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Homework Help: Direction of electric field

  1. Mar 15, 2010 #1
    1. The problem statement, all variables and given/known data

    three equal and positive charges q are located equidistant from the origin as shown http://s966.photobucket.com/albums/...n=view&current=directionofanelectricfield.jpg . Is the direction of the electric field at the origin a) along x, b)along -x, c) along +y d) along -y

    2. Relevant equations

    The electric field is directed outward away from Q when Q is positive.

    3. The attempt at a solution

    Do the field lines along the x axis emerging from the charges on the x axis cancel or enhance each other at the origin?

    Do I have to do vector addition on the x axis to solve this problem?
     
  2. jcsd
  3. Mar 15, 2010 #2

    berkeman

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    Staff: Mentor

    Just do the vector addition at the origin. What are the 3 E-field vectors at the origin? (magnitudes and directions)
     
  4. Mar 15, 2010 #3
    for the charge on the -x axis : kQ/r^3 x ri
    for the charge on the +x axis: kQ/r^3 x r-i
    for the charge on the +y axis: kQ/r^3 x r-j

    so the charges that are sitting on the x axis will cancel out. how do i write that in a more mathematical way?
     
  5. Mar 15, 2010 #4

    berkeman

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    Staff: Mentor

    Correct. In general, you would write the vector sum in rectangular coordinates. Express each individual vector as the sum of its x and y components (using the i,j vector notation, or x-hat, y-hat, etc.), and show the sum of them as a sum of x components and y components, still in rectangular vector notation.

    And in this problem, it's just multiple choice anyway. Which is your answer?
     
  6. Mar 15, 2010 #5
    The answer is d, along -y
     
  7. Mar 15, 2010 #6

    berkeman

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    Staff: Mentor

    Yep. Good work.
     
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