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Specific heat and petit's law

  1. Mar 18, 2012 #1
    Hi there,
    I have a quick question. According to duLong and petit's law, the heat capacity at constant volume for most solids at high temperatures is 3R. (I.e) Cv=3R.
    Where R is the gas constant. Does this mean that the heat capacity is the same for all materials?
    I thought each material at a unique heat capacity. Can somebody explain?
    Thank You
     
  2. jcsd
  3. Mar 18, 2012 #2

    Jano L.

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    Gold Member

    Hello thentangler,
    For solids in high enough temperatures above the Debye temperature, the heat capacaity is 3R per mole.

    Check the table of temperatures here

    http://en.wikipedia.org/wiki/Debye_model#Debye_versus_Einstein

    For long organic molecules, the heat capacity can be much larger than 3R per mole, because they have many active degrees of freedom.
     
  4. Mar 18, 2012 #3

    Borek

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    Staff: Mentor

    Note: it is specific heat capacity per mole that is (almost) identical.
     
  5. Mar 18, 2012 #4
    So suppose I encounter a material that is a classical solid, would it be safe to substitute 3R for its specific heat when im calculating its Q per mole?
     
  6. Mar 19, 2012 #5

    Borek

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    Staff: Mentor

    Define "safe". Dulong–Petit law is an approximation. Sometimes it works, sometimes it doesn't. If you can't find specific heat capacity and you use 3R you should always spell it out, as it is possible you are way off.
     
  7. Mar 19, 2012 #6
    Aahh, thank you Borek. The limitations of classical physics I suppose. :)
     
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