# Electric Fields ( i attached a file)

1. Jul 27, 2007

### trah22

1. The problem statement, all variables and given/known data
Hi, my question isnt actually over a particualr homework problem, ive attached a file of my notes on electric field, electric dipoles, i dont quite understand why the equation of the electric field is setup that way for the negative charge.

2. Relevant equations

3. The attempt at a solution

#### Attached Files:

• ###### physics3.PDF
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60
2. Jul 27, 2007

### trah22

can someone help me out

3. Jul 28, 2007

### proton

I'd love to help, but I'm unable to open the file

4. Jul 28, 2007

### trah22

weird what error comes up? maybe it requires the latest adobe, my file is in pdf. Argh im not sure what could be wrong

Is there a specific way to attach files in order for them to be posted correctly for this forum?

Last edited: Jul 28, 2007
5. Jul 28, 2007

### Staff: Mentor

There's nothing wrong with how you posted your file; attachments just need to be approved before they can be viewed. Done!

Now, what exactly is your question? What about that expression for the field of the dipole is unclear? It's just expressing the field as a vector sum of the components of the field from each charge. Since the field from a negative charge points towards the charge, both components of its field at point P are negative. (Note that $E_-$ stands for the magnitude of the field at point P contributed by the negative charge.)

6. Jul 28, 2007

### Matthaeus_

You could also have noted that the components in the $$x$$ direction cancel each other out by simmetry, without having to worry about cosines ;)

7. Jul 28, 2007

### trah22

Well to find the Electric Field u have to add the electric field for the postive and negative charge. For the xaxis i undestand that its in the positive direction so its Ecos(theta)i but howcome for the y direction its -esin(theta)j, is it because its goin down? Thats for the postive charge electric field. For the negative charge, i dont understand why its minus Ecos(theta)i and then minus Esin(theta)j. Basically could someone explain to me why for the negative charge according to the way its graphed, why its -Ecosthetai-Esinthetaj.

8. Jul 28, 2007

### trah22

Ok, i actually just figured it out, thnx for the help anyhow,

Last edited: Jul 28, 2007
9. Jul 28, 2007

### trah22

Im just having some trouble on figuring out how some of the components are and how there being used for this part of the notes ( i attached another file),

#### Attached Files:

• ###### physics4.PDF
File size:
33.3 KB
Views:
34
Last edited: Jul 28, 2007
10. Jul 28, 2007

### Staff: Mentor

Since $E_+ = E_-$, $E_+ + E_- = E_+ + E_+ = 2 E_+ = 2 kq/D^2$.

11. Jul 28, 2007

### trah22

ah i c thanx doc

12. Jul 29, 2007

### trah22

There was one more thing on that page that i dont quite get, when integrating equation 3 into equation 2, why is the a?d there? because E=F/q=kq/r^2x1/q, im not quite sure on where and why the a/D is needed.

And then for this one particular problem, its very similiar to the above problem, but i ran into a couple problems, 1 during the setup of the electric field equation, another when choosing the correct integral to use and lastly how R-L/Pie is used in the final Electric field equation.

The actual problem is:a uniformly charged insulating rod of length 14 cm is bent into the shape of a semcircle. The rod has a total charge of -7.5uC. Find the magnitude and direction of the electric field at 0, the center of the semicircle.

I wrote down on the scan on the things im talking. There just 3 parts.

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