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.
Resistance varies greatly, depending on the type of material through which the current is flowing.
A simple relationship among voltage, resistance, and electric current flow is expressed by Ohm's law . The flow of electrical current can occur through conductors in two basic ways. If the current always flows in the same direction, it is called direct current, or dc. If the current flows back and forth through the conductor, reversing its direction of travel several times each second, we have alternating current , or ac. Electrical components such as resistors can be arranged in ac and dc circuits either in series or in parallel .