Hey guys! The question is related to problem 2.26 from Electrodynamics by Griffiths (3ed). 1. The problem statement, all variables and given/known data A conical surface (an empty ice-cream cone) carries a uniform surface charge σ. The height of the cone is h, as the radius of the top. Find the potential difference between points a (the vertex) and b (the center of the top). 2. Relevant equations Here I will call the potential V. First of all, I assumed that at the vertex: V(a) = 0. (I can do that because I'm interested in V(b) - V(a), am I right?) Then I calculated V(b). So: V(a) - V(b) = - V(b) = -σh/(2ε) * ln (1 + (21/2/2)) But the book's solution didn't consider V(a) = 0, and found: V(a) - V(b) = σh/(2ε) [1 - ln (1 + (21/2/2))] Finally, my questions are: Why is my assumption wrong? How to calculate it assuming V(b) = 0?