# Homework Help: Field due to point charges - extremely strange error

1. Oct 10, 2014

### Ngineer

1. The problem statement, all variables and given/known data
Find electric field anywhere on the x axis due two two point charges
Q at (0,0,d)
-Q at (0,0,-d)

2. Relevant equations

3. The attempt at a solution
Attached

Somehow I am not getting the correct answer. I reviewed my steps with 4 people and we could not find the error!

I'm starting to think I've ruined the laws of physics.

#### Attached Files:

• ###### WP_20141010_002.jpg
File size:
27 KB
Views:
78
2. Oct 10, 2014

### Staff: Mentor

Because you know one charge to be -Q then I would expect on the diagram you'd show its field line vector directed towards the charge.

3. Oct 10, 2014

### Staff: Mentor

What is the z-component of E?

4. Oct 10, 2014

### Ngineer

N.O. - The change in sign and direction was accounted for on the second line.
berkeman - the answer I got was -2q/4piEo * 1/(x2 + d2) az which is different from the "correct" answer by a factor of d/(x2 + d2)

5. Oct 10, 2014

### Staff: Mentor

It is hard to read that image.

You set up a vector and then write "=0." If that would be true, there would be nothing left to calculate.

Where do you calculate the z-component of your force?

6. Oct 10, 2014

### Ngineer

Please discard that =0

I rewrote the solution for clarity

#### Attached Files:

• ###### WP_20141010_004.jpg
File size:
47.1 KB
Views:
90
7. Oct 10, 2014

### Ngineer

P/S. There a missing 2 one line before the end but it should be obvious

8. Oct 10, 2014

### Staff: Mentor

I don't see what you did in the numerator between the first and second line. Your new numerator has both a different magnitude and direction.
You can make this way easier by combining the fractions and sorting the numerator by x and z components.

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