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Find the magnitude of the attractive force

  1. Sep 16, 2007 #1
    [SOLVED] Coulomb's Law

    1. The problem statement, all variables and given/known data

    Find the magnitude of the attractive force exerted by the electrons in a cup of water on the protons in the second cup of water at a distance of 10 m. Assume that each cup contains 250g of water and that 6x10^23 water molecules (one mole) weighs 18 g. Do two cups of water exert any net force on each other? Why?

    2. Relevant equations

    F = k*(qq/r^2)

    3. The attempt at a solution

    250g H2O / 18g H2O * 6.022^23 = 8.36^24 molecules of water

    I don't understand how to proceed after that. Do I need to find the number of electrons/protons in all H and O atoms separately and add them? Then multiply by 1.602^-19C (electron/proton charge) and plug those numbers into the formula?
  2. jcsd
  3. Sep 16, 2007 #2
    You can approximate the all the electrons in one cup as one big charged point. Ditto for the protons.
  4. Sep 16, 2007 #3


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    Homework Helper

    You will need to find how many electrons or protons are in that many water molecules (you know they'll be the same total) and then, as genneth says, treat the total electron charge in one cup as a "lump" and the total proton charge as a "lump" in the other cup.
  5. Sep 16, 2007 #4
    Right, that's where I'm stuck.
  6. Sep 16, 2007 #5
    How many electrons are there in a water molecule? How many protons?
  7. Sep 16, 2007 #6
  8. Sep 16, 2007 #7


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    Roger that! So what is the total proton or electron charge in each cup?
  9. Sep 16, 2007 #8
    Sounds about right. And you already know the number of molecules in the beakers... so -- what's the number of electrons/protons per beaker? Can you finish the question now?
  10. Sep 16, 2007 #9
    8.36^24 * 10 * 1.602^-19?
  11. Sep 16, 2007 #10
    Go on... no need for us to be holding your hand *that* tightly...
  12. Sep 16, 2007 #11


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    And the stadium crowd goes wild! Now you're ready to apply Coulomb's Law.
  13. Sep 16, 2007 #12
    Thanks guys.
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