# Homework Help: Concentric rings and net electric fields

1. Sep 3, 2009

### MasterWu77

1. The problem statement, all variables and given/known data

The figure below shows two concentric rings, of radii R and R ' = 3.00R, that lie on the same plane. Point P lies on the central z axis, at distance D = 2.00R from the center of the rings. The smaller ring has uniformly distributed charge +Q. What must be the uniformly distributed charge on the larger ring if the net electric field at point P due to the two rings is to be zero?

_____ x Q

<a href="[PLAIN]http://s323.photobucket.com/albums/nn457/MasterWu77/?action=view&current=physics.gif" [Broken] target="_blank"><img src="http://i323.photobucket.com/albums/nn457/MasterWu77/physics.gif" [Broken] border="0" alt="physics homework"></a>[/PLAIN] [Broken]

2. Relevant equations

E= kqz/ (z^2+R^2)^(3/2)

3. The attempt at a solution

This is my first time here on this site so please bear with me as I'm not too sure how to work things haha. I'm unsure of whether the image i posted will show up or not. but it is basically 2 concentric rings, one small one (R) inside of a bigger one (R') with the radi as described. There is a line (z-axis) with the point P on it that is a distance D away from the rings. The first time I tried the problem I used the wrong formula and missed the question twice. But the last time I tried the problem, I used the equation describes the electrical field of a charged ring. I set that equation for each one of the rings and then set the equations equal to each other since the net electric field is zero. I then solved for the charge compared to the charge of the unknown (larger ring). The last answer I came up with was 13/5. And I set that number negative, because the smaller ring was a positive number so the bigger ring would have to be a negative number to make the net electric field at P = 0.

I feel like I might just missing a small little fact or maybe I'm not thinking about the end correctly. Again, this is my first time so please let me know if I'm doing anything wrong. I have also entered an answer in 3 times and I'm getting scared that I'm running out of tries.

Last edited by a moderator: May 4, 2017
2. Sep 3, 2009

### rl.bhat

Hi MasterWu77,
Welcome to PF.