Video 2 on Section 7-1 in Fundamentals of Applied Electromagnetics, 8th edition by Fawwaz Ulaby.
For time-harmonic fields, it is convenient to define a complex permittivity. This video uses Ampere's equation to offer a short derivation and explanation for the usefulness of a defining a complex permittivity.
I am teaching using the Fawwaz T. Ulaby textbook as a reference. For more information about Fundamentals of Applied Electromagnetics by Ulaby please visit this website: https://em8e.eecs.umich.edu/