## Impedance

Impedance (Z) is the term used for the combined effects of resistance along with capacitive and inductive reactance in an RC circuit (a circuit that includes a resistor and capacitor in series) passing AC current: Fig. 3. This figure depicts a simple resistance/capacitance (RC) circuit. In (A), VR is the voltage drop across the resistor and VC is the voltage drop across the capacitor. In (B), the behavior of such a circuit in response to an applied square wave pulse (Vapplied) is illustrated. The voltage across the capacitor, VC, gradually builds as charge accrues on the capacitor. This process follows a logarithmic function and has a time course shown in VC. Eventually, VC opposes the flow of current in the circuit. When the applied voltage is zero at the end of the pulse, the capacitor discharges in a reciprocal fashion. By contrast, VR is maximal at the onset of the applied pulse, but as the capacitor charges and opposes the source voltage, current flow decreases and then stops; VR then reaches zero because current, I, becomes 0 (V = IR). Note that VR and I behave similarly because they are directly proportional to one another. In this way, VC behaves as a high-frequency (low-pass) filter, whereas VR behaves as a low-frequency (high-pass) filter.

Fig. 3. This figure depicts a simple resistance/capacitance (RC) circuit. In (A), VR is the voltage drop across the resistor and VC is the voltage drop across the capacitor. In (B), the behavior of such a circuit in response to an applied square wave pulse (Vapplied) is illustrated. The voltage across the capacitor, VC, gradually builds as charge accrues on the capacitor. This process follows a logarithmic function and has a time course shown in VC. Eventually, VC opposes the flow of current in the circuit. When the applied voltage is zero at the end of the pulse, the capacitor discharges in a reciprocal fashion. By contrast, VR is maximal at the onset of the applied pulse, but as the capacitor charges and opposes the source voltage, current flow decreases and then stops; VR then reaches zero because current, I, becomes 0 (V = IR). Note that VR and I behave similarly because they are directly proportional to one another. In this way, VC behaves as a high-frequency (low-pass) filter, whereas VR behaves as a low-frequency (high-pass) filter.

The inductive reactance is subtracted from the capacitive reactance because they have opposite phase. In an AC circuit, Ohm's law takes the form V = IZ, where Z is the term for resistance in this type of circuit. This is analogous to Ohm's law as applied to DC circuits (V = IR). 