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Homework Help: Question re-post please help!

  1. Apr 9, 2005 #1
    The question shows a pic of a brayton cycle on a pressure temperature graph. The gas is helium, and there are two moles. We are given two temperatures and asked to find the other two. I did this just fine. But, then we are asked to find the heat flows per kilogram of helium in each cycle and the net work that is done per kilogram of helium in each cycle. Now, there are two adiabatic processes in the diagram, so for the heat flows, we would only have to worry about the isobaric processes. So, for an isobaricprocess,

    Q= n*Cp*change in temperature...instead of two moles, would I use 250moles/kg for n to find the heat flow per kilogram, and would i keep the temp change the same from the original temps that i found using the original two moles?

    Then, for the work, the adiabatic processes equal

    W= n*Cv*change in temp...so do I do the same thing with n again, and keep all the other info the same as what i originally found it to be?

    But, for the isobaric process,
    W=P*change in V

    so i am not quite sure how to incorporate the 250 moles/kg.....I could use PV=nRT and find the P in Pa/kg by using 250 moles/kg for n instead of the 2 moles, but then I would be using the original volumes, and wouldn't the voluems change if I changes the pressure? I am very confused...any help would be great!!!....and also, will the change in Q and the net work for the cycle be equal to each other??
  2. jcsd
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