Re: Potential of a point charge(gravitaional variety)

[SSHS KPHOX <kphox@CHERRYCREEKSCHOOLS.ORG>]

Forum for Physics Educators <PHYS-L@lists.nau.edu> writes:
Does anyone else get as confused as I do with negative signs when trying to
derive the potential for a point charge?


I have always had the same difficulty when doing this with gravity and my view of what I read in texts (HRW etc) seems like hand waving. Let me express my logic and what I think I have learned this week:

1. delta U = -work done by internal force (gravity)

2. when an object is allowed to move from infinity to some point A the gravity force and dr are in the same direction. (This is where I think the problem is in my head)

3. the work done by gravity then is GMm*integral from infinity to R of dr/r^2

4. I get the result for this of -GMm/R

5. but deltaU = -Work so should be GMm/R

As I have read some of the posts on the Electrical case I think I am seeing that my confusion is in what I am calling dr and what is dr. r is a vector directed from earth to mass m. So when it moves dr is negative. In what I wrote above, instead of dr I should have written ds which is parallel to F making my dot product positive. But when I replace ds with dr, I must say that dr = -ds?

Do I now have this right? It has bugged me for a long time.

Thank you

Ken Fox