Gauss' Law problem! Help please!!! infinite sheet with charge density? In the figure below, a small circular hole of radius R = 1.80 cm has been cut in the middle of an infinite, flat, nonconducting surface that has uniform charge density σ = 4.50 pC/m2. A z-axis, with its origin at the hole's center, is perpendicular to the surface. In unit-vector notation, what is the electric field at point P at z = 2.56 cm? Figure http://www.chegg.com/homework-help/...infinite-flat-nonconducting-surface--q1088630 Teachers Solution: "The correct electric field can be gotten by adding the infinite charged plane with uniform density σ to the disk with radius R and opposite charge density -σ." I understand how to solve the problem but I don't understand why the charge density of the disk is negative? So why is it -σ??? HELP!