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Homework Help: Calculating Torque of charges at an angle

  1. Jul 8, 2010 #1
    1. The problem statement, all variables and given/known data
    A thin wooden stick of length 12 cm has a tiny metal sphere
    glued to each end. A charge of +3 microC is placed on one
    sphere and a charge of -2 microC is placed on the other.
    The center of mass is located 7 cm from the positively-
    charged sphere. The system is mounted on a fixed
    horizontal E-W axle passing through the center of mass
    about which the system is free to rotate with no friction.
    When the system is then placed in a horizontal uniform
    southward electric field of 800 N/C, the resulting
    equilibrium position of the system is horizontal with
    the positive charge due S of the negative charge.

    What amount of torque (about the axle) is required
    to hold the system at an angular displacement of 25
    degrees away from the equilibrium position?

    2. Relevant equations
    torque = LqEsin(theta)

    3. The attempt at a solution
    I tired plugging in the values into the equation:

    torque = (0.12)(1x10^-6)(800)(sin(25)) but it isn't coming out right. What am I doing wrong?
    Last edited by a moderator: Jul 9, 2010
  2. jcsd
  3. Jul 9, 2010 #2


    User Avatar
    Homework Helper
    Gold Member

    Hello Pete_01,

    Rather than just plug some numbers into an equation, I suggest taking a step back and think about what that equation means, and in what situations it is applicable.

    The problem statement gives you two charges, one of 3 μC on one side of the stick, and -2 μC on the other. Since the spheres are oppositely charged, the electric forces obviously point in opposite directions. But when it comes to their corresponding torques on the stick, are the torques working together or against each other? (Hint: the spheres are oppositely charged, yes. But the spheres are also on opposite sides of the same stick!)

    It doesn't quite end there. The center of mass (where the axle is located) is not at the center of the stick. The positively charged sphere is 7 cm from the axle, meaning the negatively charged sphere is 5 cm from the axle. How does all of this affect the torques involved?
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