Diffusion Definition and 476 Threads

Hey, I have a question for evaporation--What is the real driving reason for evaporation? I usually believe that it is a pure diffusion process: the saturated pressure of liquid at the liquid vapor interface is higher than the partial pressure of vapor in the ambient, and this pressure difference...

Hi all, This is the situation: Solution A with 200 Osm urea, solution B with 200 Osm KCl, separated by a semipermeable membrane that is permeable to urea but impermeable to KCl. Each solution is in 1 L water. Calculate the equilibrium concentrations of urea and KCl. The numerical answer...

I'm looking for a proof of the following statement at a level an early undergrad would understand: ##J=D \vec{\nabla} \vec{n}## where ##D=\frac{v_{th}l}{3}## with ##l## being the mean free path and ##v_{th}## the thermal agitation velocity, ##J## is the particle current density. I really did...

Homework Statement Find the solution for: ({\partial{}_t}^2 -D \Delta^2)G(\vec{r},t;\vec{r}_o,t_o)=\delta(\vec{r}-\vec{r}_o)\delta(t-t_o) In two dimensions. Homework EquationsThe Attempt at a Solution Am I supposed to use bessel eqs? I'm kind of stuck in starting the problem :L

1. Homework Statement Question This is an experiment on a dilute isotopic mixture of helium gas. A spherical vessel of diameter 1 m is first filled with 4He gas to one atmosphere pressure. Then a small amount of 3He gas is introduced through a valve on one side of the vessel. Make a rough...

Hello, I just began learning mass transfer, and I am trying to use analogies from heat transfer to help me solve problems. For example, if you have one dimensional heat transfer through a plane wall, I would start with a general energy balance. $$\frac {dE}{dt} = \dot Q_{x} - \dot Q_{x + \Delta...

If we have a hole in a container with pressure equal to the pressure outside it and the concentration of gas inside the container is more than that outside will the gas effuse out?

So the rate of diffusion is dependent upon the speed of the molecules and the existence of a concentration gradient. What about the concentration of the particles themselves? Diffusion is dependent upon mean free path and concentration is inversely proportional to mean free path. So does...

I am going to do a numerical simulation of diffusion in matlab. The diffusion coefficient is concentration dependant, and i use an array operation to calculate D(x), so it is known. Based on Fick's second equation: $$ \frac{\partial C}{\partial t} = \frac{\partial}{\partial x} D \frac{\partial...

Dear scientists, For my thesis I am searching for a hydrogel that limits the diffusion of calciumions. The objective of this is to get the calciumconcentration in the hydrogel within a certain measurable (with fluorescent Cameleon) range. Unfortunately I don't know where to start looking, do...

Homework Statement i) At rest a 70 kg person with an area of 1.7 m2 consumes around 14 litres of oxygen per hour. Around 2% of this is provided by diffusion through their skin. Calculate the diffusion rate for oxygen through human skin, in millilitres per second per square metre. ii) What...

Hey guys, I have a problem that is giving me trouble. Homework Statement I have to solve time dependent diffusion equation ##D\nabla^2 T(r,t)=\frac{\partial T}{\partial t}## (##D## is diffusion constant and ##T(r,t)## is temperature function) for a spherical shell of radii ##r_1## and ##r_2##...

Hi, I want to solve the following diffusion equation: (d/dt) C(r,z,t)=D*∇^2 C(r,z,t) where C is the concentartion and D is the coefficient of diffusivity (constant) with initial condition C(r,z,0)=C0 (constant) and boundary condition dc/dr=0 at r=0; (dc/dz) at z=-L equal to (dc/dz)...

Homework Statement Plot the transient conduction of a material with k = 210 w/m K, Cp = 350 J/kg K, ρ = 6530 kg/m3 Where the material is a cylinder, with constant cross sectional area and is well insulated. The boundary conditions for the cylinder: T(0,t) = 330K T(l,t) = 299K...

Homework Statement Would appreciate any help quite confused on this topic. If a type of ion has a diffusion coefficient of 10^-8 m^2/s in water. What is average distance traveled after 30 mins? What sort of graph should you plot in order to easily test validity of r=squarer root-2Dt. How...

