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Homework Help: Electrostatics home work question

  1. Jul 10, 2011 #1
    1. The problem statement, all variables and given/known data
    Two Charges +q and -3q are separated by a distance of 1 m. At what point in between the charges on its axis is the potential is zero.

    2. Relevant equations

    I formulated the diagram as given in the attachment

    3. The attempt at a solution
    At equilibrium V1 = V2
    q/4*pi*epsilon*x = -3q/4*pi*epsilon*(r-x)
    (r-x) = -3x
    x = -r/2 -1/2 = -0.5m. I took r = 1m because distance of separation between the charges is 1 m. Am i correct? How to write the answer because i get negative distance. Could you please help where is the location of point where the potential is zero.
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution

    Attached Files:

  2. jcsd
  3. Jul 10, 2011 #2
    You should be finding the value of x where the absolute potentials from the +q and the -3q add to zero, not where these potentials are equal.
  4. Jul 10, 2011 #3
    mr. misterX, so i should use the equation v1+ v2 = 0
    am i right, sir?
    which gives me 0.5 m as the answer
  5. Jul 10, 2011 #4
    That's the right equation to use, but 0.5 m is the wrong answer.
  6. Jul 12, 2011 #5
    oh sorry! v1= -v2 so i get 0.25 m as the answer. I hope i am right. Then to how to write the answer. Can i write, the potential is zero at a distance .25m from the right of +q.
    Some more doubts.
    1) can i put -3q on left and +q on the right and solve the above problem. But i dont get 0.25 m
    2) Why we use the equation V1 + V2 = 0 at the point O which is a distance x from A?
    Thanks in advance sir
  7. Jul 12, 2011 #6


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    Staff: Mentor

    1) You have to take into account the relative position on of your "test point" with respect to any given charge in your coordinate system.

    Imagine that a point charge is surrounded by field arrows all pointing either radially outward (positive charge) or radially inward (negative charge). The field points IN OPPOSITE DIRECTIONS on opposite sides of the charge. This geometry dependence will necessitate a change of sign in your equation for the potential depending upon your test point location.

    It's always best to make a sketch of any given setup of charges, noting the directions of the field contributions of each at places where you want to determine the net field. This will guide you in writing the correct signs for the terms your equations.

    2) Electric fields obey the superposition principle. That is, the fields from individual charges can be calculated separately and then summed at a given point to find the net potential at that point.
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