Some materials may easily let electrical charges pass through them, while others obstruct their flow. The former are called conductors and the latter insulators. The atomic structures of materials determine whether they are conductors or insulators of electricity.

Most metals are conductors. Their atomic configuration is such that one or more electron(s) are loosely bound to the nucleus in each atom. Thus, a sea of mobile electrons are available in them, known as free electrons. Their easy movement neutralizes any external charge added to the conductor. Hence, metals cannot harbor excess charge and do not experience electrical forces mutually or with other materials.

Human bodies are good conductors of electricity. For example, excess charges accumulate when someone rubs their shoes against a carpet fiber or an insulator. If they then touch a charged conducting material like a doorknob, they receive an electrical shock because of the rapid flow of charges.

Unlike conductors, insulators have atomic structures that do not allow any electrons to move between atoms freely. Thus, any excess charge added to insulators remains in the material. These excess charges can then lead to electrical forces between insulators. Plastic, wood, glass, and fur are typical examples of insulators.