1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

    User Avatar

    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

    User Avatar

    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
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook