We begin with the basic physical laws, which in this case are Maxwell's equations,
In the above equations, we have done two things a little differently
than in your first class in E&M. First, we have written the equations
in the proper form so that we can describe electromagnetic fields not
only in vacuum but also inside of a material. In vacuum, 
and 
have the standard values
In a material, the values for these constants depend upon material and appear in reference books. Second, in vacuum Q and I refer to total enclosed charge and current, whereas inside a material Q and I refer to any excess, or ``free'', charges which are not a part of the material. Note that we have set these quantities to zero in both cases so that we are considering either fields in completely empty space or fields in a material where the only charges and currents are those which make up the material.
As a final comment, there is a standard notation for the combinations quantities appearing in the equation above,
