Materials such as wood or plastic, which do not allow the flow of electric current, are known as insulators . By definition, electric current is the quantity of electric charge flowing through a conductor divided by the time during which this flow occurs. If a net electric charge exists on any two objects, an electric force will always exist between them. The magnitude of these forces between charged objects can be calculated by using Coulomb's law , as given in Eq. 8.2 in the textbook. This law is ultimately responsible for such varied processes as holding atoms together and causing the flow of electrons in metallic conductors.
Electric forces can be either attractive or repulsive.
If the two charges involved are of opposite signs, they are attracted to each other. For electricity to be useful in everyday situations, we must first separate electric charges. This is because the equal numbers of electrons and protons in normal atoms make most objects electrically neutral. Several common methods of electric charge separation are by mechanical means such as rubbing a glass rod with a silk cloth or in a motor-driven electric generator, by chemical means such as is observed in batteries of various kinds, and by direct solar interaction with photocells. Even the best conductors of electricity tend to show some reluctance to having current flow through them.