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Thermal Physics Problem

  1. Feb 20, 2012 #1
    1. Determine the volume occupied by a 1 mole of an ideal gas at 27° C and 1 atm pressure. Ammonia gas at 27° C and 1 atm is passed at a rate of 41 cm3 s-1 into an apparatus where it flows over an electrically heated wire of resistance 100 . When the heating current is 50 mA the gas leaves the apparatus at 31.09° C. Calculate Cp and CV for ammonia assuming it behaves as an ideal gas.


    3. I know it a pretty simple problem but it has been a while since i practiced any thermal physics. I have determined the initial volume occupied by the ideal gas using:

    pV=nRT... and got 0.025m^2

    However i am clueless as to how to approach the rest of the problem.

    any help would be greatly appreciated.
     
  2. jcsd
  3. Feb 20, 2012 #2

    Redbelly98

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    Welcome to Physics Forums.

    Since you're told the resistance and current of the heater, you can figure out how many Joules are given off by the wire every second. You can also figure out how much gas (at 27 C and 1 atm) passes over the wire every second. Then you'll know that a certain amount of energy heats a certain amount of gas ... hopefully that gets you going on working through the problem.
     
  4. Feb 25, 2012 #3
    Okay so i found that you get 0.25J/s of energy and that in one second, 4.1x105m3s-1

    not sure where to go from here. What process is this?

    do i need to find γ to find the heat capacities?
     
  5. Feb 25, 2012 #4

    Redbelly98

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    Looks good -- assuming you really meant 4.1x10-5m3 in the volume expression :wink:

    So you have an amount of energy added by heating, an amount of gas, and an amount of temperature change ... the required information to find heat capacity.

    If I read the problem statement carefully:
    ... it sounds like a constant pressure process.

    No, not for this problem. After you figure out either Cp or Cv, there is a relation for ideal gases between Cp and Cv that you can use to get the other one.
     
  6. Feb 25, 2012 #5
    Ha yeh thats what I meant lol

    So i have ΔU from the energy per second, the temperature change,

    When you talk about the amount of gas, do i need to use the 4.1x10-5m3s-1 and the gas that is actually occupied by one mole to find the number of moles of gas that passes in 1 second.

    then use ΔU=nCvΔT to get the heat capacity?
     
  7. Feb 25, 2012 #6

    Redbelly98

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    Yes, you would use the number of moles of gas in the 4.1x10-5m3 volume.
    Is this a constant pressure or constant volume process?
     
  8. Feb 25, 2012 #7
    From what you were saying it think its a constant pressure process, so it would be ΔH=nCpΔT

    What is the ideal gas equation to find the other heat capacity?
     
  9. Feb 27, 2012 #8

    Redbelly98

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    That is my impression from the wording of the problem statement, which states a pressure value for the flowing gas.
    Yes.
    Cp and Cv differ by a constant -- it should be in your textbook.
     
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