# permittivity

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## Permittivity

A property of a dielectric medium that determines the forces that electric charges placed in the medium exert on each other. If two charges of *q*_{1} and *q*_{2} coulombs in free space are separated by a distance *r* meters, the electrostatic force *F* newtons acting upon each of them is proportional to the product of the charges and inversely proportional to the square of the distance between them. Thus, *F* is given by Eq. (1),

_{0}) is the constant of proportionality, having the magnitude and dimensions necessary to satisfy Eq. (1). This condition leads to a value for ε

_{0}, termed the permittivity of free space, given by Eq. (2), where

*c*is the velocity of light in vacuum.

If now the charges are placed in a dielectric medium that is homogeneous and isotropic, the force on each of them is reduced by a factor ε_{r}, where ε_{r} is greater than 1. This dimensionless scalar quantity is termed the relative permittivity of the medium, and the product ε_{0}ε_{r} is termed the absolute permittivity ε of the medium.

A consequence is that if two equal charges of opposite sign are placed on two separate conductors, then the potential difference between the conductors will be reduced by a factor ε_{r} when the conductors are immersed in a dielectric medium compared to the potential difference when they are in vacuum. Hence a capacitor filled with a dielectric material has a capacitance ε_{r} times greater than a capacitor with the same electrodes in vacuum would have. Except for exceedingly high applied fields, unlikely normally to be reached, ε_{r} is independent of the magnitude of the applied electric field for all dielectric materials used in practice, excluding ferroelectrics. *See* Capacitance, Capacitor, Ferroelectrics

## permittivity

[‚pər·mə′tiv·əd·ē]_{0}) is a constant appearing in Coulomb's law, having the value of 1 in centimeter-gram-second electrostatic units, and of 8.854 × 10

^{-12}farad/meter in rationalized meter-kilogram-second units. Symbolized ε.