# Uranium Diffusion and Decay

• Mar 8th 2011, 02:01 PM
juanma101285
Uranium Diffusion and Decay
Hi, I have the following problem and I am stuck in question 2. I would really appreciate it if you could check that the answer to question 1 is OK, and if you could also give me a hand with question 2.

"A tank of volume 2 $V$, filled with water, is separated into two compartments $A$ and $B$ of identical volume by a semi-permeable membrane. Uranium is introduced into compartment $A$ at a constant mass rate $S$. Uranium is exchanged between compartments $A$ and $B$ through the membrane at a rate given by $p(C_A-C_B)$, where $C_A$ and $C_B$ are the concentrations of Uranium in each compartment, and $p>0$ is a constant. Because Uranium is radioactive, its mass in each compartment also decays, at a rate proportional to mass, with proportionality constant $q>0$.

Let $M_A(t)$ and $M_B(t)$ be the masses of Uranium in compartments $A$ and $B$. Assume that there is no Uranium in the tank at $t = 0$, that is, $M_A(0) = M_B(0) = 0$, and that the water in each compartment is well mixed.

1.- Write down the differential equations for $M_A(t)$ and $M_B(t)$ describing their evolution.
2.- Find the masses at equilibrium."

So for the first one I have:

$dM_A(t)/dt=S-p(C_A-C_B)-qM_A$
$dM_B(t)/dt=p(C_A-C_B)-qM_B$

Is it correct? What about the second question, how do I work that out? Thanks!
• Mar 9th 2011, 05:05 AM
Ackbeet
Note that

$C_{A}:=\dfrac{M_{A}(t)}{V},$ and

$C_{B}:=\dfrac{M_{B}(t)}{V}.$

You need to plug this information into your DE's, which are correct as far as they go. I'm assuming the units compatibility is taken care of by the constants $p$ and $q.$