# Homework Help: Calculating the Electric Potential from an Electric Field problem

1. Jun 3, 2007

### frankfjf

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

A nonconducting sphere has radius R = 2.31cm and uniformly distributed charge q = +3.50 fC. Take the electric potential at the sphere's center to be Vo = 0. What is V at radial distance (a) r = 1.45cm and (b) r = R?

2. Relevant equations

V = the negative integral from i to f of E * ds

For a, E is kqr/R^3.

For b, E is kq/r^2

3. The attempt at a solution

When I plug all this in I get -5.37 x 10^-4 V and -.001V, respectively. However, when I look at the answer explanation in the student manual, it seems to indicate that ds (or in this case rs) = r/2. Why is it r/2? How is the book getting that for ds? At first I thought ds was equal to r, but I guess I'm not grasping something. Could someone explain why this is so? I basically have the right answer, but only if I divide both my answers by 2.

2. Jun 3, 2007

### Pythagorean

what are your limits of integration and what's your formula for V after you integrate?

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