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Homework Help: Electric Field and total energy

  1. Jan 30, 2007 #1
    1. The problem statement, all variables and given/known data

    Two point charges lie along the y axis. A charge of q1 = -12.0 µC is at y = 7.0 m, and a charge of q2 = -2.0 µC is at y = -4.0 m. Locate the point (other than infinity) at which the total electric field is zero.

    2. Relevant equations

    E = (K lql) / r^2


    3. The attempt at a solution

    I setup the two equation make E = 0
    so

    (K lq1l) / (7-y)^2 = (K lq2l) / (-4+y)^2

    It's wrong, plz correct my equation
     
  2. jcsd
  3. Jan 30, 2007 #2

    Pyrrhus

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    Homework Helper

    Why do you think that equation is wrong?
     
  4. Jan 30, 2007 #3
    I think the equation is wrong because I double checked all my math and it said that the answer is wrong.
     
  5. Jan 30, 2007 #4

    Pyrrhus

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    Homework Helper

    Ok, Let's review again:

    The point in which the electric field will be 0 is between the charges. Because of the standard convention. You picked a point inside so it's correct.

    Next we have to set our electric field vectors, such that:

    [tex] \vec{E}_{1} + \vec{E}_{2} = \vec{0} [/tex]

    [tex] \vec{E}_{1} = K \frac{|q_{1}|}{(7-y)^{2}} \vec{j} [/tex]

    [tex] \vec{E}_{2} = -K \frac{|q_{1}|}{(4+y)^{2}} \vec{j} [/tex]

    so, the error was using -4, because r will be a distance.
     
    Last edited: Jan 30, 2007
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