Hi all, I'm abit confused about diffusion - I can't seem to understand how to translate the question at hand to equation (from there it is only math...). I have a box with a gas at concentration n0 in it, which is divided from the rest of the box where you have vaccum (for simplicity, the left half part of the box is filled with a classical gas, and the right half is vaccum). Now, the barrier is taken out of the box - I need to solve the density of the gas (dependant on time). The main problem here, is why is it diffusion? The prof. said it's a diffusion problem, but won't the mean-free-path be smaller than the side of the box? therefore it's a simple flow? furthermore, when I tried to solve it I got to the point where I can't seem to write the boundry/starting conditions! 1. it's not and impulse problem 2. it's not a constant/infinite source of particles/heat. How can I treat something like a H function? (because the particles in the begining end with the barrier) Any help will be appreciated (Mind you - I don't want the whole solution! I need help stating the boundry condition in mathematical form). Thanks again.