# Homework Help: A charged segment of a ring?

1. Feb 25, 2013

### Octavius1287

Hi everyone, i have been looking at this problem for a few days now and i am pretty stuck, ill explain why

first the problem :
a charge Q is distributed over a ring section of radius r, between angles 90 and 180. find the V and E at the origin.

Now the problem im having ais everything that is kind of similar to this problem or about charged rings is only for uniformly charged rings and i cant find anything about just a segment, and in those cases i think E at the center is 0. Also every where i look im seeing a different formula for E of a charged ring. I am pretty confused

2. Feb 26, 2013

### Octavius1287

i also dont understand how only one segment and in this case the 2nd quadrant can be charged..

3. Feb 26, 2013

### sandy.bridge

$$\vec{E}=\int\frac{\lambda\hat{r}}{r^2}ds$$
Where $\lambda$ merely represents the charge distribution $Q$, and $\hat{r}$ represents a unit vector perpendicular to each segment pointing towards the origin. You want to take the line integral on the interval $(\pi/2, \pi)$.

Normally, the electric field would be zero in the center of a circle with uniform charge distribution. However, it is not in this instance.

4. Feb 26, 2013

### Octavius1287

thank you, what source did you get that equation from id like to read more about it

5. Feb 26, 2013

### Octavius1287

i see 1/4*pi*eo and to the power of 3/2 in alot of ring equations thats was confusing me is the difference between those and this one

6. Feb 27, 2013

### sandy.bridge

Not entirely sure what level you are at, but this was in both my vector calculus and electrostatics textbook. You may find variations depending on how the charge is distributed (line, surface, etc).

7. Feb 27, 2013

### Octavius1287

College, and the one in my Electromagnetics book is

E=$\hat{z}$[h/(4$\pi$ε0(b^2+h^2)^3/2)]*Q where b is the radius and h is the height, but like i said its for a uniform charge

Last edited: Feb 27, 2013
8. Feb 27, 2013

### dlgoff

This is probably what it's referring to.

I don't know if this will be helpful, but take a look.

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/elelin.html

Regards

9. Feb 27, 2013

### Octavius1287

ya all those make sense to me, but i read that page and once again it said it was only for a uniform charge

10. Feb 27, 2013

### dlgoff

Yes.

This doesn't mean it is NON-uniform for the rest of the ring. Aren't they looking for the field from the contribution of charge on just this piece of the ring?

If so,