I'm having trouble understanding a concept.
Given a conducting box with point a located outside the box, point b located within the (thick) wall of the box and point c located within the cavity of the box, I have to calculate the electric field at each of these points.
There is excess negative charge on the box and a surface density of 2.10×10^10 e/m^2.
I managed to figure out all parts, but I struggle to understand why this is the case.
Gauss' Law: ∫ E.dA = Q/ε_0
Q = Ne
The Attempt at a Solution
For point a:
Q = Ne ∴ Q = 1.6*10^(-19)*-2.10*10^(10) = 3.36*10^-9
∫E.dA = Q/ε_0 = (3.36*10^-9)/(8.85*10^-12) ≈ -379.7 N/C
I know the answer is meant to be positive, but I don't understand how. I believed it would be negative as the charge on the box is negative, and so would 'emit' a negative field.
For point b:
0 N/C because it is inside the walls of the box.
For point c:
I initially thought this would be (+ or -) 379.7 N/C to balance out point a, but found the answer to be 0 N/C. I can't find a reason why this would be, other than that the net charge must be negative; however, I don't see why this couldn't contribute a negative charge in order to increase the magnitude of the net negative charge.