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Joule Thomson Effect

  1. Jun 12, 2012 #1
    Is the equation
    JT = (1/Cp)(2a/RT - b)
    valid for adiabatic free expansion of real gases only? How was this equation derived?
  2. jcsd
  3. Jun 12, 2012 #2

    Andy Resnick

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    The expression you provide appears to be dependent on a specific equation of state (you use Van der Waals, if I may hazard a guess),but the Joule-Thompson experiment is much more general that that.

    The Joule-Thompson coefficient is defined as ∂T/∂P at constant enthalpy,and represents a process that is neither isothermal, adiabatic, cyclic, or reversible (throttling of gases). The general expression is

    [tex] (\frac{\partial T}{\partial P})_{H} = \frac{-V}{C_{P}}(1-T\alpha_{T})[/tex]

    Historically, the Joule-Thompson experiment led to the development of an absolute temperature scale.
  4. Jun 19, 2012 #3
    Look also here.

    Could you explain that a little more? Because I thought the absolute temperature scale was a consequence of Carnot theorems.
  5. Jun 19, 2012 #4

    Andy Resnick

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    They are related- the experiment was carried out to measure "Carnot's function", which is essentially the efficiency of a Carnot cycle engine (Carnot-Clapeyron theorem).

    The details are presented in Truesdell's "The Tragicomical History of Thermodynamics (1822-1854)", specifically section 9D. Briefly, the experiments measured the bath temperature, start and end pressures, and "cooling constant" (Joule-Thomson coefficient), and those were used to fit coefficients in Rankine's equation of state for air p = f(V,T), which would allow the use of air for a 'perfect gas thermometer'.

    As a consequence, it became possible to define a temperature scale that is independent of the choice of body used as a thermometer, just as the efficiency of a heat engine is independent of the choice of working fluids.
  6. Jun 19, 2012 #5
    Thank you very much for your explanation.
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