Dictionary:Impedance

{{#category_index:I|impedance}} (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 $\gamma$. These can be expressed in terms of resistance R (in ohms), inductive reactance $X_{L}=2\pi fL$, and capacitive reactance $X_{C}={\frac {1}{2\pi fC}}$:

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

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

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 E_x is the electric-field component in an arbitrary horizontal direction and H_y is the associated magnetic-field (in the orthogonal horizontal direction), the wave impedance (Cagniard impedance, field impedance, or surface impedance) Z_S is:

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

The surface impedance is a complex number because of phase differences between E_x and H_y. 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.

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