# Homework Help: Charged Cylindrical Shell

1. Sep 6, 2010

### aholtman

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

An infinite line of charge with linear density λ = 7.5 μC/m is positioned along the axis of a thick insulating shell of inner radius a = 2.9 cm and outer radius b = 4.9 cm. The insulating shell is uniformly charged with a volume density of ρ = -612 μC/m3.

What is λ2, the linear charge density of the insulating shell?

2. Relevant equations

I know that linear charge density equals q/L and that volume density equals q/V

3. The attempt at a solution

I tried multiplying the volume density times the volume of the shell and then dividing by the length of the shell. But I ran into a problem because I don't really know what the length of a shell would be. I tried finding the circumference of the outer edge and then subtracting the circumference of the inner edge from that and using that number as my length, but it didn't work. When I did that, I got -1.9 μC/m as my answer

2. Sep 7, 2010

### Mindscrape

Hmm, I actually don't quite know what the problem is asking for. Did you post verbatim what the problem is? It would make a lot more since if it was asking for say [itex]\sigma_b[/tex], the bound surface charge, of the insulating shell. Furthermore, I don't even know explicitly what shape this "shell" is.

3. Sep 7, 2010

### aholtman

yeah, I posted exactly what the problem was asking. and I tried to put the picture in, but I guess it didn't work. Its a spherical shell. I just don't know how to find the linear charge density for something that isn't linear

4. Sep 7, 2010

### gabbagabbahey

Can you upload the picture to a free image hosting site like imageshack.us and post a link to it so we can see the problem setup?

5. Sep 7, 2010

6. Sep 7, 2010

### aholtman

I've got it worked out now, but thanks anyway!

7. Sep 7, 2010

### Mindscrape

Just curious, what did the problem end up meaning by "line charge."

8. Sep 8, 2010