# Choosing Integrating constants for Electric Field

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1. Sep 9, 2015

### Jen2114

1. The problem statement, all variables and given/known data
A nonconducting disk of radius R has a uniform positive surface charge density sigma. Find the Electric field at a point along the axis of the disk at a distance x from its center. Assume that x is positive

2. Relevant equations
E=kq/r

3. The attempt at a solution
I know I'm suppose to find dEX for one ring and then integrate to find the field due to all the rings.
dEx= (k) (2πσrx)dr /(x^2 +r^2) ^3/2
Why should you integrate this component from 0 to R and not R to -R

2. Sep 9, 2015

### DEvens

Where exactly is the part of the disk from r=0 to r=-R? That is, where are the negative radius locations?

By the way, it is not an integrating constant as you suggest in the title. This is a definite integral so there is no integrating constant.

3. Sep 9, 2015

### LittleMrsMonkey

For any future reference,what you mean is called "interval of integration" and not constants.

4. Sep 9, 2015

### Jen2114

Hi,
sorry you're right I should've said that I don't understand why the limits of integration are 0 to R and not -R to R. The center of the disk is located at (0,0) and so the negative radius is at (0,-R) and the positive is at (0,R). The radius of the first ring I'm integrating is r and so then I have to integrate for the entire disk.

5. Sep 9, 2015

### Jen2114

Thank you, I will be much more clear next time

6. Sep 9, 2015

7. Sep 9, 2015

### Jen2114

So dEx=(1/4πε)*((2πσrx)/(x^2+r^2)^3/2)) is the electric field component in the x direction and so when you integrate to obtain the electric field for all the small rings in the disk , you are working your way out towards R, the radius of the entire disk. So that's why you integrate from 0 to R and not -R to R?

8. Sep 9, 2015

### LittleMrsMonkey

It's easy,see?
You've forgotten the dr in your formula.

9. Sep 9, 2015

### Jen2114

Ahhh ok I see thanks. Yeah, super clear now. Thanks I'll add the dr. Thanks again!

10. Sep 9, 2015

### LittleMrsMonkey

You're welcome.I'm studying for an E-M exam right now anyway,so it's good use of my time.