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Homework Help: Very basic form of the diffusion law for gasses

  1. Jan 5, 2015 #1
    I'm looking for a proof of the following statement at a level an early undergrad would understand:

    ##J=D \vec{\nabla} \vec{n}## where ##D=\frac{v_{th}l}{3}## with ##l## being the mean free path and ##v_{th}## the thermal agitation velocity, ##J## is the particle current density.

    I really did try google a lot but no luck. Either the proof is way too complicated for me to understand or it just tells the results without work. I'm looking for a very raw/approximating derivation that doesn't really have to be formal at all, something that can be understood in like 10 minutes, hard assumptions are allowed. I would really appreciate it if someone who knows where I can find one, could inform me.
  2. jcsd
  3. Jan 5, 2015 #2


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    To get the correct factor of 1/3 apparently requires a lot of work. The treatments that I have seen at the undergraduate level make approximations that simplify the argument but end up getting the wrong numerical factor. They then just state that a more refined treatment will yield the 1/3. See for example Feynman's lectures http://www.feynmanlectures.caltech.edu/I_43.html especially section 43-5.
  4. Jan 5, 2015 #3
    I see, this is exactly the answer I was looking for. The reason why I'm asking is that in class I remember some very very handwaving derivation that got the wrong factor. When I then looked up the law it had a different factor and I thought we were just blatantly wrong.
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