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Homework Help: Electric field and positive and a negative charge

  1. Feb 12, 2015 #1
    This is not a homework question, but a question im stuck on in my textbook. I have the answer but struggling to understand it.

    1. The problem statement, all variables and given/known data
    Picture below with answer.

    A positive and a negative charge of equal magnitude are placed at a distance 's' from each other on the x-axis as shown in Figure 1.38. Determine the direction of the electric field at point P, which is equidistant from both charges. ** I have attached the answer that is a diagram to this post** I do not understand it.

    2. Relevant equations

    3. The attempt at a solution

    Answer from textbook
    The contributions to the field at P due to thepositive and negative charges are shown in Figure 1.60.Their magnitudes are the same, because the magnitudes of the two charges are the same, as are their distances from P.The directions are along the line joining P to the chargesand away from the positive charge but towards thenegative charge. Thus, the two contributions make equal angles above and below the positive x-direction. The resultant field at P is therefore in the positive x-direction.

    What I dont understand

    Why is the elecric field from the negative charge and positive charge towards the right on x-axis but in opposite y directions. The charge of P is not known so why is it to the right, I would think it would be to the right if it was a positive charge, because +ve q would repel P and -ve q would attract P, thus both going in same direction. But if P is negative, the vectors should going left?

    My way of thinking is the fields from the -ve and +ve particles propogate from their center around them and cancel eachother out at P because the particles are of the same charge mag but opposite, therefore same field magnitude at P and equal distance from P. Therefore no resultant.

    Attached Files:

  2. jcsd
  3. Feb 12, 2015 #2


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    Education Advisor
    Gold Member

    The electric field points, by convention, away from + charges and towards - charges.

    It could have been the reverse, but that's not how it was chosen. But it has to be away from one and towards the other.
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