Various bodies, as everyone knows, are divided into non-conductors (dielectrics) and conductors according to their electrical properties. One of the features that conductors have in an electric field is that when the charges are in equilibrium on their surface, there will be no electric field inside them. How to explain this?
The thing is that conductors have special electric charges. So, metals, for example, are carriers of charges such as electrons, which have lost contact with atoms. They are called free electrons.
Such electrons in a metal conductor placed in an electric field, under the influence of the forces of this field will move in a direction that will be opposite to the electric field strength.
Take a conductor in an electric field ABCD, which is placed in a homogeneous field, with tension directed from left to right.
On the surface of the AC conductor, an excessive negative charge arises, and an excess positive charge on another DB. In this example, we see that the conductors in the electric field are electrified. The charges that appear on the surface of the conductor create an additional electric field inside it. Its lines of force have the opposite direction with respect to the lines of force of the main field. As a result, the intensity of the main field in the conductor decreases, i.e. the force that acts on free electrons, and also causes their movement to weaken. The charges that the conductors have in the electric field will stop moving when the intensity of the resulting field inside them becomes equal to zero.
So, with the equilibrium of charges on the conductor, the field inside it is absent. Its absence can be used to protect bodies from the influence of an external electric field. For this purpose, it is enough to surround the body with a thin conductive layer, for example, place it in a box of metal. There will be no field inside this box.
To prove the fact that there is no electric field in a charged conductor, in his experiment, Faraday built a large wire cage, which he installed on insulators and recharged. Being inside this cell with a supersensitive electroscope, Faraday proved that no electric forces act inside it, although a very significant charge has concentrated on the outer surface. This phenomenon is called electrification through influence or electrostatic induction. Its cause is the effect of an external electric field on unoccupied electrons in a conductor. And the charges that conductors have in an electric field are called induced charges.
The phenomenon of electrification through influence explains the attraction between electrified and non-electrified bodies, as well as the transfer of electric charge when such bodies come into contact.
When an electrified body is brought closer to a light conductor, then induced charges of both signs appear on it. So, charges of opposite signs will be attracted to the body, and charges of the same name will repel. Due to the fact that the same charges are located on the side of the light conductor, more distant from the body, the resultant force of these both forces is the force of attraction. Under the influence of this force, a light conductor will be attracted to the body. During contact, their induced charge of the opposite sign will be neutralized by a part of the induction charge, which is equal in magnitude to it. On the light conductor, the charge will remain the same sign as on the body.
Due to the fact that the light conductor now has the same charge as the body, it will push off from it; this is what we observe in experience.
Conductors and dielectrics in an electric field have various properties. So, dielectrics have virtually no free charges. When they are placed in an electric field, the phenomenon of polarization occurs.