I've done some research, and I've looked at the derivation of the diffusion equation, but I don't want to reproduce even a more simple version of it. I'd like something that can I do almost exclusively myself. The point of this is not to plot some graphs, and extract some line of best fit or...

In a forward-biased PN junction, the potential barrier decreases, allowing more majority carriers from one side to diffuse to the other side where they are minority carriers. After they cross the potential barrier, they form a diffusion current, the drift current of minority carriers is...

Hi, Could any kine soul please explain this to me: I know that diffusion is movement of impurities (dopants) into the semiconductor especially happening during heat cycles in fabrication process. but what is the dopant redistribution? Is it movement of impurities toward the surface? Thanks in...

Background: I'm currently doing a research project on diffusion. I've been thinking about how soluble gases can dissolve at higher concentrations in a colder liquid than a hot liquid. Question: If there are two connected containers, one with hot water and the other with cold water, and the cold...

Homework Statement In essence, I'm trying to make a model of how the heat from a point source diffuses through air. It should be a function of both distance and time (I'm assuming either a 1/r^2 or an exponential dependence on distance). Homework Equations I found this equation...

Hello. I know what is diffusion... chemical process when molecules from a material move from an area of high concentration to an area of low concentration... Typical example is ink in water. However, how it works? Molecules of water collide with molecules of ink? How it is happens from the...

Hello, I am having to find the rotational diffusion of a magnetic particle inside of water. I already have a diffusion coefficient but I do not know how to take into account the restoring force that the particle will feel from the magnetic field. The info that I know is: B-Field Strength...

When thinking of a spherical shaped particle moving about under Brownian motion, one describes its motion by Diffusion. The units being \frac{m^2}{s} I can understand this physically as a distance it will travel from a certain point in space averaged over x-y and z direction. Now rotational...

Q1what is ion implantation in wafer fabrication ? Ion implantation is process by which ion of material accelerated in electric field and impacted into silicon wafer Q2what is diffusion in wafer fabrication ?

Homework Statement A uniform rod of length l has an initial (at time t = 0) temperature distribution given by u(x, 0) = sin(\frac{πx}{l}), 0 \leq x \leq l. The temperature u(x, t) satisfies the classical one-dimensional diffusion equation, ut = kuxx The ends of the rod are...

I am confused about shared contacted diffusion in CMOS layout. Is it possible to use shared contacted diffusion for two nMOS transistors with different widths? BTW, two nMOS and pMOS transistors are never shared contacted diffusion, right?

Homework Statement Part (a): Under what circumstances do these equations work? Part (b): Find Temperature as a function of x, in steady state. Part (c): Show heat loss is proportional to r^1.5 Part (d):Assume A = 0, show solution works, and find γ.Homework Equations The Attempt at a Solution...

ut(x,t) + c(x) ux(x,t) = 0 I was given the equation above and c is said to be the propagation velocity and not constant what do they mean when they say i must use a method of characteristics to show that the above equation will be an ordinary diff equa

Consider a long semiconductor bar is doped uniformly with donor atoms so that the concentration is given by n = ND and is independent of position. Radiation falls upon the end of the bar at x=0, this light generates electron-hole pairs at x=0. light keeps on falling. Explanation...

I am looking at an object that diffuses randomly inside a circular plane. What is the easiest way to find the expectation value for how long would it take on average to reach the wall if it was initially placed at a random position? If it is easier: what if the initial position were the...

Homework Statement As a simple model, suppose you arrange for the intense bleaching pulse of light to illuminate a cell in a pattern of parallel planes. That is, immediately after bleaching (at time t=0), the remaining fluorescence intensity Θ(x) has the form Θ(x)= C(i) + C(1)sin(2pix/L)...

Hello, I have a certain diffusion problem I am trying to solve. Admittedly, I'm further behind on my math than I'd like, and have trouble setting it up properly, and no luck finding an exact analogue in the literature. I would like to solve for the time-dependent concentration profile...

Hello, Frustration in receiving timely responses from my teaching assistant has lead me to this website. Currently have a homework assignment on multiple group diffusion theory and one of the assigned questions is, Determine the thermal flux due to an isotropic point source, So fast...

