Drug diffusion

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Joined Oct 10, 2008
Suppose a drug is ancapsulated between two planes at x=0 and x=h. The drug diffuses out of both planes at a constant rate R, so the diffusion equation is

D d^2(c)/dx^2 = R

a) Solve for c(x) inside the tablet, subject to boundary condisions c(0) = c(h) = 0.

b) Compute the flux of the drug out of the tablet.

a) c(x) = R(x^2 + hx)/2D

b) I don't know what to do here. I tried to use Fick's 1st law (J = -D dc/dx), but this gives me J = 2Rh, while the correct answer is Rh.
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