Energy

Suppose that two ping pong balls, each carrying an electrical charge, are placed near to each other. If free to move, the two balls have a tendency either to roll toward each other or away from each other, depending on the charges. If the charges they carry are the same (both positive or both negative), the two balls will repel each other and roll away from each other. If they charges are opposite, the balls will attract each other and roll toward each other. The force of attraction or repulsion of the two balls is a manifestation of the electrical potential energy existing between the two balls.

Electrical potential energy is analogous to gravitational energy. In the case of the latter, any two bodies in the universe exert a force of attraction on each other that depends on the masses of the two bodies and the distance between them. Any two charged bodies in the universe, on the other hand, experience a force of attraction or repulsion (depending on their signs) that depends on the magnitude of their charges and the distance separating them. A lightning bolt traveling from the ground to a cloud is an example of electrical potential energy that has suddenly been converted to it "kinetic" form, an electrical current.

An electrical current is analogous to kinetic energy, that is, it is the result of moving electrical charges. An electrical current flows any time two conditions are met. First, there must be a source of electrical charges. A battery is a familiar source of electrical charges. Second, there must be a pathway through which the electric charges can flow. The pathway is known as a circuit.

An electric current is useful, however, only if a third condition is met—the presence of some kind of device that can be operated by electrical energy. For example, one might insert a radio into the circuit through which electrical charges are flowing. When that happens, the electrical charges flow through the radio and make it produce sounds. That is, electrical energy is transformed into sound energy within the radio.