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# Thread: Physics help (electric fields) REPS FOR LIFE

1. ## Physics help (electric fields) REPS FOR LIFE

I need this last answer for my homework, can't figure this one out.

The figure below shows two concentric rings, of radii R and R ' = 2.90R, that lie on the same plane. Point P lies on the central z axis, at distance D = 2.10R 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?

answer is [________] x Q

The answer comes from this, but this is using different numbers

Any ideas?

2. bump

3. E = kq/r^2 ?

maybe set E = E1

4. Originally Posted by zachstu
I'm not a member of cramster.

5. Originally Posted by O_S_U
I need this last answer for my homework, can't figure this one out.

The figure below shows two concentric rings, of radii R and R ' = 2.90R, that lie on the same plane. Point P lies on the central z axis, at distance D = 2.10R 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?

answer is [________] x Q

[[(2.10R)^2 +(2.9R)^2]^(3/2)] / [(2.1R)^2 +R^2)^(3/2)] times Q

If I didn't make any mistakes... did it kinda fast.

6. Did it work?

7. Originally Posted by Cell-Tech
Did it work?
sorry I was away, trying it now reps either way though I'll let ya know if it works

8. the answer comes from this

They used different numbers though and didn't explain it well, anybody know how to do it from here?

9. Originally Posted by O_S_U
I need this last answer for my homework, can't figure this one out.

The figure below shows two concentric rings, of radii R and R ' = 2.90R, that lie on the same plane. Point P lies on the central z axis, at distance D = 2.10R 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?

answer is [________] x Q

Possible quick answer would be to use the ratio of R:R'

R/1Q = R'/xQ

I'll go grab one of my old physics books to be sure. Might be forgetting something.

edit: lemme think about that equation you posted.

10. Originally Posted by O_S_U
the answer comes from this

They used different numbers though and didn't explain it well, anybody know how to do it from here?
I used that equation. I even plugged in R as a random number to see if the field equaled to 0 and it did.

Said it was wrong?

11. Originally Posted by Cell-Tech
I used that equation. I even plugged in R as a random number to see if the field equaled to 0 and it did.

Said it was wrong?
What would I plug in as the answer, it doesn't recognize R i'm not sure what to put in for R is the problem lol Idk i'm kinda lost here

12. Originally Posted by O_S_U
What would I plug in as the answer, it doesn't recognize R i'm not sure what to put in for R is the problem lol Idk i'm kinda lost here
IT has to be an actual number times Q?

13. Originally Posted by Cell-Tech
IT has to be an actual number times Q?
yes as you can see the number from the answer in the other one is -4.19, that's why I'm confused lol

14. I bet you get an XR/R and the R term just goes to 1 and you're left with X

15. Here is the screen shot of the cramster answer.

edit: both

16. Originally Posted by fly4navy117
Here is the screen shot of the cramster answer.

edit: both
Finally got it thanks to that ha, answer was -3.648

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