I was looking at the solution to this problem: /solution-player.aspx?solution_pr

Question

I was looking at the solution to this problem: /solution-player.aspx?solution_problem_id=282161
I cannot figure out where the formula he used came from...itlooks like the electric dipole formula with a 1/4instead of a 1/2 though. I sure it's multiplied by 2because the charges are equal, but that would make it1/ if that where the case. He gets the right answeraccording to the book, but I can't see how that formula wasderived. Also the electric dipole wasn't given to us in that partof the book yet, which is why I was trying to solve it similar tothe way this fella explained it...yet I kept getting the wronganswer. /physics-answers-5-173246-cpi0.aspx Can someone plz explain how the original solution's formulawas derived, and why I can't get the answer the other way? Thanksso much! I was looking at the solution to this problem: /solution-player.aspx?solution_problem_id=282161
I cannot figure out where the formula he used came from...itlooks like the electric dipole formula with a 1/4instead of a 1/2 though. I sure it's multiplied by 2because the charges are equal, but that would make it1/ if that where the case. He gets the right answeraccording to the book, but I can't see how that formula wasderived. Also the electric dipole wasn't given to us in that partof the book yet, which is why I was trying to solve it similar tothe way this fella explained it...yet I kept getting the wronganswer. /physics-answers-5-173246-cpi0.aspx Can someone plz explain how the original solution's formulawas derived, and why I can't get the answer the other way? Thanksso much!

Explanation / Answer

   from the given problem we can see that thevertical components of the individual fields will cancel bysymmetry    with d = 4.50 m the horizontal componentsadd to give a magnitude of    Ex,net = 2 q d / 4 o (d2 +y2)3/2             = ......... N / C
That does not seem to give me the right number...I get1.84x10^-10 with that formula. d should =4 also, although I'm notreally sure where the 4.5m came from. I just worked it from theformula like in the second example and got it right...finally :) Iwas using the wrong angle when calculating the cos of thevector. I'm still not sure where the formula you gave me came from.it's similiar to the fella's in the original example, but I stillcannot see where it came from. The formula for the electricdipole, which seems to be for a particle on the same axis at theother 2 particles is (1/2)(p/z^3) which seems to bewhat you have there, yet the particle is not on the same axis, itis perpendicular to their midpoints. It also looks similiar to the formula for a charged ring,which has to take into account the entire charge which would be 2q,which would just give you the same electric dipole formula. Neitherof which give the right answer. What I asked originally is how the guy derived his formula,because he got the right answer, but it doesn't match any otherformulas that I have. :(  

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