Impedance and admittance:
To express behavior of alternating current circuits, you require two dimensions, because alternating current has variable frequency along with the variable current. One dimension will suffice for direct current, but not for alternating current.
The presentation is rather mathematical. You can get the grasp of the general nature of the subject matter without learning how to do all the calculations presented. The mathematics can be given for those of you who wish to gain the firm understanding of how alternating current circuits behave.
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