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Homework Help: Dipole moment and net torque of an atom

  1. Oct 1, 2012 #1
    1. The problem statement, all variables and given/known data
    The HCl (hydrochloric acid) molecule has a dipole moment of about 3.4x10-30 C.m. The two atoms are separated by about 1.10-10 m. What is the net charge on each atom? What is maximal torque that this dipole will experience in a 5.104 N/C electric field? How much energy one would need to rotate one molecule 45o from its equilibrium orientation?

    2. Relevant equations
    A) Can't figure out which formula to use to find the net charge
    B) τ = P X E
    C) Not sure about this one: W = ΔU = U45° - U = - PE Cos45° - (- PE Cos 0°)

    NOTE: For B, I am assuming that the initial angle is 0° to make my life easier since the information is not provided

    3. The attempt at a solution
    A) Can't figure out which formula to use to find the net charge

    B) 3.4 X 10 -30 Cm χ 5 X 104 N/C = 2.3 X 10-27 Nm, or Joules

    C) (-2.3 X 10-27J X cos 45°) + 2.3 X 10-27J
    = -1.626 X 10-27J + 2.3 X 10-27J = 6.74 X 10-28J

    Does that make sense for B and C? And could you give me any pointers for A?

    Thank you!
    Last edited: Oct 1, 2012
  2. jcsd
  3. Oct 1, 2012 #2
    I don't know what the rule is with bumping, but here it is, anyone has any idea for this problem?
  4. Oct 1, 2012 #3
    Do you know the formula for dipole moment?
  5. Oct 2, 2012 #4
    I tried to find it in my book, but the only formula I have is the one to find the torque, which I used in B, and the formula which I used in C

    However, to find the charge in a dipole moment I do not seem to have the formula written down. I've browsed through my book but I don't seem to find it.

    Edit: Heh just did a Wikipedia search: Dipole moment can be defined as the product of magnitude of charge & distance of separation between the charges (I didn't have that formula written down)

    Knowing this, would question A be:

    Dipole = Charge X Distance

    3.4 X 10-30 Cm = Charge X 1 X 10-10 m

    (3.4 X 10-30 Cm) / (1 X 10-10 m) = Charge (meters cancel out)

    However, how can I differentiate to know whether I found the charge of which atom or the molecule?
    Last edited: Oct 2, 2012
  6. Oct 2, 2012 #5
    In dipole the charges are equal, so one of the charge will be q and the other -q.
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