Field due to point charges - extremely strange error

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Homework Help Overview

The problem involves finding the electric field along the x-axis due to two point charges, one positive and one negative, located at specified coordinates. The original poster expresses frustration over not arriving at the expected answer despite reviewing their work with peers.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • Participants discuss the expected direction of the electric field lines based on the charge types. Questions arise regarding the calculation of the z-component of the electric field and the correctness of the setup. There are mentions of discrepancies in the derived expressions for the electric field.

Discussion Status

Some participants have provided feedback on the clarity of the original poster's work, suggesting areas for improvement. There is an ongoing exploration of the calculations involved, with various interpretations of the results being discussed. No consensus has been reached regarding the correct approach or solution.

Contextual Notes

There are indications of potential missing elements in the calculations, as well as confusion regarding the signs and directions of the vectors involved. The original poster has noted a specific concern about an error that seems to contradict established physics principles.

Ngineer
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Homework Statement


Find electric field anywhere on the x-axis due two two point charges
Q at (0,0,d)
-Q at (0,0,-d)

Homework Equations

The Attempt at a Solution


AttachedSomehow 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.
 

Attachments

  • WP_20141010_002.jpg
    WP_20141010_002.jpg
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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.
 
Ngineer said:

Homework Statement


Find electric field anywhere on the x-axis due two two point charges
Q at (0,0,d)
-Q at (0,0,-d)

Homework Equations

The Attempt at a Solution


AttachedSomehow 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.

What is the z-component of E?
 
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)
 
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?
 
Please discard that =0

I rewrote the solution for clarity
 

Attachments

  • WP_20141010_004.jpg
    WP_20141010_004.jpg
    42.9 KB · Views: 403
P/S. There a missing 2 one line before the end but it should be obvious
 
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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