ABSTRACT
Electric charge is one of the indefinables in physics along with displacement, mass, time, and thermodynamic temperature. An indefinable cannot be expressed in terms of a more fundamental quantity. Electric charge is inherent in the microscopic atomic structure of matter. The “planetary” model suggests and experiments corroborate that an atom consists of a relatively compact nucleus made up of two types of fundamental particles, protons, and neutrons. Orbiting the nucleus is a cloud of diffuse matter called electrons. Quantum theory predicts that these electrons cannot be localized, but it is often a mental convenience to think of them as classical point particles. Thus, point particle electrons orbiting a compact nucleus are analogous to a planetary system where planets orbit a sun.
