<Dd> ε r = C C 0 . (\ displaystyle \ varepsilon _ (r) = (\ frac (C) (C_ (0))).) </Dd> <P> For time - variant electromagnetic fields, this quantity becomes frequency - dependent . An indirect technique to calculate ε is conversion of radio frequency S - parameter measurement results . A description of frequently used S - parameter conversions for determination of the frequency - dependent ε of dielectrics can be found in this bibliographic source . Alternatively, resonance based effects may be employed at fixed frequencies . </P> <P> The relative permittivity is an essential piece of information when designing capacitors, and in other circumstances where a material might be expected to introduce capacitance into a circuit . If a material with a high relative permittivity is placed in an electric field, the magnitude of that field will be measurably reduced within the volume of the dielectric . This fact is commonly used to increase the capacitance of a particular capacitor design . The layers beneath etched conductors in printed circuit boards (PCBs) also act as dielectrics . </P> <P> Dielectrics are used in RF transmission lines . In a coaxial cable, polyethylene can be used between the center conductor and outside shield . It can also be placed inside waveguides to form filters . Optical fibers are examples of dielectric waveguides . They consist of dielectric materials that are purposely doped with impurities so as to control the precise value of ε within the cross-section . This controls the refractive index of the material and therefore also the optical modes of transmission . However, in these cases it is technically the relative permittivity that matters, as they are not operated in the electrostatic limit . </P>

If relative permittivity of mica is 5 its absolute permittivity is