- Admittance
In

electrical engineering , the**admittance**("Y") is the inverse of the impedance ("Z"). TheSI unit of admittance is the siemens.Oliver Heaviside coined the term in December1887 . [*Ushida et al., "Immittance matching for multidimensional open-system photonic crystals", "Phys Rev B", vol 68, no. 15, Oct 2003.*]:$Y\; =\; Z^\{-1\}\; =\; 1/Z\; ,$

where :"Y" is the admittance, measured in siemens :"Z" is the impedance, measured in ohms

Note that the synonymous unit

mho , and the symbol ℧ (an upside-down Omega Ω), are also in common use.Admittance is a measure of how easily a circuit or device will allow a current to flow. Resistance is a measure of the opposition of a circuit to the flow of a steady current, while impedance takes in to account not only the resistance but dynamic effects (known as reactance) as well. Likewise, admittance is not only a measure of the ease with which a steady current can flow (conductance, the inverse of resistance), but also takes in to account the dynamic effects of

susceptance (the inverse of reactance).**Conversion from impedance to admittance**The impedance, Z, is composed of real and imaginary parts,:$Z\; =\; R\; +\; jX\; ,$where:"R" is the resistance, measured in ohms:"X" is the reactance, measured in ohms

:$Y\; =\; Z^\{-1\}=\; frac\{1\}\{R+jX\}\; =\; left(\; frac\{1\}\{R+jX\}\; ight)\; cdot\; left(\; frac\{R-jX\}\{R-jX\}\; ight)\; =\; left(\; frac\{R\}\{R^2+X^2\}\; ight)\; +\; jleft(frac\{-X\}\{R^2+X^2\}\; ight)$

Admittance, just like impedance, is therefore a complex number, made up of a real part (the conductance, "G"), and an imaginary part (the susceptance, "B"), shown by the

equation ::$Y\; =\; G\; +\; j\; B\; ,$

:$Y\; =\; G\; +\; jB\; =\; left(\; frac\{R\}\{R^2+X^2\}\; ight)\; +\; j\; left(\; frac\{-X\}\{R^2+X^2\}\; ight)$

Then "G" (conductance) and "B" (susceptance) are given by:

:$G\; =\; Re(Y)\; =\; left(\; frac\{R\}\{R^2+X^2\}\; ight)$

:$B\; =\; Im(Y)\; =\; left(\; frac\{-X\}\{R^2+X^2\}\; ight)$The magnitude and phase of the admittance are given by:

:$left\; |\; Y\; ight\; |\; =\; sqrt\; \{G^2\; +\; B^2\}\; =\; frac\; \{1\}\; \{sqrt\; \{R^2\; +\; X^2\}\; \}\; ,$

:$angle\; Y\; =\; arctan\; left(\; \{frac\{B\}\{G\; ight)=\; arctan\; left(\; \{frac\{-X\}\{R\; ight)$where :"G" is the conductance, measured in siemens:"B" is the susceptance, measured in siemens

**Admittance in mechanics**In

**mechanical systems**(particularly in the field ofhaptics ), an admittance is a dynamic mapping from force to motion. In other words, an equation (or virtual environment) describing an admittance would have inputs of force and would have outputs such as position or velocity. So, an admittance device would sense the input force and "admit" a certain amount of motion.Similar to the electrical meanings of admittance and impedance, an impedance in the mechanical sense can be thought of as the "inverse" of admittance. That is, it is a dynamic mapping from motion to force. An impedance device would sense the input motion and "impede" the motion with some force.

An example of these concepts is a virtual spring. The equation describing a spring is

Hooke's Law ,:$F\; =\; -kx\; ,$

If the input to the virtual spring is the spring displacement, x, and the output is the force that the virtual spring applies, "F", then the virtual spring would be classified as an impedance.If the input to the virtual spring is the force applied to the spring, "F", and the output is the spring displacement, "x", then the virtual spring would be classified as an admittance.

**Admittance in geophysics**The geophysical conception of admittance is similar to that described above for mechanical systems. The concept is primarily used for describing the small effects of atmospheric pressure on earth gravity. Studies have also been carried out regarding the gravity of Venus. [

*Hoogenboom, Smrekar, Anderson, Houseman, "Admittance survey of type 1 coronae on Venus", "Journal of Geophysical Research", Vol 109, E03002, 2004*] Admittance in geophysics takes atmospheric pressure as the input and measures small changes in the gravitational field as the output. Geophysics admittance is commonly measured in μgal/mbar. These units convert according to 1m/s^{2}= 100 gal and 1 bar 100kPa (kN/m^{2}) so in SI units the measurement would be in units of;$frac\{m/s^2\}\{N/m^2\}$ or $frac\{m^3\}\{Nmbox\{-\}s^2\}$ or, in primary units $frac\{m^2\}\{kg\}$

However, the relationship is not a straightforward one of proportionality. Rather, an "admittance function" is described which is time and frequency dependent in a complex way. [

*Abd El-Gelil, Pagiatakis, El-Rabbany, "Atmospheric Air Density Admittance Function for High Precision Gravity Reduction", "Elements: The newsletter of the Canadian Geophysical Union", vol 24, no. 2, July 2006*]**ee also**SI electromagnetism units **References****External links*** [

*http://www.geocities.com/SiliconValley/2072/eleccsa.htm Conductance, Susceptance, and Admittance*]

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