1. The problem statement, all variables and given/known data Take the expression 21.11 (pictured below, specifically the bottom one) for the electric field above the center of a uniformly charged disk with radius R and surface charge density σ, and show that when one is very far from the disk, the field decreases with the same square of the distance as it would for a point charge, E ~ q/4πεx2, where q is the total charge on the disk. 2. Relevant equations 3. The attempt at a solution I solved one like this pretty easily where you prove that a long wire acts as an infinite wire, but I've been looking at this one for about an hour and I'm stumped. I know I need to get x2 on the bottom of the equation without being inside a square root, but I don't know how. The only approximation I can think of is that when R is much bigger than x, √(x2 + R2 + 1) goes to 1, but then I don't have an x anymore.