Impedance may refer to:
  • Electrical impedance
    Electrical impedance
    Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...

    , the ratio of the voltage phasor to the electric current phasor, a measure of the opposition to time-varying electric current in an electric circuit
    • Characteristic impedance
      Characteristic impedance
      The characteristic impedance or surge impedance of a uniform transmission line, usually written Z_0, is the ratio of the amplitudes of a single pair of voltage and current waves propagating along the line in the absence of reflections. The SI unit of characteristic impedance is the ohm...

       of a transmission line
  • Wave impedance
    Wave impedance
    The wave impedance of an electromagnetic wave is the ratio of the transverse components of the electric and magnetic fields . For a transverse-electric-magnetic plane wave traveling through a homogeneous medium, the wave impedance is everywhere equal to the intrinsic impedance of the medium...

    , a constant related to electromagnetic wave propagation in a medium
    • Impedance of free space, a universal constant
  • Mechanical impedance
    Mechanical impedance
    Mechanical impedance is a measure of how much a structure resists motion when subjected to a given force. It relates forces with velocities acting on a mechanical system. The mechanical impedance of a point on a structure is the ratio of the force applied at a point to the resulting velocity at...

    , a measure of opposition to motion of a structure subjected to a force
  • Acoustic impedance
    Acoustic impedance
    The acoustic impedance at a particular frequency indicates how much sound pressure is generated by a given air vibration at that frequency. The acoustic impedance Z is frequency dependent and is very useful, for example, for describing the behaviour of musical wind instruments...

    , a constant related to the propagation of sound waves in an acoustic medium
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