Understanding Eq. 2.18 & Eq. 2.5 in Jackson's ED Book

[rbwang1225]

In Jackson's ED book, he mentioned eq. 2.18 is essentially the same as eq. 2.5. What I don't understand is that in eq. 2.18 it is the position of the unit source to be differentiated while in eq. 2.5 it is the observation point. Is this O.K.?

 

[Drakkith]

It might help if you provided the examples you were talking about. I know I don't have this book that you are referring to.

 

[Meir Achuz]

The two equations are for different things. That is why there are the two equations.
(2.5) is to get the surface charge for a given potential.
(2.18) is what is needed for the Dirichlet boundary condition using a GF.
That is one of the (many) difficult sections of J.