Find out, what happens in the dielectric, when it comes to electric field. As you know, in the dielectric there are no free charge carriers. Electric charges of the dielectric are included in the composition of its molecules and can move only very small distances; within the molecule or atom.

Since the dielectric reduces the force of interaction between charges, t. e. weakens the electric field, we can conclude, the displacement of charges within the molecules of the dielectric is really happening. Find out the mechanism of this phenomenon.

First, let us imagine the atom, the diameter of a nucleus which has a size of order 10^-15 m. Then its electron cloud (as a first approximation assume, that it is spherical) will have a radius of about 10^-10 m. From a comparison of the sizes of the nucleus and the electron cloud can be seen, the nucleus of an atom can be taken as a point, located in the center of the cloud. If the atom will be in an electric field with a strength E, the cloud will move against the direction E for a distance lregarding the kernel (rice. 15.19).

Because the nucleus is several thousand times more massive than the electron, α-latest moves in the atom at very high speed (order 10⁶ m/s), the core only responds to the average force of attraction to the electrons in the atom. So we can assume, that the whole negative charge of the cloud is concentrated at its center, and the whole atom, in an electric field, can be likened to a system of two equal in magnitude and opposite in sign of charge q=Ze, located at a distance of l. Such a system is called the dipole. Therefore, when an atom is adjudged to be in an external electric field, then it is converted into an electric dipole, which creates its own electric field, weaken the external field in the dielectric (rice. 15.20).

Work g_E=lqcalled electric moment of a dipole. Electric point g_E is the vector, directed along the l from negative charge to positive (RNS. 15.21), the module which is determined by the ratio:

g_E=lq, (15.15)

It turns out, that electric moment of molecules, due to the displacement of the electronic cloud relative to the nuclei, proportional to the field intensity E, t. e.

g_E=αE, (15.16)

(α is called the electronic polarizability of the molecule). Then the more the strength of the external field E, the more moments become electric dipoles in the dielectric. In this case, all the vectors of electric moments of molecules of the dielectric are orientated parallel to the E. Such a dielectric is called polarized, and its dipoles are called soft and, because of their length l depends on E.

Dielectric polarization, due to the displacement of electron clouds in molecules, relative to nuclei, called electronic polarization. It occurs at any dielectric and interesting topics, that does not depend on temperature.

If the molecule has no centre of symmetry, it has its own electric point and in the absence of the field in the dielectric (rice. 15.22). Since the atoms in this molecule are rigidly connected, can be considered, that electric moment does not depend on the external field in the dielectric. Such dipoles are called hard. In figure. 15.22 depicts two possible configurations of molecules of type a₂In: α — nonpolar molecule, the resultant dipole moment is zero, b — polar molecule, the resultant dipole moment is determined by the vector sum of the dipole moments of the individual links. Natural dipoles are, for example, water molecules, in which the atoms are arranged, as in figure. 15.22, b (connection HE form an angle of 105°).

In the absence of external fields of natural dipoles are randomly distributed, therefore, their fields are mutually compensated. However, if to make such a dielectric in an external field, then each dipole will act a force couple (rice. 15.23, and). Therefore, the dipoles are rotated, and even in a strong field lining up chains along the lines of the field intensity (rice. 15.23, b). The dipoles create their own field (rice. 15.23, in), weaken the external field in the dielectric. This phenomenon is called orientational or dipole polarization of dielectric. Easy to understand, what is orientational polarization should decrease with increasing temperature of the dielectric, since chaotic motion of dipoles violates their ordered arrangement in the polarized dielectric.

In crystalline dielectrics, ion having the structure, there is even a third type of polarization. Under the influence of an external field the positive ions of the dielectric are displaced in the direction of the intensity vector, and negative ions in opposite direction. This phenomenon is called ionic polarization of a dielectric.

In figure. 15.23, b seen, that oppositely charged ends of neighboring dipoles should mutually neutralize their impact on other charges. Only the uncompensated charges on the ends of dipoles, the speakers on the surface of the dielectric. With the side, where is the line tension of the external field are included in the dielectric, are negative charges of the dipoles, and on the opposite end of the positive charges. All the charges on the surface of a polarized dielectric are related, t. e. part of the molecules. They are called polarization charges. All the impact of a polarized dielectric the electric field reduces to the action of only the polarization charges. This is true for all types of polarization.

Field inside the dielectric, created by polarization charges, directed towards external field (rice. 15.23, in), t. e. weakens the external field, but completely destroys it (compare with the Explorer). Unlike Explorer, there is even I! in, what, separating a polarized dielectric on the side, it is impossible to separate positive charges from negative. On opposite sides of each piece of polarized dielectric've always been charges of different signs. This proves, that polarization charges of the dielectric are indeed related, t. e. part of the dipoles.

The weakening of the field in the dielectric, due to its polarization, explains the influence of the dielectric on the interaction force between electrified bodies. Indeed, if two charges q₁ and q₂ placed in the dielectric, it is polarized and around a charge q₁ and q₂ polarization charges appear, which is equivalent to the reduction of charges q₁ and q₂ (rice. 15.24), so, and the forces of their interaction.

Now it becomes clear, why the force of interaction between charges was highest in vacuum and why the formula of Coulomb's law is the dielectric permeability of the medium.

Note, that at a sufficiently large value of the electric field in the dielectric can be a destruction of dipoles. While inside the dielectric, there are free charges, who in their motion cause mechanical destruction of the dielectric. The phenomenon is called dielectric breakdown. Example of breakdown can serve as the electrical discharge of a lightning bolt during a thunderstorm.