Polarization of dielectric is the phenomenon of emergence of charges under the influence of the external field. The charges appearing at the same time are called polarizing. There are two types of dielectrics and also mechanisms of their polarization.
Dielectrics and their types
Dielectrics consider substances which do not carry electric current. Carry many pure liquids, for example oils, gasolines and the distilled water and also ceramics, glass, seasoned wood, crystals of salts and gases under the influence of weak external fields to them. There is no clear boundary between conductors and dielectrics as all substances to some extent carry electric current. However, if the conductivity is expressed poorly, it can be neglected and considered substance the ideal insulator.
Under the influence of electric field the charges in dielectrics can be displaced only on insignificant distance, the size of this shift does not exceed the sizes of molecules and atoms. These shifts bring in emergence of the induced charges, unlike conductors, such charges can arise both on a surface, and in dielectric.
Mechanism of polarization of unpolar dielectrics
Carry substances which consist of atoms and molecules without own dipolar moment in lack of the field to unpolar dielectrics. These are gases with symmetric diatomic molecules — hydrogen, oxygen and nitrogen, plastic, organic liquids and gasolines. In them the centers of positive charges of kernels coincide with negative charges of electronic clouds. The mechanism of polarization of unpolar dielectrics is called induction. Under the influence of the external field the centers of charges are displaced slightly, at the same time each atom gets the induced dipolar moment. Its direction coincides with the direction of the field, and size depends on his tension. As each molecule got the dipolar moment, all dielectric also it got. Unlike conductors in which influence of the field is characterized by the size of the induced charges the important parameter of dielectrics is the dipolar moment of unit of volume — a polarization vector.
Mechanism of polarization of polar dielectrics
Molecules of some substances have own dipolar moment in lack of external electric field, call such dielectrics polar. Electronic density in molecules of polar dielectrics are displaced to one of atoms, the polarization mechanism other here. In lack of the external field the dipolar moments of molecules are focused chaotically, and their total moment is equal to zero. External electric field influences the rotating moment of each molecule therefore they begin to be guided so that their dipolar moment is built along a vector of tension of the external field. This mechanism of polarization is called orientation. At the same time dielectric gets the induced dipolar moment.