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I don't understand an approximation in an expression in stat. mech.

  1. Oct 5, 2014 #1

    fluidistic

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    1. The problem statement, all variables and given/known data
    Hello guys, I fail to understand a mathematical approximation I see in a solved exercise.
    The guy reached a partition function of ##Z=\sum_{l=0}^\infty (2l+1) \exp \left [ -l(l+1) \frac{\omicron}{T} \right ]## and he wants to analyze the case ##T>> \omicron##.
    He states that with the change of variables ##x=l(l+1)\frac{\omicron}{T}##, ##Z\approx \frac{T}{\omicron} \int _0^\infty e^{-x}dx##.
    I really don't understand this last step.
    2. Relevant equations
    Sum becomes integral. The change of variables.

    3. The attempt at a solution
    Making the change of variables, I understand that the sum transforms into an integral and I also understand why the limits of the integral are 0 and infinity (because l goes from 0 to infinity and thus x too).
    I am unable to perform the change of variables and get rid of l's outside the exponential.
    I'd appreciate if someone shed some light. Thanks.
     
  2. jcsd
  3. Oct 5, 2014 #2

    vela

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    I think the l's get sucked into what becomes dx since the derivative of l(l+1) is 2l+1.
     
  4. Oct 5, 2014 #3

    RUber

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    Notice that once you insert the substitution, ## Z=\sum_{l=0}^\infty (2l+1) \exp \left [ -l(l+1) \frac{\omicron}{T} \right ]##, ## =\sum_{l=0}^\infty (2l+1) \exp [-x] ##.
    Then see that ## dx=(2l+1)\frac{\omicron}{T} ##.
    So, ## 2l+1=dx\frac{T}{\omicron} ##.
    Since ##T>>o ## the integral is a good approximation, since it is the limit as ## l(l+1)o/T \to 0 ##.
    Putting this all together, ## Z = \int_{l=0}^\infty \frac{T}{\omicron} \exp [-x] dx ##.
     
  5. Oct 6, 2014 #4

    fluidistic

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    Thank you guys. I missed the dx part indeed.
     
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