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This part from University Physics turned my world upside down. What is the difference between ions, free electrons, and excess charge? Also, ions and free electrons make up a neutral conductor? Bloody how?

In practical problems we often encounter situations in which we want to know the electric field caused by a charge distribution on a conductor. These calculations are aided by the following remarkable fact: When excess charge is placed on a solid conductor and is at rest, it resides entirely on the surface, not in the interior of the material. (By excess we mean charges other than ions and free electrons that make up the neutral conductor.)

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I'm not an expert, but this is my idea on what they are and how are related:

  • Free electrons. Electrons that are not bound to the nucleus of an atom.
  • Ions. Are atoms which have lost or gain electrons respect the number of protons in their nuclei.
  • Excess charge. A neutral conductor could have free electrons and ions occurring naturally but is still neutral if you count the whole thing. For example, an electron from a copper atom "escape" its atom ("becoming" an ion + a free electron) and go (the free electron) to the near atom, the sum of charges of both atoms are still neutral. The excess charge is produced when more electrons are added to the conductor letting the conductor with an imbalance of charges.

The statement you quoted specifies that the extra charges (excess of electrons) reside in the outer surface which sometimes when you imagine them you may think they spread uniformly through the volume but it isn't true. Although it can still have this natural ions and electrons appearing and disappearing in the whole volume.

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