1. The problem statement, all variables and given/known data http://imgur.com/VzZjSEH I took a screen shot so I did not leave anything out. 2. Relevant equations Surface area, and volume of a sphere. 3. The attempt at a solution I am trying to find the linear charge density from the charge volume density. I got it explained from a friend. It was right, but made no sense. He did it by taking the volume density, and multiplying it by the charge volume density by pi, and (outer r2 - inner r2). So he did -657 * pi * (.0422 - .0262) Can someone explain to me how this is right?