Reactance and Complex Numbers, The reactance of a capacitor (Xc) is given by the formula Xc=1/(jwC), where C is the capacitance and w=2(pi)f (and f is the frequency). j is mathematical operator, which shift any signal or function by 90 degree. ZC = 1/(jwC ) = -j/(wC) The impedance of a resistor is simply its resistance (a real value) but the impedances of a capacitor or an inductor are both imaginary. You create two versions of the circuit and drive each one with one of the two. The j stands in for the imaginary number √(-1).