impedance

(noun)

A measure of the opposition to the flow of an alternating current in a circuit; the aggregation of its resistance, inductive and capacitive reactance. Represented by the symbol Z.

Related Terms

Examples of impedance in the following topics:

  • Impedance

    • The phase of the complex impedance is the phase shift by which the current is ahead of the voltage.
    • This quantity is known as the element's (complex) impedance.
    • The impedance of a resistor is R, while that of a capacitor (C) is $\frac{1}{j \omega C}$.
    • In the case of the circuit in , to find the complex impedance of the RC circuit, we add the impedance of the two components, just as with two resistors in series: $Z = R + \frac{ 1}{j \omega C}$.
    • The (real value) impedance is the real part of the complex impedance Z.

  • RLC Series Circuit: At Large and Small Frequencies; Phasor Diagram

    • By combining Ohm's law (Irms=Vrms/Z; Irms and Vrms are rms current and voltage) and the expression for impedance Z, from:
    • If the frequency is high enough that XL is much larger than R as well, the impedance Z is dominated by the inductive term.
    • This response makes sense because, at high frequencies, Lenz's law suggests that the impedance due to the inductor will be large.
    • The impedance Z at small frequencies $(\nu \ll \frac{1}{\sqrt{2\pi LC}})$ is dominated by the capacitive term, assuming that the frequency is high enough so that XC is much larger than R.

  • Resonance in RLC Circuits

    • Combining Ohm's law, Irms=Vrms/Z, and the expression for impedance Z from
    • At some intermediate frequency $\nu_0$, the reactances will be equal and cancel, giving Z=R —this is a minimum value for impedance, and a maximum value for Irms results.

  • Phase Angle and Power Factor

    • As we studied in a previously Atom ("Impedance"), current, voltage and impedance in an RC circuit are related by an AC version of Ohm's law: $I = \frac{V}{Z}$, where I and V are peak current and peak voltage respectively, and Z is the impedance of the circuit.

  • Resistance and Resistivity

    • Resistance and resistivity describe the extent to which an object or material impedes the flow of electric current.
    • Resistance is the electric property that impedes a current.

  • Diffraction

    • Similar effects occur when a light wave travels through a medium with a varying refractive index, or a sound wave travels through one with varying acoustic impedance.

  • Current and Voltage Measurements in Circuits

    • The electric property that impedes current (crudely similar to friction and air resistance) is called resistance R.

  • Inductors in AC Circuits: Inductive Reactive and Phasor Diagrams

    • Because the inductor reacts to impede the current, XL has units of ohms (1 H=1 Ωs, so that frequency times inductance has units of (cycles/s)(Ωs)=Ω), consistent with its role as an effective resistance.

  • Ohm's Law

    • Recall that while voltage drives current, resistance impedes it.