In order to successfully analyze AC circuits, we need to work with mathematical objects and
techniques capable of representing these multi-dimensional quantities. Here is where we need to
abandon scalar numbers for something better suited: complex numbers. Just like the example of
giving directions from one city to another, AC quantities in a single-frequency circuit have both
amplitude (analogy: distance) and phase shift (analogy: direction). A complex number is a single
mathematical quantity able to express these two dimensions of amplitude and phase shift at once.
Complex numbers are easier to grasp when they’re represented graphically. If I draw a line with
a certain length (magnitude) and angle (direction), I have a graphic representation of a complex
number which is commonly known in physics as a vector : (Figure 2.1)