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Homework Help: Bulk melting point

  1. Dec 24, 2011 #1
    1. The problem statement, all variables and given/known data
    Hi all, I have a question which is regarding melting point. I know that lindemann's criterion doesn't actually describe the phenomena of melting but is an approximation of what maybe occurring/ an intuitive approach.

    So the question is in a tabulated form for lead and tungsten ( assuming crystals) with Youngs modulus, Molar volume and melting temperature given and they want me to describe the terms / details quantitatively.

    I have assumed that they want me to essentially find out the root mean square amplitude of the vibrations which will eventually give me a separation distance (not equlibirum) that needs to be at least 10% of a fraction to the inter atomic distance.

    2. Relevant equations

    Root mean square of amplitude of atomic vibrations is given by the following equation:

    [itex]\sqrt{<u^2>}[/itex] = [itex]\sqrt{KbT/D}[/itex] where D = spring constant.

    D = Y[itex]r_{0}[/itex]

    3. The attempt at a solution

    Data given for lead is:

    [itex]T_{m}[/itex] = 3650 K , Molar volume : 0.0953m^3 , Young's modulus: 36*10^10 N/m^2

    My approach involved finding out the vibrational energy by kbT. Then equating that to 1/2kx^2 ( here 'x' = r-r0) but first I need to find out spring constant k and separation at equlibirum.

    To get 'k' I have used the following equation:
    k= Y[itex]r_{0}[/itex]
    and for [itex]r_{0}[/itex] , Molar volume = [itex]N_{a}[/itex]b* ([itex]r_{0}[/itex])^3

    Is the above approach right ? is this how I am mean't to do it ?

    Thanks for your replies and merry christmas !

    Edit: Also one more thing which confuses me slightly .. I know that lindemann's constant is a value which's usually same i.e doesn't vary but I have seen different variation to get it... is the rms of the amplitude of the vibration = C * interatomic distance ? can someone clarify this for me as well. Thanks!

    EDIT2: If you have a look on the following links.. you will notice that there is a difference of powers for the mean square amplitude value <u^2> = ..
    Last edited: Dec 24, 2011
  2. jcsd
  3. Dec 28, 2011 #2
    Can someone clairfy to me when exactly does melting occur. Is it when the root mean square of the amplitude gets to about 10% of the inter atomic distance or is it just the mean square of the amplitude ?

    Also to find the inter atomic distance.. is that same as 'r0' separation of atoms at equilibrium ?

    <u^2> = Cl^2 * a^2 is this the right expression ? Google hasn't helped much on this but has shown some conflicting form.
    Thank you..
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