Symbol ε. The ratio of the electric displacement 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/r24πε0
In this statement of Coulomb's law using SI units, ε0 is called the absolute permittivity of free space, which is now known as the electric constant. 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/r24πε
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.