When a dielectric is placed in an electric field, it develops induced charges that weaken the field. The dielectric constant describes this ability of a material, and it is also an important basis for judging the polarity of a polymer.

Definition of relative permittivity

Relative permittivity (also called the relative dielectric constant, written εr and sometimes κ, K or Dk) is the ratio of the material's permittivity εs to the permittivity of vacuum ε0:

εr = εs / ε0

In vacuum εs = ε0, so εr = 1. Put another way, fill the gap between the plates of a capacitor with a dielectric and compare it with the same capacitor in vacuum: the factor by which the capacitance increases is the εr of that material, εr = Cx / C0, where Cx is the capacitance with the dielectric and C0 the capacitance in vacuum. At the same voltage, the larger the εr, the more electrical energy the capacitor stores.

Because permittivity varies with frequency, the test frequency must be stated whenever values are compared.

Do not confuse it with dielectric strength: the dielectric constant describes how much electrical energy a material can store, whereas dielectric strength is the highest field it can withstand before breaking down. They are different properties.

Judging polarity from εr

• εr greater than 3.6: polar material.
• εr between 2.8 and 3.6: weakly polar material.
• εr below 2.8: nonpolar material.

Looking up the dielectric constant of a material therefore gives a rough idea of its polarity, and with it a hint of which materials and solvents it is likely to be compatible with.