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Homework Help: Force and Torque exerted on a dipole system from a point charge

  1. Mar 21, 2012 #1
    I have attached a diagram.
    Three charge are at corners of an triangle. The + and - 5.00*10^-6C charges form a dipole.
    a) Find the magnitude and direction of the force exerted on the dipole from the -10*10^-6C charge.
    b) For an axis perepndiculat to the line connected the + and - 5.00*10^-6C charges , find the torque exerted on the dipole by the -10*10^-6 C charge, direction and magnitude?

    My attempt:
    a) The net force = 0.
    Its magnitude on both charges = 1125 N, (from Coloumb's law and using r as 2cm.

    However the answer is 1680N on both charges respectively.

    b) Torque = Force * distance perpendicular to the line of action.

    Considering a single charge, the force exerted on it is 1125N, and the distance perpendicular to the line of action 1.5cm.

    The net torque will then = 2(1.5*10^-2 * 1125) , as the force acts in opposite directions on both charges, consequently rotating the dipole system in the same way.

    T = 33.75N.m

    (The correct answer is however 22.3N.m)F

    Attached Files:

  2. jcsd
  3. Mar 21, 2012 #2


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    Homework Helper

    The question is the force exerted on the whole dipole. You have to add the forces exerted on the separate charges, taking their directions into account.

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