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 (ε_r ) of a medium, formerly called the dielectric constant, is given by ε_r = ε / ε₀.