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!

Internal energy + entropy for molecule

  1. Dec 9, 2015 #1
    Internal energy can be defined theoretically for one molecule (U = 1/2 Kb T) for example but entropy is defined for a system thus for many molecules. Then we define temperature equal to δU / δS but here U can be defined for one molecule, so S can also be defined for one molecule? How?

    Thank you
  2. jcsd
  3. Dec 9, 2015 #2
    Do you really feel that the equation you presented for U describes the kinetic energy of each and every molecule of an ideal gas, or is it just the average kinetic energy over all the molecules, consistent with the Boltzman distribution?
  4. Dec 9, 2015 #3
    No like I said U can theoretically describe both. For example if U can describe one molecule thus the temperature can describe one molecule. So S can describe one molecule? Then if S cannot describe one molecule, a temperature cannot describe one molecule, so there is a contradiction in the U defintion for one molecule
    Last edited: Dec 10, 2015
  5. Dec 10, 2015 #4
    The average number of children per family in the US was 2.7 in 1961.
    Can you say that any specific family had 2.7 children?
  6. Dec 11, 2015 #4


    User Avatar
    Science Advisor

    In statistical mechanics, there is nothing wrong with defining entropy for a single molecule.
  7. Dec 11, 2015 #5
    Great. So how the temperature of one molecule in statistical mechanics is related to macroscopic temperature (T= dU/dS) of a volume containing this molecule and many others?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook