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Homework Help: Place a 3rd charge so net force is 0

  1. Mar 26, 2012 #1
    1. The problem statement, all variables and given/known data

    A charge of +2q is placed at the origin and a second charge of -q is placed a x = 3cm. Where can a third charge +Q be placed so that it experiences a zero force?

    2. Relevant equations


    3. The attempt at a solution
    OK I thought I had this until the roots of the quadratic I am solving come out to be imaginary...

    Q1 = 2q
    Q2 = -q
    Q3 = +Q

    I am assuming that Q3 must be placed to the right of Q2.
    the distance between Q1 and Q3 = x+0.03m and the distance between Q2 and Q3 = x

    If F on Q3 = 0 then FQ1=FQ2 at this point

    F1on3 = k*Q1*Q3/r^2 = F 2on3 = k*Q2*Q3/r^2 ;; cancel k and Q3

    Q1/r1to32 = Q2/r2to32


    Q1*x2 = Q3*(x+0.03m)2
    2q*x2 / -q = (x+0.03m)2
    -2x2 = (x+0.03m)2
    -2x2 = x2 + 0.06x + 0.0009

    0=3x2 + 0.06x + 0.0009

    Can anyone tell me where I've gone wrong here? Is my assumption that Q3 is to the right of Q2 incorrect?
  2. jcsd
  3. Mar 26, 2012 #2


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    Homework Helper

    Your initial assumption is correct.

    I would be looking as follows:

    The charges of 2q and q [never mind the signs] means the force form the forst charge is potentially twice the size.
    The way to balance that is to have it √2 times as far from your Q

    so in your nomenclature

    x + 0.03 = √2x [remember that is only √2 the x is not under the √ sign]

    That leads to 0 = x2 - 0.06x - 0.0009 - slightly different to your equation.
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