dielectric    A  dielectric is an insulator, with a large band gap  (>  3 eV)  between the  conductor  and  the valance band. A dielectric  plate inserted between two capacitor plates increases the  capacitance by a factor of the dielectric constant, k of the material.  The dielectric constant is the ratio of permittivity of  the  material  to the  permittivity  of  free  space. When a dielectric  is  placed  in  an electric field, a physical process called  polarization  takes place. In a polarized dielectric the positive  and the negative charges are displaced producing dipole moment throughout   the dielectric   (see   polarization   of dielectric).  Polarization  P,  is defined  as the total dipole moment  per  unit  volume of  the  substance.  The polarization reduces  the electric field, by a factor of k . The Gauss law* in free space is modified as

Ñ .( k E)= r / e ₀

where E is the electric field in the dielectric. Vector  D º  e _0k E is called the electric flux density (earlier  called  as electric displacement). The SI unit of D is C m^-2. The vectors D, P and E are related as,

D = e _oE + P

Using the definition of D,

P = e _o( k -1) E (d7)

which gives polarisation P for an electric field E.(See also ferroelectric materials).