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Electrical Forces, Torque & Flux

  1. Mar 3, 2009 #1
    1. The problem statement, all variables and given/known data

    A long, thin rod (length = 4.0 m) lies along the x axis, with its midpoint at the origin. In a vacuum, a +6.0 C point charge is fixed to one end of the rod, and a -6.0 C point charge is fixed to the other end. Everywhere in the x, y plane there is a constant external electric field (magnitude 5.0 x 103 N/C) that is perpendicular to the rod. With respect to the z axis, find the magnitude of the net torque applied to the rod.

    2. Relevant equations

    Torque = Fl

    3. The attempt at a solution

    F1 = (5E3N/C)(+6.0E-6C)
    F1= 0.03N

    F2= (5E3N/C)(-6.0E-6C)
    F2= -0.03N

    T1= (0.03N)(4.0m)
    T1= 0.12 N/m

    T2= (-0.03N)(4.0m)
    T2= -0.12N/m

    I'm not sure where to go from here in order to find the net torque, I don't think it is as simple as they cancel out but I can't figure out what to do next.

    1. The problem statement, all variables and given/known data

    A charge Q is located inside a rectangular box. The electric flux through each of the six surfaces of the box is: 1 = +4400 Nm2/C, 2 = +3530 Nm2/C, 3 = +1310 Nm2/C, 4 = -4520 Nm2/C, 5 = -5930 Nm2/C, and 6 = -4580 Nm2/C. What is Q?

    2. Relevant equations

    I'm not even sure where to start with this one, maybe one of these two equations:
    Flux = Summation(Ecosphi)A
    Flux = Q/Permittivity of free space

    3. The attempt at a solution
  2. jcsd
  3. Mar 3, 2009 #2


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    Homework Helper

    When you have 2 equal and opposite forces then I think the Torque on the object can be taken about one of the points of the application of the forces, and the torque would be the moment of the other force about that point. If your force is correct, (I didn't do the math), then either of your answers is correct, except that they should both end with the same direction, clockwise or counterclockwise and not different as you apparently show.
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