INSULATORS

If the people refuse to pass balls along the line in the preceding example, the line represents an electrical insulator. Such substances prevent electric currents from flowing, except in very small amounts under certain circumstances. Most gases are good electrical insulators (because they are poor conductors). Glass, dry wood, paper, and plastics are other examples. Pure water is a good electrical insulator, although it conducts some current when minerals are dissolved in it. Metal oxides can be good insulators, even though the metal in pure form is a good conductor.
An insulating material is sometimes called a dielectric. This term arises from the fact that it keeps electric charges apart, preventing the flow of electrons that would equalize a charge difference between two places.
Excellent insulating materials can be used to advantage in certain electrical components such as capacitors, where it is important that electrons not be able to flow steadily. When there are two separate regions of electric charge having opposite polarity (called plus and minus, positive and negative, or + and -) that are close to each other but kept apart by an insulating material, that pair of charges is called an electric dipole.