Dielectrics are non-conductors. Their electric charge will not flow when an electric field is applied, but there will be a displacement of electric charges. This displacement of charges can occur at both the atomic level and molecular level.
At the atomic level, electrons are all confined in atoms in dielectrics. Electrons and nuclei carry negative and positive charges respectively. In the absence of an electric field, electrons move symmetrically around the nucleus inside the atom. The atom appears as a neutral particle. When an electric field is applied, electrons will be pulled toward the higher voltage end, while nuclei will be pulled toward the lower voltage end. Total charge of the atom is still zero, but there is a nonzero distance between the average position of electrons and nucleus. This is called a dipole. Under the influence of the electric field, each atom becomes a dipole lined up in the direction of the field. The sum of them is a nonzero dipole. The stronger the field is, the larger the displacement and the dipole are.
At the molecular level, some materials' molecules have an uneven charge distribution. As a result, each molecule is already a dipole. These dipoles point to random directions in space in the absence of an external field, summing up to zero on average. When an external field is applied, they become more or less aligned with the field direction, summing up to a nonzero dipole pointing in the direction of the field. The stronger the field is, the more aligned the molecules are, thus the larger the dipole.
The dipole produced at the atomic level is usually much weaker than that at the molecular level. When a material's molecules have dipoles at the molecular level, the atomic level effect can be ignored. Such a material is called a polar compound. Otherwise, the material is called a nonpolar compound. Water is a polar compound while methane is nonpolar. To quantify how easily a dipole can be produced when an electric field is applied, permittivity or relative dielectric constant (e) is defined. The larger the displacement is, the larger the permittivity becomes. Permittivity is generally a complicated complex tensor. It is approximated by a real number in many cases.
Any material has a certain permittivity, including the conductor. Permittivity of a dielectric can often be approximated as a real number while that of a conductor is always a complex number with a large imaginary part. The imaginary part is called conductivity. Permittivity is a macroscopic quantity. It is only meaningful in a large region as an average property. In today's semiconductor technology, where several layers of atoms can be sandwiched in a bulk material, what the suitable value to use as the permittivity in that thin layer remains a research problem. We can always do the calculation on an atom-by-atom basis, but that is usually complicated.
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