1. The problem statement, all variables and given/known data Suppose we have an outer shell with charge -2Q and radius b. We also have an inner shell with charge Q and radius a. what is the potential when a<r<b. 2. Relevant equations E=kQ/r^2 V=kQ/r E=-dV/dx 3. The attempt at a solution The method I have been using according to my professor is a mathematical coincidence and the proper method to solving this is to use integrals. How I solve it is that I sum the potential of the outer shell with the inner shell. So since we are inside the -2Q shell we have a constant V=-2Q/b. Since we are outside the Q shell we have V=Q/r. Thus the resulting potential is V=Q/r-2Q/b. I would like to learn the proper method where you have to take infinity as a reference and integrate. And then you get +C somewhere... lol Could someone please explain this? Thanks!