Editing Polarization density (section)

=== Amorphous materials ===
Another problem in the definition of '''P''' is related to the arbitrary choice of the "unit volume", or more precisely to the system's ''scale''.<ref name="def_P_M_Maxwell_eqs" /> For example, at ''microscopic'' scale a plasma can be regarded as a gas of ''free'' charges, thus '''P''' should be zero. On the contrary, at a ''macroscopic'' scale the same plasma can be described as a continuous medium, exhibiting a permittivity <math>\varepsilon(\omega) \neq 1</math> and thus a net polarization {{math|'''P''' ≠ '''0'''}}.