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ehabmozart
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Homework Statement
Positive charge +Q is distributed uniformly along the +x axis from x=0 to x=a. Negative charge - Q is distributed uniformly along the +x axis from x=0 to x=-a. A positive point q lies on the positive y axis, a distance y from the origin. Find the force (mag and dir.) that the +ve and -ve charge distributions together exert on q.
Homework Equations
F= Eq
The Attempt at a Solution
I am so fine with the understanding and setting up the answer to this question. Initially, due to symmetry, I know that the resultant force would be just like the double of the field produced by the negative rod (or positive) in the negative x axis. So the answer should be - i direction. That's fine! ... My problem now is the final answer. Now let us consider the negative rod. I know originally that my answer is negative and what i usually did with point charges too if the direction is negative is that i take the absolute value of every single thing including q and then i add a negative sign at the end. Now by taking the negative rod to examine, I reached to the formula of KQ ( -1/(x^2+y^2)^.5) with the integral limits of -a to 0. Is this right... Is it right to take the integral from the small to the negative?? ... Now after doing the calculation I end up with a negative sign which is ( - K Q ( 1/y - 1/(y^2+a^2)^.5) .. The final answer is in NEGATIVE... I am confused what to do next... Should I multiply it again by negative i restor in every question.. I need general rules for this and not just an answer to this..
Questions.
1) Should my answer of the integral be inline with my physics asumptions... I mean I know that the answer should be + i and the integral comes out in negative.. WHat should I do?
2) Should the integral always to be from the smallest to the largest
THANKS A LOT FOR YOUR PATIENCE