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Homework Help: Protein diffusion

  1. Apr 6, 2009 #1
    1. The problem statement, all variables and given/known data

    You release a billion protein molecules at position x=0 in the middle of a narrow capillary test tube. The molecues' diffusion constant is 10^-6 cm^2s^-1. An electric field pulls the molecules to the right (larger x) with a drift velocity of 1 micrometer per second. Nevertheless, after 80 s you see that a few protein molecules are actually to the left of where you released them. How could this happen? What is the ending number density right at x=0?

    2. Relevant equations

    Suppose N molecules all begin at the same location in 3D space at time zero. Later the concentration is

    c(r,t)=\frac{N}{(4 \pi Dt)^{3/2}} e^{-r^2/(4Dt)}

    3. The attempt at a solution

    From a similar problem, it looks like the formula in 1D is

    c(r,t)=\frac{N}{(4 \pi Dt)^{1/2}} e^{-r^2/(4Dt)}

    Is this correct?

    If so, we have

    c(0,80)=\frac{1 \cdot 10^9}{(4 \pi 1 \cdot 10^-6 \cdot 80)^{/2}} e^0 = 3.15E10 cm^-1


    This clearly must be wrong, or what?
  2. jcsd
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