Editing Permittivity (section)

=== Classification of materials ===
{{Classification of materials based on permittivity}}
Materials can be classified according to their complex-valued permittivity {{mvar|ε}}, upon comparison of its real {{math|''ε''′}} and imaginary {{math|''ε''″}} components (or, equivalently, [[Electric conductivity|conductivity]], {{mvar|σ}}, when accounted for in the latter). A ''[[perfect conductor]]'' has infinite conductivity, {{math|''σ'' {{=}} ∞}}, while a ''[[perfect dielectric]]'' is a material that has no conductivity at all, {{math|''σ'' {{=}} 0}}; this latter case, of real-valued permittivity (or complex-valued permittivity with zero imaginary component) is also associated with the name ''lossless media''.<ref>{{cite book|url=https://rutgers.app.box.com/s/rwzifofsu9slf8xy38f6uwhjd5gmn2q7|title=Electromagnetic Waves and Antennas|chapter=1: Maxwell's Equations|publisher=Rutgers University|first=Sophocles J.|last=Orfanidis}}</ref> Generally, when <math>\frac{\sigma}{\omega \epsilon} \ll 1</math> we consider the material to be a ''low-loss dielectric'' (although not exactly lossless), whereas <math>\frac{\sigma}{\omega \epsilon} \gg 1</math> is associated with a ''good conductor''; such materials with non-negligible conductivity yield a large amount of [[dielectric loss|loss]] that inhibit the propagation of electromagnetic waves, thus are also said to be ''lossy media''. Those materials that do not fall under either limit are considered to be general media.