Permittivity

In electromagnetism, the permittivity ε of a medium is the ratio D / E where D is the electric displacement in coulombs per square metre (C/m²) and E is the electric field strength in volts per metre (V/m).

Permittivity is specified in farads per metre (F/m). It can also be defined as a dimensionless relative permittivity, or dielectric constant, normalized to the absolute vacuum permittivity ε₀ = 8.854 10^-12F/m.

When an electric field is applied, a current flows. The total current flowing in a real medium is in general made of two parts: a conduction current and a displacement one. A perfect dielectric is a material that shows displacement current only.

The permittivity ε and magnetic permeability μ of a medium together determine the velocity of electromagnetic radiation through that medium.

In a vacuum, these are given by

where μ₀ is the magnetic constant, or permeability of free space, equal to 4π × 10^-7 N·A^-2, and c is the speed of light, 299,792,458 m/s.

In case of lossy medium (i.e. when the conduction currents are not negligible) the total current density flowing is:

where , σ is the conductivity (responsible for conduction current) of the medium and ε_d is the relative permittivity (responsible for displacement current).

In this formalism the complex permittivity ε^* is defined as: