# Electrical Field of a Charged set of Spheres

1. Feb 19, 2009

### Snowman2526

1. The problem statement, all variables and given/known data

Two spherical shells have a common center. A -1.37 x 10-6 C charge is spread uniformly over the inner shell, which has a radius of 0.0422 m. A +5.86 x 10-6 C charge is spread uniformly over the outer shell, which has a radius of 1.60 m. Find the magnitude of the electric field at a distance (measured from the common center) of (a) 0.200 m (b) 0.100 m, and (c) 0.025 m.

2. Relevant equations

Honestly I'm not sure how to set this equation up.

I have calculated the Electric Field of each sphere. the smaller is 6.92*106 and the larger is 2.06*104

I also calculated the Electric Flux each. The smaller being 6.54*104 and the larger being 6.63*105.

Now i'm not quite sure if I have to add the two spheres together, or what to do since the inside sphere is negatively charged.
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Feb 19, 2009

### Delphi51

This is a Gauss' Law question.
For (a) you imagine a sphere at with radius 0.2 and consider only the charge contained in it. The whole thing is spherically symmetric, so the E field will be the same everywhere on the surface of the sphere.

3. Feb 21, 2009

### Snowman2526

i'm sorry to necro bump my old thread but I still cannot get this problem. When you say imagine it with the radius .2...do i add it to the radius of the two circles? or is it a fresh radius and I just plug .2 into r for my equation.

On top of that, I am sitting with two variables because I can't find out what E is. Do I add the Coulomb charge of the two spheres or combine them somehow? I'm honestly not looking for someone to do it...I just can't grasp the concept for this problem.

4. Feb 21, 2009

### Delphi51

Imagine a sphere with radius 0.2. The Law says E times the surface area of the sphere equals some constant times the charge INSIDE the sphere. So you just ignore the outer charge for part (a).