1. 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 question is why outside the boards the ElecField is 0? I though that i dont waste energy to bring an electric charge from x=∞ to x=d because the potential in ∞ is 0 (from my theoretical assumption - but in the question nobody told me that). So, what is the best answer for that? Thanks!