# Why integrate at these points? (Electric potential of a sphere)

1. Sep 28, 2013

### tappling

1. The problem statement, all variables and given/known data
Find the potential inside and outside a uniformly charged solid sphere of radius R and total charge q.

2. Relevant equations
V(r) = -∫E dl

3. The attempt at a solution

I just have a question about finding the potential inside the sphere. Why integrate from infinity to the surface of the sphere (infinity to R) and add the integral of inside the sphere (R to r, whatever radius is inside the sphere). I'm just having trouble visualizing this integration, and why the integral is structured the way it is. Thanks in advance!

2. Sep 28, 2013

### TSny

This is tied to the question: Where are you choosing V = 0?

Remember, when you integrate -∫E$\cdot$dl between two points, you get the potential difference between those two points. So, if you want the potential at a point P, you can integrate from the point of zero potential to the point P.

3. Sep 28, 2013