When applying Kirchhoff’s loop rule to an AC circuit, one assumes the current in the loop is the same through all circuit elements at any particular instant. Changes in current travel at the speed of light (see Chapter 23), so a certain amount of time is actually required for the AC changes to propagate from one part of a circuit to another.
(a) Estimate how much time Δt is needed for these changes to travel across a circuit 5 cm in size. Assume the propagation speed in a metal wire is approximately 3x10⁸.
(b) Use your value of Δt to find the lowest frequency at which the current can be a maximum in one part of the circuit and a minimum at another part. Cases like this one at high frequencies are called distributed circuits and must be analyzed using a generalized version of Kirchhoff’s laws, which accounts for the propagation time.