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# property>electrical permittivity

## What is Electrical Permittivity?

Permittivity is a property of a dielectric medium that quantifies how that particular medium affects an electric field or is affected by a field. It depends on the ability of the medium to polarize as a result of the electric field, and thus transmit (i.e. permit) that field. The SI units of permittivity are farads per metre (F/m).

Permittivity is not a constant but varies throughout the medium, and is also dependent on the frequency of the field applied, on temperature, or on the humidity (in the case of air). It can also depend upon on the strength of the field.

Mathematically the permittivity, ε, is given by the relation:

D = ε E

where, E is the electric field intensity or strength, and D is the electric displacement field which represents how the field affects the electric charges in a medium, both of these being vector quantities. This assumes that the material is homogeneous and isotropic.

The electric constant, ε0, is the permittivity of free space (i.e. a vacuum). This constant can itself be derived from the speed of light and the magnetic constant: ε0 = 1 / (c020) . The electric constant is also included in the Coulomb force constant group in Coulomb's law.

The permittivity of other materials are often defined in terms of the electric constant by means of a relative permittivity such that ε = ε0r. The relative permittivity is also referred to as the dielectric constant. A perfect dielectric has no conductivity, and could be used to build an ideal capacitor that would provide lossless storage of electrical energy.

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