Question

A charged paint is spread in a very thin uniform layer over the surface of a plastic sphere of diameter 10.5 cm, giving it a charge of -22.3 µC.

(a) Find the magnitude of the electric field just inside the paint layer
(b) Find the magnitude of the electric field just outside the paint layer
(c) Find the magnitude of the electric field 5.00 cm outside the surface of the paint layer.

Answer 1

I got (a) right, saying that E=0
Here is what I tried for (b) and (c):

Using the equation \[E=\frac{ q }{ 4\pi r^2 \epsilon_0 }\]
For (b), the radius is 0.0525m, so:
\[E=\frac{ -22.3*10^{-6} }{ 4\pi (0.0525)^2 (8.854*10^{-12}) }=-7.272*10^7\]

For (c), r=0.0525+0.05=0.1025, so:
\[E=\frac{ -22.3*10^{-6} }{ 4\pi (0.1025)^2 (8.854*10^{-12}) }=-1.908*10^7\]