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Math Help - Thermodynamics problem dealing with series

  1. #1
    corr0105
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    Thermodynamics problem dealing with series

    This is for my biothermodynamics class:
    MOLECULAR WEIGHT DISTRIBUTION IN POLYMERS:
    NOTE: nk=n(subscript k) wk=w(subscript k). NOT multiplication.
    While the fraction of polymer chains having length k is P(k) = n/∑_(k=1)^∞nk), the fraction of chains having molecular weight proportional to k is w=k*n/(∑_(k=1)^∞k*nk).
    Q: a. Show that w = k(1-p)*n
    b. Compute the average molecular weight, <k> = ∑_(k=1)^∞k*wk


    A: a. Well... I can't figure out the answer yet despite the amount of time I have spent trying to figure this out. It is given in the book that nk=(1-p)p^(k-1). I have attempted to plug this into the expression for wk and manipulate the sum so as to solve for a closed form that would hopefully take a form that would simplify to the expression required to be proven. My attempts have failed thus far and I would appreciate any help I can get. I could easily simplify to a closed form if nk were a steady constant, however the subscrip k tells me that although it is just a number, it will change with every value of k.

    b. I know that the average can be computed like so: (∑_(k=1)^∞k*P(k)), but I might need a little help simplifying this expression to a closed expression as opposed to a summation.

    Any help is greatly appreciate, thanks so much!!




    Just for clerification, here are all the term definitions:
    p: the probability that a monomer unit is reacted and connected in the chain.
    nkThe most probable distribution. It is the statistical weight (unnormalized probability) for having a chain k monomers long. The factor of (1-p) arises in the definition for n because the terminal end unit must be unreacted for the molecule to be exactly k units long.
    P(k): fraction of polymer chains having length k.
    wk: the fraction of chains having molecular weight proportional to k.
    Last edited by corr0105; February 1st 2008 at 12:53 PM.
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