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Torque on a dipole in an electric field

  1. Feb 21, 2014 #1
    1. The problem statement, all variables and given/known data
    An electric dipole has opposite charges of 6.55⋅10−15C separated by a distance of 0.65mm. It is oriented at 54.0° with respect to a uniform electric field of magnitude 10.0⋅103N/C. Determine the magnitude of the torque exerted on the dipole by the electric field.

    q+=6.55*10-15C
    q-=-6.55*10-15C
    r=0.00065m
    [itex]\vec{E}[/itex]=10,000N/C
    Θ=54.0°

    2. Relevant equations
    p=qd
    [itex]\tau[/itex]=p[itex]\times[/itex][itex]\vec{E}[/itex]


    3. The attempt at a solution
    The angle of the dipole is 54.0° with respect to the uniform electric field. Using trig I can see that the torque can be calculated by the cross product of the dipole moment and the magnitude of the electric field multiplied by sin(Θ).

    [itex]\tau[/itex]=p[itex]\times[/itex][itex]\vec{E}[/itex]

    The dipole moment is given by charge times the distance(r)

    p=qd

    p=(6.55*10-15C)(0.00065m)=4.26*10-18C*m

    If we plug in that value into our torque equation...

    [itex]\tau[/itex]=p[itex]\times[/itex][itex]\vec{E}[/itex]sin(Θ)

    [itex]\tau[/itex]=(4.26*10-18C*m)*(10,000N/C)*sin(54°)

    [itex]\tau[/itex]=3.44*10-14N*m

    Which is not correct according to my online homework. This calculation seems simple...too simple. Am I missing something?

    Also, if the torque results from a cross product operation, which is an operation between vectors, then in this case I just multiply the magnitude of the dipole moment and the magnitude of the electric field because we are not working with vectors in this problem?

    As always, any help is appreciated. Thanks in advance.
     
  2. jcsd
  3. Feb 22, 2014 #2

    haruspex

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    I get the same.
    No, you must include the sin(). Is that what you meant?
     
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