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Universal gas, Boltzmann's constant

  1. Jul 8, 2005 #1
    I understand the formulas involved and numerical values of these constants and in the respective units . However I'm still having troubling understanding exactly what it is these two constants define. Does Universal gas constant say that For every mol times Kelvin there is 8.315J ? I don't think that makes sense since an increase in kelvin should actually decrease the amount joules, since K is in the denominator, which I believe then defies a portion of kinetic theory. Then Boltzmann's constant is it saying that every atom will have 1.38x amount of joules per kelvin?
  2. jcsd
  3. Jul 8, 2005 #2


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    R is defined by this relationship going back to the definition of enthalpy:

    [tex]h = u + \frac{p}{\rho}[/tex] and the form of ideal gas equation of state [tex]p =\rho RT[/tex] You get:

    [tex]h = u + RT[/tex]

    If you differentiate with respect to temperature:

    [tex]dh = du + R dT[/tex]

    [tex]\frac{dh}{dT} = \frac{du}{dT} + R[/tex]

    From relations for ideal gases...

    [tex]C_v = \frac{du}{dT}[/tex] and [tex]C_p = \frac{dh}{dT}[/tex]

    we can say that [tex]C_p = C_v + R[/tex] or

    [tex]C_p - C_v = R[/tex]

    This is important because that says that for all ideal gasses, the difference between [tex]C_p[/tex] and [tex]C_v[/tex] is a constant.
  4. Jul 9, 2005 #3
    oh, I don't think came across any formula concerning enthalpy yet but it makes sense in equation form.
  5. Jul 9, 2005 #4


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    Enthalpy is a very useful term. If you get into thermodynamics you will become very familiar with it.
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