Three boards are in the x axis. the middle one is in x=0 and charged in a positive sign density is: σ. the other 2 boards are in : x=d and x=-2d. This 2 boards are grounded. The area of the boards is A (A>>d). I found that on the middle plate the charge is divided in 2:1 ratio: The electric field from X=0 to X=d is:E1 = (8/3)*∏*k*σ and from X=0 to X=-2d is: E2 = (-4/3)*∏*k*σ. If I want to find the total force on the middle plat should I do: F=σ*(E1-E2) ?? I get (4/3)∏*k*(σ^2)*A but the answer says: F=(2/3)∏*k*(σ^2)*A thanks!