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Homework Help: What is the electric force of the molecule on the proton?

  1. Oct 20, 2012 #1
    1. The problem statement, all variables and given/known data

    An ammonia molecule (NH3) has a permanent electric dipole moment 5.0 x 10^-30cm. A proton is 2.50nm from the molecule in the plane that bisects the dipole.

    What is the electric force of the molecule on the proton?

    2. Relevant equations

    F = QE = eE_yj = (-kpe)/(d^3)j

    3. The attempt at a solution

    ((9*10^9 N*m^2/C^2)(5.0*10^-30 C*m)(1.6*10^-19 C))/(2.5*10^-9)^3

    This gives me an answer of 5.0*10^-13 N, but that seems to be wrong. Anyone know where I went wrong here? Thank you!
  2. jcsd
  3. Oct 20, 2012 #2
    1. You need to use more digits. My answer is about 10% off yours.
    2. Why does this seem wrong? Is it because it's very small? Try converting this to atomic units i.e. eV/nm (electron charge*1V/nm) or calculate the resulting acceleration assuming no other forces (in m/s^2 or nm/ns^2).

    The thing about atomic scale systems is that using human scale units is not appropriate. N,m,s,kg just don't make sense as good units, and it makes large quantities seem small and small quantities large.
  4. Oct 21, 2012 #3
    Ah, okay I see what you mean. I finally got it, thanks!
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