1. The problem statement, all variables and given/known data Suppose you design an apparatus in which a uniformly charged disk of radius R is to produce an electric field. The field magnitude is most important along the central perpendicular axis of the disk, at a point P at distance 2R from the disk. Cost analysis suggests that you switch to a ring of the same outer radius R but with inner radius R/2. Assume that the ring will have the same surface charge density as the original disk. If you switch to the ring, by what percentage will you decrease the electrric feield magnitude at P? 2. Relevant equations Electric field due to disk = σ/2ε*(1-z/(z^2+r^2) 3. The attempt at a solution Now what I tried was finding the electric field of the big disk, then finding the electric field of the ring by subtracting the electric field of the missing disk that was taken out of radius R/2. (Edisk-Ering)/Edisk. For E of the large disk I get σ/2ε(1-2R/(5R^2)^(1/2)) for the E of the ring I get σ/2ε((1-2R/(5R^2)^1/2)-(1-2R/(17R^2/4)^(1/2)). Which I simplify to σ/2ε(2R/(17R^2/4)^(1/2)-2R/(5R^2)^(1/2)). It seems I'm making a mistake in calculating (Edisk-Ering. I've seen then answer and my Edisk is correct. Am I making an algebra mistake somewhere? I did this problem correct two days ago with no problem so I don't know how I'm getting it wrong now, I lost the paper :(. Sorry for the ugliness of all the type. I tried to use this itex but I wasn't able to input the equations in correct apparently. I'd really appreciate any help as this problem is driving me nuts.