The first thing to know is the difference between conductor and dielectric. There are free electrons in the conductor, but there are no free electrons in the dielectric. Are bound electrons. When these two substances are put into an electric field, the free electrons in the conductor are immediately electrostatically induced to the surface of the conductor, forming surface charges, and the negative charges (free electrons) move against the direction of the electric field. Relatively speaking, positive charge moves along the direction of electric field, which leads to positive and negative charges on the surface of conductor, which also produces electrostatic field. The electric field excited by surface charge is opposite to the external electric field. When the electric field generated by the free charge is exactly the same intensity, the electrons stop moving, which is electrostatic balance. After electrostatic balance, the sum of the electric fields in the conductor is zero. However, the dielectric is full of bound charges and cannot move. Under the action of external electric field, it can only be polarized. Insulating materials, such as plastic, rubber, etc. After polarization, the internal dielectric and field strength are not zero.