The substances containing no free charges are celled dielectrics. Each dielectric possesses positive and negative charges both, but they are not free and their algebraic sum is zero. [i.e. Σq = 0 for a dielectric and any charge q is not free]. The dielectrics is of two types
(i) Polar dielectrics: In some dielectrics the centres of positive and negative charges do not coincide, so that dielectric has permanent electric dipole moment, such dielectrics are called polar dielectrics. The examples are H2o, HCl, CO, etc.
(ii) Non polar dielectrics: In some other dielectrics the centres of positive and negative charges coincide, so that the dielectric does not possess a permanent electric dipole moment; such dielectrics are called non polar. The examples are carton dioxide CO2, hydrogen H2, etc.