Change of variable in electrostatic potential energy.

[Azorspace]

Homework Statement

Hello everyone i have a problem in visualizing why a a change of variable happens. To evaluate a volume integral. I hope that someone could give me a hint of the reasoning that makes you think in that specific change of variable.

Homework Equations

See the attachment for this part, or the image link.

The Attempt at a Solution

I thought that it was changing from cartesian coordinates to spherical, but that was not right, so now i have a problem because i don't know how is the reasoning that lead you to think in that change of variable.

http://img266.imageshack.us/img266/5585/equations.jpg

 

[gabbagabbahey]

Many people here (including myself) will not open .doc files since they are known to easily hide viruses.

You'll have an easier time getting assistance if you type out the equations usins $\LaTeX$ or post a link to an image file (you can host them for free at imageshack.us) instead.