1. The problem statement, all variables and given/known data http://scitec.uwichill.edu.bb/cmp/online/P10D/Hunte/Electric%20fields_files/image122.jpg Find the Electric Field at Point P at height z over a uniformly charged disc of radius a with uniform charge Q. 2. Relevant equations 3. The attempt at a solution I'm looking at the solution online, on multiple sites, only to be stuck with the very first step. Why on earth is [tex]dA=2\pi*r*dr[/tex] and what exactly is dA?