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Homework Help: Electric Field

  1. Oct 26, 2005 #1
    I am posed with a question, there are two charges, both positive, and two neutral points.

    I am asked to compare the electric field strengths at the two points.

    Here is a pictorial representation:

    [tex]\bigoplus[/tex]------->[tex]\bullet 1[/tex]<-------[tex]\bigoplus\bigoplus\bigoplus[/tex]------->[tex]\bullet 2[/tex]

    The three +s in a row represent one big positive charge.

    So I drew my electric field lines (represented by the arrows), and now I have to analyze whether the field strengths at the two points are different, or equal.

    At first I thought that 1 > 2, since 1 has respectively four positive charges at it, while 2 only has three positive charges.
    However, then I thought:
    In the case of 1, the two electric fields will repel each other, and then there will no ( ? ) field lines around 1, in which case 2 > 1.

    But I don't have much proof to back up my answer, and my textbook doesn't explain this in great detail (neither did my teacher). Any suggestions on how to compare electric field strengths?

  2. jcsd
  3. Oct 26, 2005 #2


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    Gold Member

    If the two points are of equal distance to the big charge then the field at point 2 will be bigger, here's why:
    the total field at point 1 is (k/r^2)*3q - (k/r^2)*q = (k/r^2)*2q the (-) is to show that they are in oppisite directions
    the total field at point 2 is (k/r^2)*3q + (k/9r^2)*q = (k/r^2)*3.111q
    anyway in point 1 the small charge takes away from the big one, at point 2 it adds to it.
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