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Circuits with Capacitors and Inductors

Module by: Don Johnson

Summary: Introducing when a circuit has capacitors and inductors other than resistors and sources, the impedance concept will be applied.

circuit5.png
Figure 1: A simple RCRC circuit.
Let's consider a circuit having something other than resistors and sources. Because of KVL, we know that v in = v R + v out v in v R v out . The current through the capacitor is given by i=Cddt v out i C t v out , and this current equals that passing through the resistor. Substituting v R =Ri v R R i into the KVL equation and using the v-i relation for the capacitor, we arrive at
RCddt v out + v out = v in R C t v out v out v in (1)
The input-output relation for circuits involving energy storage elements takes the form of an ordinary differential equation, which we must solve to determine what the output voltage is for a given input. In contrast to resistive circuits, where we obtain an explicit input-output relation, we now have an implicit relation that requires more work to obtain answers.
At this point, we could learn how to solve differential equations. Note first that even finding the differential equation relating an output variable to a source is often very tedious. The parallel and series combination rules that apply to resistors don't directly apply when capacitors and inductors occur. We would have to slog our way through the circuit equations, simplifying them until we finally found the equation that related the source(s) to the output. At the turn of the twentieth century, a method was discovered that not only made finding the differential equation easy, but also simplified the solution process in the most common situation. Although not original with him, Charles Steinmetz presented the key paper describing the impedance approach in 1893. The impedance concept is very important, extending its reach far beyond circuits.

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