stream of free electrons.

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