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Classical thermodynamics Q's.

  1. Jan 8, 2010 #1
    Classical thermodynamics Q's.
    Hello !
    I have some conceptual problems that are confusing me, hope someone could help.
    Book Being used: Fundamentals of Engineering Thermodynamics. (Micheal J. Moran & Howard N. Shapiro).

    1) Analytical Kelvin-Planck statement. (pg. 224)
    Wcycle <= 0. ( Single Reservoir).

    A) How come we have any cyclic work being done while only ONE single reservoir feeding the system? In fact, Wcycle doesn't make any sense in this scenario.

    B) If we include the cold OR hot reservoir in the analysis, i.e. as a part of the system, wouldn't Kelvin-Planck be violated? ( or as in this case, since the cold reservoir is included, the cycle is interacting with the external hot reservoir and the included cold reservoir.)

    C) Let’s assume that we have a reversible cycle. Furthermore, the cycle is being fed some heat Q and delivering some work W.
    Now, Kelvin-Planck statement is satisfied by having hot/cold reservoirs. Now, to investigate the cycle, the first law will be used. dE= DQ-DW , dE= 0 ( cycle). DQ=DW , integrating, Q=W. What I don’t understand is that why is the Wrev = 0 in this case ?

    D) Kelvin-Planck statement is sort of a common sense, for to complete a cycle, a cycle is needed to be restored to its initial state. However, how is this same for Carnot efficiency? To be more exact, according to Carnot findings, the only things affecting a reversible cycle are the Tcold res. and Thot res., however, I can NOT seem to see this intuitively. Is there an intuitive approach to understand this?

    2) In finding the coefficient of performance of any refrigerator, we consider Q leaving the cold reservoir and Win. However, to find any efficiency we just require Eout / Ein. Why in refrigerators case we consider different scenarios? Is it just that we avoid the confusion resulting in having efficiencies higher than 1 ?

    3) COPH.P. = COPref +1
    Is there any physical interpretation for (+1) ? or it’s just the work of derivations ?

    4) Clausius statement (pp. 241-242).
    Cyclic integral of (DQ/T) = -Sgen
    Shouldn’t this integral cover ,only, entropy generation due to heat and NOT due to irreversibilities.(pg. 242)

    5) Is there any nice article about the violation of Kelvin-Planck and how that violation would lead to Clausius statement violation ? I couldn't really grasp the whole picture.

    Thanks !
  2. jcsd
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