Gauss: Conducting Spherical Shell w/ Point Charge

[calvert11]

Homework Statement

Consider a conducting spherical shell with inner radius 0.8 m and outer radius 1.3 m. There is a net charge 3 μC on the shell. At its center, within the hollow cavity, there is a point charge 3 μC.

Determine the flux through the spherical Gaussian surface S, which has a radius of 0.4 m. Answer in units of N · m2/C.

a= 0.8m
b= 1.3m
q1 = q2 = 3e-6 C

Homework Equations

Flux = E*A
E = (kQ)/r^2
A = 4pir^2

The Attempt at a Solution

I thought I could approach this in a very straight forward way: finding E of the point charge and multiplying it by the spherical shell's surface area (using the outer radius for both r's).

So, E = (8.99e9*3e-6)/1.3^2 = 15958.6
A = 4*pi*1.3^2 = 21.237

EA = 3.389e5 N*m2/C, which is wrong. I'm not sure what I should do.

 

[Doc Al]

Your solution looks good to me. (Not sure why you used the outer radius, but it doesn't matter. For that matter, you could have just used Gauss's law directly.)