Electrical impedance. An AC supply applying a voltage V {\displaystyle \scriptstyle V} , across an impedance Z {\displaystyle \scriptstyle Z} , driving a current I {\displaystyle \scriptstyle I} . Electrical impedance is the amount of opposition that a circuit presents to current or voltage change. The two main ways to write an impedance are: (see the 2nd figure, complex impedance plane) with the resistance R (real part) and the reactance X (imaginary part), for example Z = 1 + 1 j {\displaystyle Z=1+1j}.