# Dictionary:Impedance

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(im pēd’ ∂ns)

1. The apparent resistance to the flow of alternating current, analogous to resistance in a dc circuit. Impedance is (in general) complex, of magnitude Z with a phase angle ${\displaystyle \gamma }$. These can be expressed in terms of resistance R (in ohms), inductive reactance ${\displaystyle X_{L}=2\pi fL}$, and capacitive reactance ${\displaystyle X_{C}={\frac {1}{2\pi fC}}}$:

${\displaystyle Z={\sqrt {R^{2}+\left(X_{L}-X_{C}\right)^{2}}}}$,

${\displaystyle \gamma =\tan ^{-1}\left({\frac {X_{L}-X_{C}}{R}}\right)}$.

Z is in ohms when frequency f is in hertz, L is inductance in henrys, and C is capacitance in farads.

2. Acoustic impedance, the product of density and velocity, the ratio of the pressure to the volume displacement at a given surface in a sound-transmitting medium. Because reflection coefficients change with angle, elastic impedance is sometimes used for non-normal incidence.

3. In magnetotelluric exploration, if Ex is the electric-field component in an arbitrary horizontal direction and Hy is the associated magnetic-field (in the orthogonal horizontal direction), the wave impedance (Cagniard impedance, field impedance, or surface impedance) ZS is:

${\displaystyle Z_{S}={\frac {E_{x}}{H_{y}}}}$.

The surface impedance is a complex number because of phase differences between Ex and Hy. Measurements of surface impedance versus frequency can be interpreted in terms of the electrical properties of the subsurface. The surface impedance is a tensor if the conductivity structure is nonlayered. The reciprocal of impedance is called admittance.