Now let’s fast forward to the 21st century. The theory of electricity was developed without knowledge of atoms, but there is no reason for us to continue to overlook this important part of our contemporary perspective. FIGURE 22.1 shows that an atom consists of a very small and dense nucleus (diameter $\sim {10}^{-14}\text{m}$) surrounded by much less massive orbiting electrons. The electron orbital frequencies are so enormous ($\sim {10}^{15}$ revolutions per second) that the electrons seem to form an electron cloud of diameter $\sim {10}^{-10}\text{m,}$ a factor ${10}^4$ larger than the nucleus.

FIGURE 22.1 An atom.

Experiments at the end of the 19th century revealed that electrons are particles with both mass and a negative charge. The nucleus is a composite structure consisting of protons, positively charged particles, and neutral neutrons. The atom is held together by the attractive electric force between the positive nucleus and the negative electrons.

One of the most important discoveries is that charge, like mass, is an inherent property of electrons and protons. It’s no more possible to have an electron without charge than it is to have an electron without mass. As far as we know today, electrons and protons have charges of opposite sign but exactly equal magnitude. (Very careful experiments have never found any difference.) This atomic-level unit of charge, called the fundamental unit of charge, is represented by the symbol e. TABLE 22.1 shows the masses and charges of protons and electrons. We need to define a unit of charge, which we will do in Section 22.4, before we can specify how much charge e is.