Why Does My Gaussian Integral of Maxwell-Boltzmann Not Equal n?

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Discussion Overview

The discussion revolves around the Gaussian integral of the Maxwell-Boltzmann distribution as presented in a specific context of the lattice Boltzmann method. Participants are examining the claim that the integral of the equilibrium distribution equals the number density, n, and are exploring discrepancies in their calculations.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant references a specific form of the Maxwell-Boltzmann distribution and questions why their integral does not yield n, but rather \(\frac{n}{2\pi\theta}\).
  • Another participant agrees with the first participant's calculation, suggesting that the integral should yield n only if the leading denominator were \(\sqrt{2 \pi \theta}\).
  • A third participant notes a different approach from a Wikipedia version of the Maxwell-Boltzmann distribution, which leads to a result of N, indicating potential confusion regarding the distinction between velocity and speed in the equations presented.
  • A later reply highlights the lack of clarity in the original text regarding the notation used for velocities, suggesting that clearer vector notation could have prevented misunderstandings.

Areas of Agreement / Disagreement

Participants express differing views on the interpretation of the integral and the notation used in the original source. There is no consensus on the correct interpretation or resolution of the discrepancies in the calculations.

Contextual Notes

Participants mention a lack of familiarity with statistical mechanics, which may influence their understanding of the equations and concepts discussed. The discussion also reflects potential ambiguities in the notation used in the original text.

Monty Hall
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I'm reading http://www.ndsu.edu/fileadmin/physics.ndsu.edu/Wagner/LBbook.pdf (pg 12, 3.2.2) about the lattice Boltzmann method. I'm speaking of his specfic form of the Maxwell-Boltzmann and his claim that the integral of equilibrium distribution is equal to n

[tex]\int f^0 = n[/tex]

[tex]f^0(v)=\frac{n}{(2\pi\theta)^{3/2}}e^{-(v-u)^2/2\theta}[/tex]

But when I use gaussian integrals I don't get n but rather [tex]\frac{n}{2\pi\theta}[/tex]. What am I missing?
 
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From a quick glance, your answer seems correct; if the integral was to be just n, then the leading denominator would have to be

[tex] \sqrt{2 \pi \theta}[/tex]
 
Thanks for your reply. When I look at the wikipedia version of the maxwell-Boltzmann, I see they do a triple integral over basically the same equation, there I get N. From how the author wrote the equation, there was no indication that he was speaking of velocity and not speed like bold face or overarrows. Plus it doesn't help that I'm not familiar with statistical mechanics either).
 
That makes sense; poor form that they didn't make that more clear--either they should have had all 3 serifs, or vectors over their velocities or something.
 

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