Skip to main content
Physics LibreTexts

13.3: Complex Numbers

I am now going to repeat the analyses of Sections 13.1 and 13.2 using the notation of complex numbers.   In the context of alternating current theory, the imaginary unit is customarily given the symbol j rather than i, so that the symbol i is available, if need be, for electric currents.  I am making the assumption that the reader is familiar with the basics of complex numbers; without that background, the reader may have difficulty with much of this chapter.


We start with the inductance.   If the current is changing, there will be a back EMF given by File:C:/Users/DELMAR~1/AppData/Local/Temp/msohtmlclip1/01/clip_image002.gif   If the current is changing as


                                                            File:C:/Users/DELMAR~1/AppData/Local/Temp/msohtmlclip1/01/clip_image004.gif                                                                  13.3.1


then  File:C:/Users/DELMAR~1/AppData/Local/Temp/msohtmlclip1/01/clip_image006.gif Therefore the voltage is given by


                                                            File:C:/Users/DELMAR~1/AppData/Local/Temp/msohtmlclip1/01/clip_image008.gif                                                                 13.3.2


The quantity jLw is called the impedance of the inductor, and is j times its reactance.  Its reactance is Lw, and, in SI units, is expressed in ohms.   Equation 13.3.2  (in particular the operator j on the right hand side) tells us that V leads on I by 90o.


Now suppose that an alternating voltage is applied across a capacitor.  The charge on the capacitor at any time is Q  =  CV, and the current is  File:C:/Users/DELMAR~1/AppData/Local/Temp/msohtmlclip1/01/clip_image010.gif If the voltage is changing as


                              File:C:/Users/DELMAR~1/AppData/Local/Temp/msohtmlclip1/01/clip_image012.gif                                                                13.3.3


then File:C:/Users/DELMAR~1/AppData/Local/Temp/msohtmlclip1/01/clip_image014.gif Therefore the current is given by



                                          File:C:/Users/DELMAR~1/AppData/Local/Temp/msohtmlclip1/01/clip_image016.gif                                                                13.3.4


That is to say                                       File:C:/Users/DELMAR~1/AppData/Local/Temp/msohtmlclip1/01/clip_image018.gif                                                              13.3.5


The quantity File:C:/Users/DELMAR~1/AppData/Local/Temp/msohtmlclip1/01/clip_image020.gifis called the impedance of the capacitor, and is j times its reactance. Its reactance is File:C:/Users/DELMAR~1/AppData/Local/Temp/msohtmlclip1/01/clip_image022.gif, and, in SI units, is expressed in ohms.    Equation 13.3.5  (in particular the operator -j on the right hand side) tells us that V lags behind I by 90o.


In summary:


            Inductor:    Reactance = Lw.                 Impedance = jLw.          V  leads on I.


            Capacitor:  Reactance = File:C:/Users/DELMAR~1/AppData/Local/Temp/msohtmlclip1/01/clip_image022.gif        Impedance = -j/(Cw).    V lags behind I.


It may be that at this stage you haven't got a very clear idea of the distinction between reactance (symbol X) and impedance (symbol Z) other than that one seems to be j times the other.  The next section deals with a slightly more complicated situation, namely a resistor and an inductor in series.  (In practice, it may be one piece of equipment, such as a solenoid, that has both resistance and inductance.)  Paradoxically, you may find it easier to understand the distinction between impedance and reactance from this more complicated situation.