1. The problem statement, all variables and given/known data There is a washer (inner radius is a and outer radius is b) on the x-y axis with a constant surface charge density σ . What is the force felt on charge q on z-axis distance D from center of washer. 2. Relevant equations 3. The attempt at a solution I don't even know where to begin... I was thinking about drawing a triangle to a random spot on the washer from the origin to the charge q...but I don't know if that will work since it's a washer. I don't think I can pick a gaussian surface...can I? Please give some hints!