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Potential energy between two oppositely charged ions in a crystal

  1. Aug 12, 2009 #1
    Hi, I am having trouble with the following:

    attachment.php?attachmentid=20081&stc=1&d=1250100045.jpg

    All I have really done is differentiate the function to give an expression for F(r), but I am a bit clueless about the rest. How can there be equilibrium if the two ions are attracted to each other?

    Cheers,
    GeoffreyThelm
     

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  2. jcsd
  3. Aug 12, 2009 #2

    kuruman

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    Your potential has two terms, are they both attractive?
     
  4. Aug 12, 2009 #3
    I see.. So one (the first term?) is due to the attractive force from one ion, and vice versa for the other? So c is gonna end up being something which looks like electric potential should. So is the point of equilibrium the point between the ions where the first term= the second term?
     
  5. Aug 12, 2009 #4

    kuruman

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    The point of equilibrium is where the sum of the forces is zero. When you have a potential, it is where dU/dx = 0, i.e. where the potential has an extremum (maximum or minimum). In this case it had better be a minimum. Consider plotting this potential to see what it looks like.
     
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