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Structure and polarization of dielectrics

The basic characteristic of dielectrics is that they are polarized under the action of an external electric field. When the external electric field exceeds a certain limit value, the dielectric is broken down and loses its dielectric properties.

According to the structure of dielectrics, they can be divided into two categories. General dielectrics, such as hydrogen, its atoms (H), have only one electron rotating around a positively charged nucleus. The movement speed of this electron is very high. From a macroscopic point of view, it seems to be a charge evenly distributed on the surface of a sphere. The electric field generated by this distributed charge outside the atom can be regarded as the electric field of a point charge concentrated at the center of the sphere. Therefore, under normal conditions, the center of action of the electron of the hydrogen atom coincides with the positively charged nucleus. However, under the action of an external electric field, the electron and the nucleus are pulled apart by the forces f and f? of the electric field E and deviate from their original positions. At this time, each hydrogen atom can be regarded as a pair of positive and negative charges that are very close to each other, called electric dipoles. The directions of these electric dipoles are all along the direction of the external electric field. Therefore, positive and negative bound charges will appear on the surface of the dielectric, which is called the polarization of the dielectric. The stronger the external electric field, the more bound charges appear on the surface of the dielectric, and the higher the degree of polarization. This polarization is an instantaneous process. When the external electric field is removed, the action centers of the positive and negative charges will overlap and return to their original state. It has nothing to do with the energy loss in the dielectric. For dielectrics such as hydrogen, the action centers of the positive and negative charges of their atoms or molecules overlap when there is no external electric field, and they are called non-polar molecules. In addition to hydrogen atoms, this type of dielectric also includes methane, paraffin, polystyrene, etc.

There is another type of dielectric, such as water, plexiglass, etc., the action centers of the positive and negative charges of their molecules do not overlap even when there is no external electric field. Each molecule is equivalent to an electric dipole, so this type of molecule is called a polar molecule. However, in the absence of an external electric field, due to thermal motion, the arrangement of the electric dipoles is very chaotic, and the entire dielectric is still neutral and has no effect on the outside. When this dielectric is placed in an external electric field, the electric dipole will turn in the direction of the external electric field, and bound charges will appear on the surface of the dielectric. The stronger the external electric field, the more orderly the electric dipoles are arranged, and the higher the degree of electric polarization. The polarization process of this type of dielectric is more significant in liquid dielectrics than in solid dielectrics, because in liquids, molecules are easier to rotate. For non-uniform dielectrics, the result of electric polarization is that in addition to the bound surface charge, there is also bound body charge inside.

The thermal motion of molecules will prevent the orientation of dipoles, so this type of polarization is accompanied by energy dissipation. Compared with the previous polarization, it is a long process because it takes a certain amount of time for molecules to rotate.

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