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 P: 45 1. The problem statement, all variables and given/known data The charge is distributed with uniform surface density σ on the disk of radius R. Find the potential at the axis of the disk. 2. Relevant equations Coulomb's law and the definition of a electric potential at point x 3. The attempt at a solution I have a solution in front of me but can't understand some step inside it: The potential can be defined now phi(x)= (1/4pi*epsilon0)Integral[(sigma(x')/|x-x'|)dS'] and the solution for the integral from 0 to R is: (sigma/2*epsilon0)(sqrt(x^2+R^2)-z) Now, the electric field at this point is: E(z)=(sigma/2*epsilon0)(1-(z/sqrt(z^2+R^2)) I can clearly follow until now, but then the book says that for z>>R we get E=Q/(4pi*epsilon0*z) where Q is the total charge of the disc. How can it be proportional to 1/z?? when I take z>>R - I get E(z)=0...