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Homework Help: Force of atoms in different molecules?

  1. Jul 17, 2006 #1

    In DNA, thymine and adenine molecules are attracted to each other. The solid lines show bonds between atoms in the same molecule and the dotted line shows bonds between atoms in different molecules. Atoms in the same molecule are seperated by 1e-10 m. If the net charge on the H and N atoms is 0.2e and the net charge on the O atom is 0.4e, what is the force between the H and O atoms in different molecules?

    my workings:
    charge for H atom = 0.2 X (1.6e-19)
    = 3.2e-20 C

    charge for O atom = 0.4 X (1.6e-19)
    = 6.4e-20 C

    k = 9e9 Nm^2/C^2

    I need to work out the distance between the O and H before i can work out the force. Can someone point me in the right direction?

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    Last edited: Jul 17, 2006
  2. jcsd
  3. Jul 17, 2006 #2
    use the law of Coulomb:

    F_{el} = f \frac {Q_1Q_2} {r^2}

    everything is given.
    Last edited: Jul 17, 2006
  4. Jul 17, 2006 #3
    i thought so too but the correct ans according to the book is 5.7e-10 N but my ans i got is 1.8e-9
  5. Jul 17, 2006 #4
    i think you need the figure to get to know the distance between H and O in the AT-bridge in the DNA (thus not in 1 single molecule)

    In you first message i hope you mean the N atoms (not the B atoms). Maybe you should do something with that ;)

    The answer for the [tex]F_{el}[/tex] between O and H in the same molecule is correct though, but it's not what is asked here.
    Last edited: Jul 17, 2006
  6. Jul 17, 2006 #5

    basically its asking for the force of attraction between H and O atoms which are not on the same molecule.
  7. Jul 17, 2006 #6


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    I think you're going to have to wait till the attachment gets approved before you get any more help.
  8. Jul 17, 2006 #7


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    I'm not really sure about distances in a DNA molecule but like others have said i'd just use the 1x10-10 intermolecular distance that was given above. It will provide a good enough approximation.
  9. Jul 17, 2006 #8
    i did, but the answer isnt correct. the book says the ans should be 5.7e-10 N. my answer of 1.8e-9 doesnt even come close to that.
  10. Jul 17, 2006 #9


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    Hmm thats strange that they did not include basic information as to how far apart they are. Well if you work backwards the separation should be 1.8x10-10m. Are you sure there was no other information given with the question?
  11. Jul 17, 2006 #10
    nope...only the ans is given

    5.7e-10 N
  12. Jul 17, 2006 #11
    that was my point too ... :confused:
  13. Jul 18, 2006 #12


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    Giancoli gives the distance between the O in thymine and the N in adenine atoms as 2.80 angstrom. This means the OH distance is 1.80 angstrom , which gives the required answer.
    Last edited: Jul 18, 2006
  14. Jul 18, 2006 #13
    oh yea...damn i forgot all about it. i thought i'm suppose to find the distance with what is given :redface: thanks :smile:
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