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Relating internal energy to highest temp

  1. Jul 16, 2007 #1
    1. The problem statement, all variables and given/known data
    one mole of He, H2, CO2 goes through a process such that the change in their internal energies are the same. WHich of the gases has a higher temp change?


    2. Relevant equations
    Eint=3/2nRdeltaT=nCvdeltaT (for monatomic gases)
    Eint=5/2nRT (diatomic gases)
    polytamoic gases have higher values for eint,Cv,CP



    3. The attempt at a solution

    is it helium because the change of temp has to be quite large in order to have equal internal energies with the others due to its 3/2 ratio?
     
  2. jcsd
  3. Jul 16, 2007 #2

    Dick

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    You general thinking is correct. In general it takes a larger temperature change to excite helium to a higher net energy. Why? Think temperature corresponds to energy per degree of freedom. Why would helium be different in terms of degrees of freedom?
     
  4. Jul 16, 2007 #3
    it has less degress of freedom because its a monatomic gas and they only have 3....
     
  5. Jul 16, 2007 #4

    Dick

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    I guess the interesting thing would be why diatomic has 5.
     
  6. Jul 16, 2007 #5
    so its basically due to the degrees of freedom, im sorry im not sure if theres anything else im not understanding
     
  7. Jul 16, 2007 #6
    i just got one more question i dont quite fully understand, the actual problem in the text is: suppose you pour hot coffee for your guests. TO have the coffee that will stay warm for the longest period (i.e. the coffee which takes the longest time to reach room temp.) one should:
    a) add cream just after the coffee is poured
    b) add cream jsut before drinking but a few minutes after the coffee is poured
    c) can add the cream at anytime

    This question is for section 17.9through18.1 in my text but all that seems to cover is molarheatcapacity, second law of thermodynamics, conduction and convection, and some quant

    How do i approach this problem?
     
  8. Jul 16, 2007 #7

    Dick

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    You could have said that helium is spherically symmetric so has no rotational degrees of freedom. This is pure QM. On your second question, you've hit a legal snag of the forum. You have to give us your thoughts first. I'll give you a hint. Suppose it's a pure conduction problem. How is the difference from equilibrium related to the rate of cooling?
     
  9. Jul 17, 2007 #8
    um, im not quite certain but from what i understand conduction only occures if the temps differs in the medium and also vary depending on wether or not whatever is being looked at is a good or bad thermal conductor, im not sure how to apply this into the problem, the coffee is obviously extremely hot at the beginning and cream is usually poured in cold, or even if it isnt it will be significantly colder still compared to the coffee.
     
  10. Jul 17, 2007 #9

    Dick

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    Mixing the fluids does not change heat content relative to equilibrium. How does temperature difference from equilibrium affect the rate of heat transfer? And the coffee could be extremely cold and the cream extremely hot and it wouldn't affect the answer. This is a think problem.
     
  11. Jul 17, 2007 #10
    aright, so the hotter coffee is initially the heat transfer would then take/last longer, i hope i didnt just reverse that. So youd want to add it at its hottest point just after its poured according to this. Does that seem correct?
     
    Last edited: Jul 17, 2007
  12. Jul 17, 2007 #11

    Dick

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    I think you did reverse the first statement. But I think you get the gist of it. The answer could be stated much more clearly.
     
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