I have a general question about the solution to the Diffusion equation using the explicit finite difference method. Now, it is known the solution is stable when D*dt/dx^2 is less than 0.5, based on the choice of time and space steps. However, how does the choice of the time and space steps...

I'm trying to implement a numerical code for the diffusion equation with moving boundaries. I have no problems with the numerical implementation, but with the transformation of coordinates. In spherical coordinates, the diffusion equation is \frac{\partial c}{\partial t} = D...

I have two cylindrical tubes connected at a 90 degree junction. One tube has a constant flow of a laminar fluid going through it and the fluid is saturated by a soluble particulate with a known concentration. The other tube has a much smaller radius, initially has no particles and is closed at...

The problem of the homework states: Mono-energetic sources of neutrons emitting S neutrons/cm3 are distributed throughout an infinite moderator with diffusion coefficient, D (cm), and absorption cross section Ʃa (cm-1) Show that the neutron flux in the moderator is given by: \Phi = S/Ʃa...

This is killing me; not sure if I'm over-complicating or under-complicating it. I’m trying to understand if there is any difference between acoustic scattering and diffusion, and how that differs from attenuation. When particles collide, a small amount of heat is created, since the collision...

Homework Statement Ants are release from the center point of a smooth table and after 1 minute a snapshot is taken and the ants are counted at concentric 5cm circles. There are 10 outer circles ( The most outer circle being 50cm from origin). The number of ants at each ring starting from...

Hey everybody, first post here. Having some trouble with a biophysics questions. So, the question is basically this: You have a certain number of ants, you place them in the center of a large smooth table and after 1 minute a snapshot is taken and the number of ants in 5cm concentric circles is...

Hello... My main problem is to find the Green's function of this pde: \frac{∂T(x,t)}{∂t}=\frac{∂}{∂x}(D(x)\frac{∂T(x,t)}{∂x}) . This is the well-known diffusion equation, but here, the diffusion coefficient is coordinate-dependent. My main focus is to find the Green's function, but i know...

"Consider a solution of the diffusion equation ##u_{t} = u_{xx}## in {0 ≤ x ≤ L, 0 ≤ t ≤ ∞}. a) Let M(T) = the maximum of u(t,x) in the closed rectangle {0 ≤ x ≤ L, 0 ≤ t ≤ T}. Does M(T) increase or decrease as a function of T? b) Let m(T) = the minimum of u(t,x) in the closed rectangle {0 ≤ x ≤...

Hi everyone! Homework Statement We're given a three dimensional cone with perimeters d0 at the top and d1 at the bottom and a substance that diffuses through the cone with diffusion constant D from top to bottom. The concentration of the substance is held constant at the top plane of the...

Hi everyone! I need help getting started in this project. Homework Statement I am trying to simulate the diffusion of a substance in a pipette/cell/cell-membrane geometry. Homework Equations Heat Equation The Attempt at a Solution I think the best way to tackle this...

I'm wondering how this actually can be solved since i think there is one piece missing. Homework Statement Determin the diffusion length L on a P+N diode with Nd = 10^16 cm^-3 in the N region. Homework Equations Known is that Diffusion length can be found from L=sqrt(Dt) The...

Which process occur faster in semiconductors? drift or diffusion

Hi I'd appreciate any help on identifying the type of PDE the following equation is... *This is NOT homework, it is part of research and thus the lack my explanation of what this represents and boundary conditions. I have a numerical simulation of the solution but I'm looking to have a math...

Hi guys, This is my first post here, this place looks like a great resource. Well, jumping straight in, I have a couple of questions on diffusion. At work, I did a couple of experiments with Ion Exchange Resins. Not getting the results we wanted, my boss asked me to do an analysis of...

Hi, I want to ask if diffusion current can exist in the case: Assuming that small circles are electrons.

Can you help me explain this part from my book? In equilibrium, the pn junction has no external connections, i.e., the terminals are open and no voltage is applied across the device. If so, I wonder why there is drift current in the equation above. In equilibrium, the total current is zero...