Electric circuits are essentially paths through which electrical current moves. The two basic types of electric circuits are the direct-current, or DC circuit, in which the current flows only in one direction, and the alternating-current, or AC circuit, in which current alternates at a given frequency (usually 60 Hz in North America and 50 Hz in Europe). Circuits consist of localized circuit elements connected by conducting paths (e.g., wires). The three basic circuit elements are resistors, capacitors, and inductors. An example of the simplest circuit possible is provided by resistors connected across an emf source either in series or in parallel. When resistors are connected in series, all the current flowing through one resistor also flows through each of the other resistors and the total resistance of the circuit is the sum of the individual resistances of the resistors. A parallel circuit, however, is one that contains...