# Questions tagged [diffusion]

Diffusion is the net movement (spreading out) of molecules or atoms down a concentration gradient: from a region of high concentration to a region of low concentration.

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### Neutron diffusion equation cylinder question

Hi there I was wondering if anyone could help me with how to approach this problem that I have. I'm looking to find the neutron flux outside of a cylindrical container which is made of steel, however ...
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### The roles of convection and advection in the absolute flux for species $i$ in mass transfer

I am confused as I have been reading on mass transport on multiple sources and they seem to use "advection" and "convection" interchangeably in the equation for the absolute flux ...
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### Does diffusion current "consume" thermal energy?

My (possibly erroneous) understanding of solid state physic has led me to the belief that diffusion current may "consume" thermal energy. Here is my (possibly erroneous) understanding. When ...
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### Diffusion from a rod with constant concentration

Suppose I have an infinite rod that is suddenly brought into a medium where some substance starts to diffuse radially outwards from the rod. During this, the concentration in the rod is kept constant....
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### Need help identifying missing boundary condition in charge transport problem

The system I am considering consists of a conducting liquid sandwiched between two electrodes. The electrodes supply a constant flux of electrons $J_{ext}$. In the liquid there are also uncharged ...
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### Time constant for diffusive process with radiation and convection

I'm studying the thermal comportement of a "mass stove". I would like to do a materials selection. My first constraint is to have a temeperature of my "mass" between a lower and an ...
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### How to calculate windowed mean squared displacement for multiple particles?

I'm new to molecular dynamics and I find this equation pretty confusing: $MSD(\tau)=<r(t+\tau)-r(t)>$ For example take a structure with N mobile ions (e.g. oxygen particles). In different ...
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### Modeling contamination diffusion in a draining container

I have a 250-mL bottle filled with pure water and a sampling line at the bottom slowly draining the bottle to analyze, say 2.5 mL/min. The bottle is on the order of 10 cm high. Since the bottle is ...
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I am trying to understand the following equation about the diffusion Green's function ("Principles of condensed matter physics" by Paul Chaikin, chapter 7.4 Diffusion) $$n(x,t)=\int G(x-x',t-... • 11 0 votes 0 answers 17 views ### How does the rate of heat exchange relate to the rate of particle exchange, when opening my office window? We know that opening the window lets in some fresh air. But in the winter, this fresh air could be uncomfortably cold. To what extent does the room being cold imply that the air is fresh? For ... • 101 1 vote 1 answer 449 views ### Calculating upwards drift velocity of hydrogen in air, where is the error in my calculations? I have the following problem: I'm charging a lead-acid battery in a container. The charger could fail, producing too many amperes at too many volts. Too many amperes I can easily handle in a fuse, so ... • 123 0 votes 1 answer 40 views ### Diffusion current because of internal field of semiconductor Suppose we have an isolated n-type semiconductor having varying concentrations of carriers over x. After some finite time, the concentration becomes uniform for a given constant temperature. We ... • 21 3 votes 1 answer 115 views ### How fast do \rm CO_2 molecules transfer between air and water at equilibrium? Suppose a bath of water is in equilibrium with the air above it at temperature T and pressure P. The concentration of \rm CO_2 in the air is, say, 400ppm, and the concentration of \rm CO_2 in the ... • 523 0 votes 0 answers 12 views ### Expansion and visibility of particles because of diffusion A cloud forms as a point source containing a fixed quantity of particles and then expands according to the diffusion equation. Let's analyze how the visible size of the cloud changes with time. When ... 0 votes 0 answers 36 views ### Most general Green-Kubo relation for transport coefficient - \beta/V dependence of diffusion Hansen & McDonald write that a the most general Green-Kubo formula for some microscopic dynamical variable A can be written as: K=\frac{\beta}{V} \int_0^\infty \bigg\langle \dot{A}(t) \cdot \... 0 votes 0 answers 53 views ### How to numerically solve non-linear 1D diffusion equation in time-domain? The equation to be solved is this:$$\frac{\partial H_y(x,t)}{\partial t}=\left(\frac{1}{\sigma\:\mu(H_y)}\right)\frac{\partial^{2}{H_y(x,t)}}{\partial{x^{2}}}$$But note that the magnetic ... 1 vote 1 answer 30 views ### Jet of gas striking a disc, what fraction of the total volume is in contact with the disc [closed] In a catalysis experiment, I have a jet of gas coming through a nozzle of cross-sectional area A_1 at a volumetric flow rate F. This strikes a circular disc of area A_2 coated with catalyst, at right ... • 11 0 votes 1 answer 17 views ### What would the measurement device show me if I measured the dye concentration at the pipe location where all of the mass is concentrated at? I was studying the methods for solving the diffusion equation. Here I found an example problem with the Dirac delta distribution initial condition and zero boundary conditions (example 9.5.1):$$ {\...
The Einstein relation says that the diffusion coefficient $D$ and it's mobility $\mu$ under a force are related to each other as follows: $$D = \mu k_B T$$ I don't completely understand the ...