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Homework Help: Electric field and gravitational field question

  1. Aug 11, 2008 #1
    1. The problem statement, all variables and given/known data

    making the worst possible combination of errors, an oxygen atom has a charge of 3.8x10^-39C. Treating the atoms as point particles, compare eletric force of two such oxygen atoms with gravitational force between atoms. is the net force repulsive or attractive?

    2. Relevant equations

    F = kqq/r^2

    F = GmM/r^2

    1 amu = 1.66x10^-27 kg

    3. The attempt at a solution

    oxygen atom = 32 amu

    32 amu * (1.66x10^-27 kg) = 5.31x10^-26 kg

    F(electric) = [ (9x10^9) * (3.8x10^-39)^2 ] / r^2

    F(gravity) = [ (6.67x10^-11) * (5.31x10^-26 kg)^2 ] / r^2

    F(gravity)/F(eletric) = 1.4x10^6

    attractive because F(gravity) > F(electric)

    but according to the answer key the answer should be 2.8x10^6
    what am I doing wrong?
  2. jcsd
  3. Aug 11, 2008 #2


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    Homework Helper

    I don't see anything wrong. Furthermore the question doesn't even ask for any numerical answer, only which force is greater.
  4. Aug 12, 2008 #3


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    Science Advisor
    Homework Helper

    I think you mean oxygen molecule (32 amu), not atom (16 amu). But even so, I don't find anything terribly wrong with your ratio.
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