Susceptance

In electrical engineering, susceptance ("B") is the imaginary part of admittance, where the real part is conductance. The reciprocal of admittance is impedance, where the imaginary part is reactance and the real part is resistance. In SI units, susceptance is measured in siemens.
Origin.
The term was coined by Charles Proteus Steinmetz in a May 1894 paper. In some sources Oliver Heaviside is given credit for coining the term, or with introducing the concept under the name "permittance". This claim is mistaken according to Steinmetz's biographer, Ronald R. Kline. The term "susceptance" does not appear anywhere in Heaviside's collected works, and he used the term "permittance" to mean capacitance, not susceptance.
Formula.
The general equation defining admittance is given by
where,
The admittance () is the reciprocal of the impedance (), if the impedance is not zero:
and
where
The susceptance formula_5 is the imaginary part of the admittance formula_6.
The magnitude of admittance is given by:
And similar formulas transform admittance into impedance, hence susceptance () into reactance ():
hence
The reactance and susceptance are only reciprocals in the absence of either resistance or conductance (only if either or , either of which implies the other, if or equivalently ).
Relation to capacitance.
In electronic and semiconductor devices, transient or frequency-dependent current between terminals contains both conduction and displacement components. Conduction current is related to moving charge carriers (electrons, holes, ions, etc.), while displacement current is caused by time-varying electric field. Carrier transport is affected by electric field and by a number of physical phenomena, such as carrier drift and diffusion, trapping, injection, contact-related effects, and impact ionization. As a result, device admittance is frequency-dependent, and the simple electrostatic formula for capacitance, formula_10 is not applicable. A more general definition of capacitance, encompassing electrostatic formula, is:
where formula_6 is the device admittance, evaluated at the angular frequency in question, and formula_13 is the angular frequency. It is common for electrical components to have slightly reduced capacitances at extreme frequencies, due to slight inductance of conductors used to make capacitors (not just the leads), and permittivity changes in insulating materials with frequency: is very nearly, but not quite a constant.
Relationship to reactance.
Reactance is defined as the imaginary part of electrical impedance, and is "analogous" but not generally equal to the reciprocal of the susceptance.
However, for purely-reactive impedances (which are purely-susceptive admittances), the susceptance is equal to negative the inverse of the reactance.
In mathematical notation:
The negation is not present in the relationship between electrical resistance and the analogue of conductance "G", which equals formula_15.
If the imaginary unit is included, we get
for the resistance-free case since,
Applications.
High susceptance materials are used in susceptors built into microwavable food packaging for their ability to convert microwave radiation into heat.