1. The problem statement, all variables and given/known data 2. Relevant equations 3. The attempt at a solution I'm able to arrive at the correct solution here, but I'm fairly sure that my line of reasoning is not proper. 1. For the outermost shell, I discard the inner two shell/charge, since their effective charge is zero. Is this a correct assumption? 2. In computing the potential between R1 and R2, I discard the space between the point charge and spherical shell since there is no net electric field emanating from them. This gives a potential of 0 for that middle Then, I add on the contribution from the outmost shell, which was previously computed. Is adding on the electric potential of the outermost shell without respect to the innermost shell appropriate here at the end? 3. Here is where I get very confused. I start by taking the potential from the point charge to R1. Simple enough. However, we previously calculated the potential from R1 to infinity in step 2, so my instinct would be to simply add that on since potential is a scalar. However, the final answer has a contribution from the middle component (-2Qk/R1) in it. Why does the middle layer suddenly start contributing?