a board charged with superficial density σ is in YZ and X=0. another board with a thickness of D(0<X<D) sticks to him with a volumed density ρ(x,y,z)=μ*X. The electric field out of the charged area is zero. I have to find the elec field inside the board:i made a little gauss cylinder inside the board and outdise him. and so the answer is: E=-2∏Kμ(D^2 - X^2). but, why dont I add to that expresion the electric field of the board chaged with σ? (2∏Kσ) My second quetion if I have to find the difference between the potentials between (2D,0,0) and (-2D,0,0) what should I do If the elec field is 0 in this areas?