Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: Potential of a point charge

I agree that the minus sign in the definition is confusing. Especially
when the author tells us "between two points A and B" as if any
arbitrary two points could be chosen. In reality (and this should be
clearly stated before the definition is introduced) the point B is
"downhill" electrically speaking, with respect to the point A. In other
words, the potential energy increase is negative when the dot product
is positive (work done 'by the field" and not "on the field").
Ludwik Kowalski

On Thursday, February 26, 2004, at 12:52 PM, Rick Tarara wrote:

Does anyone else get as confused as I do with negative signs when
trying to
derive the potential for a point charge?

I finally figured out my confusion but the process of :

1) delta PE = (negative) of Work done by the field

2) so that F = -qE

3) so that delta V = - integral (E dot ds)

4) BUT now dr = -ds (this is what took me a while to see)

5) so that delta V = - integral (Edr)

6) which is (for point charge) deltaV = - integral (kQ/r^2) dr

7) which integral generates another negative so delta V = kQ/r

That dr = -ds is always my stumbling block because most texts don't
really
spell it out. HR&K just jump from E dot ds to Edr.

Anyway--just venting and acknowledging that this HAS to be confusing to
students who are just coming off of Gauss' Law which always confuses
the
hell out of them--for reasons I don't quite understand!

Rick

*********************************************************
Richard W. Tarara
Professor of Physics
Saint Mary's College
Notre Dame, Indiana
rtarara@saintmarys.edu
********************************************************
Free Physics Educational Software (Win & Mac)
NEW: U.S. Energy 2004 -- updated/simplified
energy management simulator
SIMLAB-THE SIMPLE PENDULUM lab simulation
www.saintmarys.edu/~rtarara/software.html
Energy 2100--class project
www.saintmarys.edu/~rtarara/ENERGY_PROJECT/ENERGY2100.htm
********************************************************