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Ranking the force of point charges

  1. Sep 21, 2009 #1
    1. The problem statement, all variables and given/known data

    5 point charges;
    q1; charge = +q distance = d
    q2; charge = +2q distance = 3d
    q3; charge = -3q distance = 2d
    q4; charge = -4q distance = 3d
    q5; charge = -5q distance = 2d
    are placed in the vicinity of an insulating spherical shell with a charge (+Q), distributed uniformly over its surface.
    Rank the point charges in order of the increasing magnitude of force exerted on them by the sphere. Indicate all ties where appropriate. Show all calculations of force.

    (there is a diagram attached, but I don't have a scanner and my teacher siad it is only to visualise the problem.)

    2. Relevant equations

    Coulomb's law;
    F= K (q1)(q2)
    r squared

    3. The attempt at a solution
    My teacher said to apply coulombs law to each of the charges, than compare each.

    I tried this, but have no idea of what values I should use for q1 and which I should use for q2.
    And do I need the distance of each charge from the edge of the sphere?
    The more I look at it, the less sense it makes.
  2. jcsd
  3. Sep 21, 2009 #2
    You see that F = qE and F = kq1q2/r^2. From Gauss's Law the total charge enclosed is Q. You are still taking r as being the distance from the center of the sphere to the point charge. Simply make q1 = Q and substitute the different q2 values and r values and compare those magnitudes. If you haven't seen Gauss's Law this will probably make more sense when you do.
  4. Sep 21, 2009 #3
    No, we haven't touched on Gauss's law yet.

    I'm still not sure how to rank the equations after I've substituted the values.
    so far I have;

    q1=> F= K Qq

    q2=> F= K 2Qq

    q3=> F= K -3Qq

    q4=> F= K -4Qq

    q5=> F= K -5Qq

    how would I rank these?
    Or is there more I need to do first?

  5. Sep 21, 2009 #4
    i hope those equations still make sense;
    it deleted the spaces in the bottom row.
    The first one should be;
    K Qq
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