Surface charges on circuit wires and resistors play three roles

The significance of the surface electric charge densities associated with current‐carrying circuits is often not appreciated. In general, the conductors of a current‐carrying circuit must have nonuniform surface charge densities on them (1) to maintain the potential around the circuit, (2) to provide the electric field in the space outside the conductors, and (3) to assure the confined flow of current. The surface charges and associated electric field can vary greatly, depending on the location and orientation of other parts of the circuit. We illustrate these ideas with a circuit consisting of a resistor and a battery connected by wires and other conductors, in a geometry that permits solution with a Fourier–Bessel series, while giving flexibility in choice of wire and resistor sizes and location of the battery. Plots of the Poynting vector graphically demonstrate energy flow from the battery to the resistive elements. For a resistor with a large resistance, the potentials and surface charge densities around the current‐carrying circuit are nearly the same as for the open circuit with the resistor removed. For such resistors, the capacitance of a resistor and its adjacent elements, defined in terms of the surface and interface charges present while current flows, is roughly the same as the capacitance of the adjacent elements of the open circuit alone. The discussion is in terms of time‐independent currents and voltages, but applies also to low‐frequency ac circuits.