Electrons, and protons, and some other particles ("charged" particles) exert a long-range force on one another. Like gravitation, this force is inversely proportional to the square of the distance between the particles; but unlike gravitation, this force is attractive or repulsive, depending on what particles are involved. Furthermore, this force is much stronger than the force of gravitation between an electron and a proton, it is about 2 10 ³⁹ times stronger. This force is called the electric force, and particles that exert these forces are said to carry electric charge. By definition, we will regard the electric charge of an electron as negative, and we will regard the charge of any other particle as positive or as negative, depending on whether it attracts or repels electrons. Macroscopic electric forces, such as the electric forces between two capacitor plates, are the result of the superposition of the attractive and repulsive electric forces of a large number of electrons and protons within these plates.

In this chapter, we will study the description of the electric force in terms of the electric field and potential. Throughout this chapter and the next three, we will assume that the charges exerting these forces are either at rest or moving very slowly; the electric forces produced under these conditions are called electrostatic forces. In later chapters, we will see how these forces are modified when the particles move at high speed or with acceleration.