Most substances found in nature are electrical insulators. If a voltage is applied to such an insulator, such as glass or ceramics, the electrons in the material cannot move freely. However, they can shift slightly within their respective atoms or molecules. As a result, the negative centers of charge of the electrons are displaced relative to the positive centers of charge of the atomic nuclei, creating electric dipoles. These are usually referred to as induced dipoles, because they are generated by an external electric field. The entire process is called polarization.
Polarization partially compensates for the external electric field, which reduces the field strength inside an insulator. Because of this dipole formation, an insulator that develops polarization in an electric field is also called a dielectric.
In a vacuum, no charges exist, so polarization cannot occur. In gases like air, the difference compared to a vacuum is noticeable only in the fourth decimal place under room temperature and atmospheric pressure. In contrast, polar liquids such as water or alcohol exhibit a relatively high polarizability.
Experiment: Polarized Water
If a rubbed and electrostatically charged glass or plastic rod is brought close to a thin stream of water, the water visibly bends toward the rod. The reason is the polarization of the water molecules and the resulting forces exerted on them by the charged rod.