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## Homework Statement

A nonconducting sphere of radius r_2 contains a concentric spherical cavity of radius r_1. The material between r_1 and r_2 carries a uniform charge density rho_E(C/m^3). Determine the electric potential V, relative to V=0 at r= infinity, as a function of the distance r from the center for r_1 < r < r_2

## Homework Equations

## The Attempt at a Solution

http://i.imgur.com/bOMD7ej.png

The link is just a screenshot of the answer and steps given in the solution manual. I have similar work but I am confused about why the solution involves integrating from r_2 to r. How is that the potential relative to V=0 at r=infinity?

And what happened to V_R_2? Since it disappears later on, I assume it's because it's equal to 0V but how come?