permittivity (redirected from permittivities)

permittivity [‚pər·mə′tiv·əd·ē]

(electricity)

The dielectric constant multiplied by the permittivity of empty space, where the permittivity of empty space (ε₀) is a constant appearing in Coulomb's law, having the value of 1 in centimeter-gram-second electrostatic units, and of 8.854 × 10^-12farad/meter in rationalized meter-kilogram-second units. Symbolized ε.

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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 q₁ and q₂ 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),

(1) 

where 1/(4&pgr;ε₀) is the constant of proportionality, having the magnitude and dimensions necessary to satisfy Eq. (1). This condition leads to a value for ε₀, termed the permittivity of free space, given by Eq. (2), where

(2) 

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 ε₀ε_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