https://www.smartphysics.com/Content/Media/Images/EM/03/h3_lineD.png [Broken] In the image above, i was asked to find the electric field at point P. since the y-components cancel due to symmetry, i used he equation [itex]\Phi[/itex]=[itex]\int[/itex]E dA=Qenclosed/[itex]\epsilon[/itex] . I found q1 and q2 by multiplying (charge density x h). then from that, i added the charges up to get Q(enclosed). I found my E by [itex]\Phi[/itex]/(2πah). This method was wrong apparently but i don't know why. can someone explain? Is it because the electric field through the surface is not uniform? I later used superposition instead and i got the right answer.