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Distance between 2 charged protons (i'm desperate)

  1. Sep 2, 2009 #1
    I'm sure this is so easy and just 1 step. I can do questions like this, I just don't understand, this theory one, or whatever it is.

    1. The problem statement, all variables and given/known data
    How far apart are two protons if they repel each other with a force of 1.0 mN?

    2. Relevant equations
    F=k * Q1Q2/r^2


    3. The attempt at a solution
    F=k * Q1Q2/r^2
    .001N = 9*10^9 * 1.6*10^-19 * 1.6*10^-19/ r^2
    .001 = 2.304 *10^28/r^2
    r^2= 2.304*10^28/.001

    r= 4.8 * 10^-13 m.

    ***the answer key says: 1.5 x 10-11 m.

    please if you could help me quick it would be amazing.
    Last edited: Sep 3, 2009
  2. jcsd
  3. Sep 3, 2009 #2


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    Homework Helper

    Your calculation is correct.
  4. Sep 3, 2009 #3
    sorry i think i was confusing. i got 4.8 * 10^-13 m. the answer key says 1.5*10^-11. are you telling me the answer key is wrong?
  5. Sep 3, 2009 #4
    I think your approach and solution looks correct to me; I got the same answer. You correctly applied coulomb's Law for this case since the two protons exert an electrocstatic force on each other. Perhaps the book answer could be wrong;
  6. Sep 3, 2009 #5
    i'm doubting the answer key is wrong. there was another question that was similar in the studies (this is the review), that i also couldn't get.
  7. Sep 3, 2009 #6
    If mN = milli-Newtons(10^-3) then you are correct in your answer of 4.8*10^-13 m.

    If mN = micro-Newtons (10^-6) then you get 1.5*10^-11 m
  8. Sep 3, 2009 #7
    aahhhh.. thanks, i wasn't sure about mN and i googled it and that's what it said.
  9. Sep 3, 2009 #8
    That's because m is the standard prefix for milli- (10^-3). The micro- (10^-6) prefix is usually represented with a [tex]\mu[/tex].
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