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How do I check if it is a positive charge or negative charge?

  1. Mar 1, 2015 #1
    1. The problem statement, all variables and given/known data
    s5kahj.png

    2. Relevant equations


    3. The attempt at a solution

    The electric field at the origin O, due to charges Q, q1 and q2, is equal to zero
    Hence , EQ = Eq1
    q = cos30 x 300 x10^-9/4 * 1.8^2 = 210 x10^-9 C

    So I am able to find out that q = 210 x 10^-9 C and that mean that B and C cannot be the answer which left me with A and D.

    How do I check if it is a positive charge or negative charge?

     
    Last edited by a moderator: Mar 1, 2015
  2. jcsd
  3. Mar 1, 2015 #2

    rude man

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    The x component at the origin due to Q is what polarity? So the polarity of Ex due to q1 has to be the opposite, right? So what then does the polarity of q1 have to be?
     
  4. Mar 1, 2015 #3
    Why does it has to be opposite for the polarity of Ex due to q1?
     
  5. Mar 1, 2015 #4

    rude man

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    because the net E field at the origin is zero so q1 has to cancel out the x component of Q.
     
  6. Mar 1, 2015 #5
    What about the q2 at the y component?
     
  7. Mar 2, 2015 #6

    rude man

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    q2 does not produce an x compnent of the E field at the origin. You are summing x components of E at the origin.

    By the way, you can also solve this problem with summing y components of E = 0 at the origin. I suggest you do it for increased understanding.
     
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