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What is the attractive electric force between the ions?

  1. Feb 5, 2008 #1
    [SOLVED] Electric Force Question

    The distance between neighboring singly charged sodium and chloride ions in crystals of table salt (NaCl) is 2.82 x 10^-10 m. What is the attractive electric force between the ions?





    By coulomb’s law,
    Electric force F = [1/4πεo] q1 q2/ r^2
    = 9.0 x 10^9 (1.6 x 10^-19) / (2.82 x 10^-12)^2
    = 2.9 x 10^9 N (force of attraction)


    Correct????
     
  2. jcsd
  3. Feb 5, 2008 #2

    Kurdt

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    One is positively charged and the other is negatively charged. Is the distance 2.82x10-10m or 2.82 x10-12m?
     
  4. Feb 5, 2008 #3
    The distance is supposed to be 2.82 x 10^-10. So........ the answer should be 1.26 x 10^10 N??? Did I work this question correctly or is it all wrong?
     
  5. Feb 5, 2008 #4

    Kurdt

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    Your first answer was in the right area, but I got it to be -2.9x10-9N.
     
  6. Feb 5, 2008 #5
    Could you show me how you got that answer? Because I'm still coming up with the same answer.
     
  7. Feb 5, 2008 #6

    Kurdt

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    I think you've just forgot to square the charge. Remember its [itex]q_1q_2=(1.6x10^{-19})^2[/itex].
     
  8. Feb 5, 2008 #7
    I'm just not getting this for some reason and it seems so simple...now i'm getting 2.89 x 10^-1. Where am I making a mistake? Could you please help me work through this problem?
     
  9. Feb 5, 2008 #8
    Why do you have that as a negative?
     
  10. Feb 6, 2008 #9

    Kurdt

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    An attractive force is negative. One of the ions will be negatively charged and the other will be positively charged giving a negative answer.

    (9x109*1.6x10-19*1.6x10-19)/(2.82x10-10)2
     
  11. Feb 6, 2008 #10
    Ok...I reworked it and got 2.9 x 10^-11 (so do I just add a negative since an attractive force is negative?) Is this right?

    Edit: 2.9 x 10^-9...or -2.9 x 10^-9
     
    Last edited: Feb 6, 2008
  12. Feb 6, 2008 #11

    Kurdt

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    Yes. That would be correct. the negative comes from the fact that one of the charges has to be negative. Rather than include it in the calculations i just remember that the answer will be negative once I've plugged all the numbers in.
     
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