Permittivity (ε) is the ratio of the electric displacement (electric flux density) in a medium to the intensity of the electric field producing it. It is important for electrical insulators used as dielectrics.


If two charges q1 and q2 are separated by a distance r in a vacuum, the force F between the charges is given by:


   F = q1q2 / 4πε0 r 2.


In this statement of Coulomb's law using SI units, ε0 is called the electric constant, or, in older terminology, the absolute permittivity of free space. It has the value 8.854 × 10-12 F m-1.


If the medium between the charges is anything other than a vacuum the equation becomes


   F = q1q2 / 4πε r 2.


and the force between the charges is reduced. ε is the absolute permittivity of the new medium. The relative permittivity (εr ) of a medium, formerly called the dielectric constant, is given by εr = ε / ε0.