Spherical Capacitor: Calculating Capacitance in pF

[professordad]

Homework Statement

A spherical capacitor consists of a spherical conducting shell charge -Q concentric with a smaller conducting sphere of radius 4.0 cm and charge Q. The larger conducting shell has inner and outer radii of 11.0 cm and 13.0 cm, respectively. What is the capacitance of the system in pF?

Homework Equations

C = \frac{Q}{\Delta V}

The Attempt at a Solution

I don't know what to do when the outer shell has a thickness. I know that when it doesn't have a thickness you would do
-\int_{r_1}^{r_2} kQ\frac{dr}{r^2}
where r_1 and r_2 are the inner and outer radii, respectively. But here what do you do for r_2?

Thanks.

 

[Tanya Sharma]

How would -Q be distributed on the outer shell ?