1. Physics: conductivity, resistivity

Metals have their outer electrons loosely bound to the nucleus, so they can move around groupings of atoms in the metal mass. This gives the metallic bond, which involves the formation of an "electron sea" around the atoms. Metals are therefore a collection of positive ions surrounded by a cloud of valence electrons that move around them. However, only a small number of these electrons, whose energy is close to that of a certain level, known as the Fermi level, are mobile under the effect of an electric field. Under the action of an external electric field, these electrons will be able to occupy higher, unoccupied energy levels, increasing their speed and thus facilitating the passage of electric current.

The influence of temperature should be noted here: electrons are scattered by the thermal vibrations of the ions, which become more and more important as the temperature...