Simple Circuit Concepts versus Physics
Circuits as shown in Figures 1(a) and (b) consist of electric current flowing along closed conducting paths. The upper horizontal line in Figure 1(a) is called the "hot" or "signal" path, while the lower horizontal line is the "return" path. As shown in Figure 1(b), two or more circuits may share a common return path which carries the algebraic sum of the return currents. While easy to understand, the simple circuit concept sketched in Figure1 omits certain subtle physics issues which can turn out to be very important and result in signal degradation or interference. These physics issues include distributed capacitance between the signal and common conductors, distributed capacitance between the two signal wires in Figure1(b), self-inductance of the combined signal and hot conductor, mutual inductance with other circuits, capacitive coupling to other circuits, pickup of electromagnetic waves radiated by distant emitters, and conductive coupling from other circuits.