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A +'ve charge Q is fixed to the origin. A -'ve charge 9Q

  1. Jun 16, 2015 #1
    1. The problem statement, all variables and given/known data

    A positive charge Q is fixed to the origin. A negative charge with magnitude 9Q is fixed along the positive x-axis a distance d to the right of the origin. Determine a point where a small positive test charge q will experience zero electrical force from the two charges, or show that there is no such point. Draw diagrams to help you.

    2. Relevant equations

    FE = kq1q2 / r2

    3. The attempt at a solution


    I know the test charge must be to the left of the positive charge Q, and I tried to solve by rearranging the equation to solve to r, but that leaves you with dividing the force, which in this case is zero which does not work.

    r = √kq1q2 / FE
    But FE = 0 so this does not work.
  2. jcsd
  3. Jun 16, 2015 #2


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    Your equation is for the force between two charges. There are three in the problem. The net force must sum to zero not the two individual forces.
  4. Jun 16, 2015 #3
    So would the equation for three charges just include a q3 at the end? Thus making the small positive test charge 8Q to have it equal zero?
  5. Jun 16, 2015 #4


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    Not sure I follow that.

    The test charge q is +ve so there are two forces on it..

    1) A repulsive force between +Q an +q
    2) An attractive force between -9Q and +q

    The question asks where must +q be for these two forces sum to zero. Have you studied vector addition?
  6. Jun 16, 2015 #5
    How would vector addition work for this question as there are no values for the variables?
  7. Jun 16, 2015 #6


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    The values are irrelevant, only the relative size of the charges is important (and you know that one has 9 times more charge than the other).

    Place the test charge at an arbitrary position on the x-axis, then write an equation for F1 the force on the test charge from the one charge and another equation for F2 the force on the test charge from the other charge. The net force is F1+F2. The problem asks you to find where this net force is zero.
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