Dielectrics in an electric field behave according to their internal structure. They are also called non-conductors, because, as you know, they are substances that do not conduct almost electric current. They do not contain free charge carriers that would be able to move inside this dielectric.
A molecule is the smallest particle of a substance that preserves its chemical properties. It, in turn, itself consists of atoms with a positively charged nucleus and negatively charged electrons. Molecules are generally neutral. As the theory of covalent bonds says , one or several pairs of electrons formed in them, becoming common for connecting atoms, ensure the stability of molecules.
For each type of charge - positive (nuclei) and negative (electrons) - there is a point that, as it were, is for them a "center of gravity" (electric). These points are called the poles of the molecule. If the electric centers of gravity of opposite charges coincide in the molecule: positive and negative - it will be non-polar (without a dipole moment).
The structure of the molecule can be asymmetric, say, there can be two dissimilar atoms in it, then to some extent a shift of the common pair of electrons in the direction of one of the atoms should occur. It is clear that in this case, the uneven distribution of opposite charges (positive and negative) inside the molecule will lead to a mismatch of their electric centers of gravity. The resulting molecule is called polar or having a dipole moment.
The main property of dielectrics is their ability to polarize.
Dielectrics in an electric field are polarized. This means that in their atoms, the electrons begin to move in elongated orbits. As a result, some of their surfaces are negatively charged, while others are positively charged. Thus, an electric field arises in dielectrics, which, accordingly, is called internal. That is, dielectrics are simultaneously affected by electric fields (external and internal), which are in this case oppositely directed.
The resulting electric field has a strength equal to the difference in strength between the larger and smaller of the fields. It should be noted that the field strength in the dielectric, regardless of its type, is always less than the strength of the external electric field, which caused its polarization.
The polarization intensity is directly proportional to the dielectric constant of the dielectric. The smaller it is, the less intense polarization occurs in the dielectric and the stronger the electric field in it.
Charges appear not only on the surface, but also at the ends of the dielectric, but their transition upon contact with the electrode is impossible, since the nonconductor is attracted to the electrode by Coulomb forces.
Dielectrics in an electric field, if it is strong and its intensity can be increased, at certain values of intensity, they will begin to break through, that is, electrons will begin to break away from the atom. This will lead to the process of ionization of the dielectrics, as a result of which they will become conductors.
The magnitude of the external field, which leads to the breakdown of the dielectric, is called its breakdown intensity. And the corresponding ultimate voltage at which the dielectric breaks through is the breakdown voltage. Another name for the ultimate voltage is known - the dielectric strength.
It should be noted that only dielectrics in an electric field have an internal field, which basically disappears if the external is removed.