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Heat and Work interactions

  1. Sep 11, 2006 #1


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    The question verbatim is:

    "A portable device for pressurising tyres consists of a small container of 1.5 cft volume, initially filled with, say, CH4 at 500 psia. When a tyre (volume=0.5 cft) is to be filled, the gas is allowed to flow into it from the container through a valve till the pressure in the tyre reaches 50 psia. The process is essentially adiabatic and the initial pressure in the tyre may be taken as 15 psia. Let the subscripts 1 & 2 and i & f refer to the initial and final states of the gas in the container and tyre respectively. Assume that P1=500 psia, Pi=15, T1=Ti=80 F

    a) Assume that the filling process is reversible. Use the chart to read v,s and h for the gas (i)initially in the container (ii) initially in the tyre and (iii) finally in the tyre."
    b) Calculate m1, mi and mf
    c) What's the value of s2? Read t2 and h2 from the chart
    d) Use the first law for container + tyre to find m2(h2-p2v2)
    e) Solve for P2 (:confused:) by trial and error so as to satisfy the first law
    f) Calculate the change in entropy of
    (i)The closed system (container + tyre)
    (ii)The environment

    First of all, I think there's some ambiguity in the statement of the question on the notations. Leaving that aside, I have a doubt regarding the final state of the air in the tyre. The thing is that T_f for the tyre isn't known. So how can I calculate T_f (so that I can then calculate h,v,s from the chart)?
    Now the process is adiabatic and reversible, so the entropy change for the whole system (ie, container + tyre, neglect valve) is 0. But, that doesn't help me find T_f because the entropy change in the tyre alone wouldn't be 0. I think there's some information which needs to be given, or some assumption to make to calculate the final state. Once I do that, the rest of the problem seems easy.
    Last edited: Sep 11, 2006
  2. jcsd
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