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A Simple Question about Diffusion and ion speed

  1. Jan 3, 2009 #1
    I'm trying to get an intuitive understanding about a process involving an ion mobility that (I'm told) has D = 9.3 x 10-5 cm2/s. I'm assuming that D is a diffusion coefficient.

    I'd like to know approximately how long it takes for the ion to travel about 10 cm. Would this be something that happens in a matter of femtoseconds, minutes, hours, weeks, years.... or what? Unfortunately I don't have any more information than that, so is there some way to calculate (or just roughly guess) the ion speed based on D?

    many thanks,
  2. jcsd
  3. Jan 3, 2009 #2


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    The general approximation for diffusion time is [itex]t=L^2/D[/itex]. The exact answer depends upon the boundary conditions, but this estimate is good to within an order of magnitude. In your case: about 12 days.
  4. Jan 4, 2009 #3

    That's great! Thanks very much for your help on this!

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