Electric field in a spherical shell

[yjk91]

Homework Statement

A -5-nC point charge is located at the center of a conducting spherical shell. The shell has an inner radius of 2 m, an outer radius of 4 m, and a charge of +7 nC. (Let the radially outward direction be positive.)
(a) What is the electric field at r = 1 m? (Indicate the direction with the sign of your answer.)




(b) What is the electric field at r = 3 m? (Indicate the direction with the sign of your answer.)


(c) What is the electric field at r = 5 m? (Indicate the direction with the sign of your answer.)



(d) What is the surface charge distribution, σ, on the outside surface of the shell? (Indicate the direction with the sign of your answer.)

The Attempt at a Solution

a) you get this by doing -5 * 10^-9 * 9 * 10^9 / (1) ^2 = -45 N/C


(b) 0 N/C

i' not sure of this i thin it's because it's on the middle. Middle meaning middle of 2 M and 4 M which is 3M
i would like a clear answer

(c) What is the electric field at r = 5 m? (Indicate the direction with the sign of your answer.)
Units are required for this answer. 0.719 N/C

this i have no idea.. i think there is a formula but not sure how the charge of 7nC affects the field

(d) What is the surface charge distribution, σ, on the outside surface of the shell? (Indicate the direction with the sign of your answer.)

i think you use E = σ/epsilon0

 

[ehild]

Do you know Gauss' Law?

ehild