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Andrew mason ideal gas law and adiabatic expansion problem

  1. Feb 9, 2009 #1

    ADIABATIC FREE EXPANSION(unrestricted and free)
    in vaccum gas a volume of gas is released
    expected expansion proceeds withouut change in internal energy therefore temperature is contant (joule's law) . there is no pressure acting on this sytem threfore no work is done.
    now out of 3 thermodynamic variables 2 (p & t) are constant with only v changing
    what causes this change ? ie what factor accounts for this change of volume

    Ive read about joule thompson effect , where releasing gases thru a narrow orifice into vaccum causes rapid fall in temperature leading to the condensation of the gas.(used in liquefaction of air)
    Isnt this case similar to adiabatic expansion of gases in vaccum? If so there ought not be a temperature change.
    So what causes the condensation of gases?

    Ideal gas laws
    Most books define conditions for ideal behaviour as
    --> no intermolecular interactions
    --> negligibility of molecular dimensions(ie diameter of moleculke<<< inter molecular distances)

    So we would expect real gases to approximate to ideal gases at high temperature and low pressures

    However, I came across a book where it says another condition in the list
    --> the kinetic energy of particles >>> inter molecular interactions
    And example cited is dense gases with high temperatures
    ie real gases approximate to ideal behaviour at high temperatures and high pressures

    So which approach is correct? there is a lot of misconception on this subject at the place where i live and this is a question frequently asked in exams please help.
  2. jcsd
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