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nchin
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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/questions-and-answers/figure-small-circular-hole-radius-r-180-cm-cut-middle-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!
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/questions-and-answers/figure-small-circular-hole-radius-r-180-cm-cut-middle-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!