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Susceptance (symbolized B ) is an expression of the ease with which alternating current ( AC ) passes through a capacitance or inductance .

In some respects, susceptance is like an AC counterpart of direct current ( DC ) conductance . But the two phenomena are different in important ways, and they can vary independently of each other. Conductance and susceptance combine to form admittance , which is defined in terms of two-dimensional quantities known as complex number s.

When AC passes through a component that contains a finite, nonzero susceptance, energy is alternately stored in, and released from, a magnetic field or an electric field. In the case of a magnetic field, the susceptance is inductive. In the case of an electric field, the susceptance is capacitive. Inductive susceptance is assigned negative imaginary number values, and capacitive susceptance is assigned positive imaginary number values.

As the inductance of a component increases, its susceptance becomes smaller negatively (that is, it approaches zero from the negative side) in imaginary terms, assuming the frequency is held constant. As the frequency increases for a given value of inductance, the same thing happens. If L is the inductance in henries ( H ) and f is the frequency in hertz ( Hz ), then the susceptance - jB L , in imaginary-number siemens , is given by:

- jB L = - j (6.2832 fL ) -1

where 6.2832 is approximately equal to 2 times pi , a constant representing the number of radian s in a full AC cycle, and j represents the unit imaginary number (the positive square root of -1).

As the capacitance of a component increases, its susceptance becomes larger positively in imaginary terms, assuming the frequency is held constant. As the frequency increases for a given value of capacitance, the same thing happens. If C is the capacitance in farads ( F ) and f is the frequency in Hz, then the susceptance +jB C , in imaginary-number ohms, is given by:

+jX C = + j (6.2832 fC )

Also see admittance , conductance , reactance , resistance , impedance , ohm , siemens , henry , and farad .

This was last updated in March 2010


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