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Homework Help: Finding magnitude of torque on a molecule

  1. Sep 10, 2009 #1
    1. The problem statement, all variables and given/known data

    Molecules of carbon monoxide are permanent electric dipoles due to unequal sharing of electrons between the carbon and oxygen atoms. The figure shows the distance and charges. Suppose a carbon monoxide molecule with a horizontal axis is in a vertical electric field of strength 15000 N/C.

    The picture they provide is one of 2 circles connected together in a linear line with C on the left and O on the right. The diagram shows them to be .11 nm apart from each other. The C has a +3.4E-21 charge and the O has a -3.4E-21 charge.

    A. What is the magnitude of the net force on the molecule?

    B. What is the magnitude of the torque on the molecule?

    2. Relevant equations
    F = eE
    Torque = qEL

    3. The attempt at a solution

    Ok, so part A I got right. I just used the F = eE equation and got the answer 2.4E-15 and thought that I would be using this for part B.

    In part B my book shows the equation qEL = torque. It says that qE is the magnitude of the electric force and L is the length they are from each other.

    Torque = (2.4E-15 N) * (1.1E-10 m) = 2.64 E -25

    This isn't right though. What am I doing wrong? I really appreciate the help!
  2. jcsd
  3. Sep 11, 2009 #2

    Doc Al

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    Staff: Mentor

    Is the electric field uniform? I don't understand how you got your answer for the net force on the molecule. (What's the net charge on the molecule?)
  4. Sep 11, 2009 #3
    Oh wait, I just realized the answer to part A is zero because they cancel each other out. I was wrong. So the torque tho I'm supposed to use not the net force but the force on the individual atoms? 3.4E-21

    I'm still lost...
  5. Sep 11, 2009 #4

    Doc Al

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    Staff: Mentor

    The maximum torque on a dipole equals the force on one charge times the distance between them. Read about it here: http://hyperphysics.phy-astr.gsu.edu/HBASE/electric/diptor.html" [Broken]
    Last edited by a moderator: May 4, 2017
  6. Dec 7, 2010 #5
    to find the force on each molecule, you divided by the radius squared. don't do that - multiply by the distance instead--traditonally torque is distance * force

    so it is 15000 * 3.4*10^-21 * .11*10^-9 = 5.6*10^-27
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