AC circuits have a demon called phase. To tackle AC circuit analysis we need to use complex numbers.

A complex number is a number of the form , where is the real part and is the imaginary part.

This form is called the rectangular form. Complex numbers also have their polar form, where they are instead written as their magnitude and the angle it makes with the positive real axis(x-axis), with

The magnitude of a complex number is called the modulus or the absolute value, and is denoted .

The angle is called the argument, sometimes also written as , and is the imaginary number, the square root of -1. . It doesn't make sense, but we need it. So it does.