1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Specific heat for a triatomic gas

  1. Aug 8, 2016 #1


    User Avatar
    Gold Member

    1. The problem statement, all variables and given/known data
    Using equipartition law, find specific heat of gas containing triatomic linear molecules. Will the result be different if the molecule was non- linear? In what way?

    2. Relevant equations
    According to equipartion theorem, each degree of freedom gets (1/2)kT kinetic energy and (1/2)kT potential energy.

    3. The attempt at a solution
    For a linear arrangement,

    • number of translational degrees of freedom is ##3##
    • number of rotational degrees of freedom is ##2## (or is it 6 because the atom can rotate about the central atom or about one of the atoms at the end)
    • number of vibrational degrees of freedom is ##1##
    At room T, i will neglect point 3.

    My second doubt is, the internal energy is ##\frac{1}{2}NkT\times f##

    Why is it only 1/2 the value of kT (which is the sum of kinetic and potential energies as my prof says in one of the slides which i have attached below)?

    In the slide, $$U=\frac{h\nu}{e^{\frac{h\nu}{kT}}-1}$$

    Why isn't it $$U=\frac{h\nu}{e^{\frac{h\nu}{\frac{1}{2}\times kT}}-1}?$$

    Attached Files:

  2. jcsd
  3. Aug 8, 2016 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Total number of degrees of freedom is 3n.
  4. Aug 18, 2016 #3


    User Avatar
    Gold Member

    How did you get that expression?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted