1. The problem statement, all variables and given/known data A cylinder of radius a and length l has charge distribution ρ=Cr2 where C is a constant and r is radial distance in cylindrical coordinates. Derive an expression for the average charge density within the cylinder. 2. Relevant equations Well, charge density given is within the volume, I think. So for a point on the axis of the cylinder should be ρ divided by the length shouldn't it? Or is that being too simple? 3. The attempt at a solution ρ = Cr2 / l Can someone confirm this or point me in the right direction, please.