Electrostatic force between a charged ring and charged rod.

[qpham26]

Homework Statement

a) Calculate the electrostatic force on an uniformly charged rod of length 2L and charge q, which lies along the axis of an uniformly charged ring of radius R and charge q'. The centers of the charged rod and the rings are displaced at a distance z= z₀.
b)Show that if z₀ >> L then the expression from (a) reduces to that between point charges


2. The attempt at a solution
This is how I attacked this problem:
First I try to pick a point charge on the rod, I choose the one that is closest to the ring. Then the electric force between the ring and the point charge will be ...the formula that was driven b/t point charge and ring(sry I don't know how to type out the formula, but you know what I meant)

Then that result would become dF for the whole system, then I just need to expand the problem to get the expression between the rod and ring.

Is this the right way for doing problem like this?

I used that method to attack the problem but at the end I don't seem to get the right answer or be able to reduces to the case of that b/t point charge.

Thanks for reading.
p/s: I will try to work on it again and then I will scan my paper.

 

[SammyS]

...
Thanks for reading.
p/s: I will try to work on it again and then I will scan my paper.

Yes, that is a good idea. It's hard for us to help you if we don't know what you have tried.