Stack Exchange Network

Stack Exchange network consists of 175 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers.

Visit Stack Exchange

Questions tagged [mean-free-path]

The average length that a molecule will travel in a fluid before colliding with another molecule.

1
vote
1answer
36 views

How to (hypothetically) calculate $\tau$, the mean free time?

Referring to the Drude Model, I've seen a lot of excellent questions on whether $\tau$ should be thought of as the "average time between collisions" or the "average time until the next collision", and ...
0
votes
0answers
29 views

Electron mean free path as a function of pressure

How do I calculate electron beam mean free path as a function of vacuum pressure? For example, if I am pumping a vacuum system down and have an electron beam of known energy, what is the mean free ...
8
votes
2answers
194 views

Rigorous derivation of the mean free path in a gas

Can anyone supply me with a derivation of the mean free path, of particles in a Maxwell Boltzmann Gas? Cited in various literature is the formula, \begin{align} \begin{split} \ell&=\frac{1}{\...
1
vote
0answers
34 views

Mean free path for $3\rightarrow 1$ scattering

I want to calculate the mean free path of an antineutrino in nuclear matter where it can undergo the reaction $p+e^-+\bar{\nu} \rightarrow n$, which I imagine will involve calculating the rate of that ...
1
vote
2answers
234 views

Most probable free path length

The distribution of scattering time is $nv\sigma e^{-nv\sigma t}$ where $\sigma$ is the collision-cross section, $n$ is the number density of molecules and $v$ is the average relative velocity. This ...
1
vote
2answers
36 views

How is the waveform deviation from the equilibrium related to the air molecule movement?

I'm not sure if this is a stupid question. I've been considering the deviation from the equilibrium reflects the air pressure, with larger deviations reflecting higher air pressure. But in Reetz and ...
0
votes
0answers
45 views

Average distance travelled before absorbtion 253 nm photon in air

At first I wanted to use term "Mean free path" in title but I know it means other kinds of interactions too,not just absorbtion.What I really mean is mean free path but only for absorbtion. For ...
0
votes
1answer
50 views

How to interpret 'electron energy above fermi level (eV)'?

In some mean free path graphs, I see 'electron energy above Fermi level (eV)' on the x-axis See figure: What does it mean? Is it electrons kinetic energy in the material? What happens if the graph ...
2
votes
1answer
456 views

Mean free path of gas mixture

I come cross a question about calculate the MFP of a gas mixture, which contains several different kinds of molecules, each has different size, velocity and number density. My question is: How can I ...
0
votes
2answers
115 views

Is penetration of a fast particle deep into a medium a quantum mechanical effect?

Consider the cosmic rays. The muons can travel 15000 meters through the atmosphere and get down to sea level. If this is not a quantum mechanical effect, its penetration depth is determined by the ...
3
votes
0answers
223 views

Ion charge exchange mean free path

I am using the following equation to calculate the mean free path between a deuteron ion and background gas: $$\lambda_{d-gas} = \frac{kT_{gas}}{p_{gas} \, \sigma_{10}}$$ Where $\sigma_{10}$ is the ...
1
vote
1answer
3k views

Viscosity and mean free path

I have troubles understanding how to derive the formula for viscosity in terms of the mean free path $$\eta\sim \rho \lambda \bar v$$ where $\bar v$ is the average molecular velocity of the gas, $\...
0
votes
1answer
474 views

speed distribution and mean free path of gases at pressure

I know that at 18,000 ft. above mean sea level, the atm. pressure ~half of what it is at seal level (760 Torr). The temperature also decreases by 70C. a. How would this change the speed distribution ...
1
vote
1answer
410 views

Mean free path in 2d?

Let us say we have a collection of $n$ stationary balls per square meter each of diameter $d$ and we roll an identical ball between them. Then what is the free mean path of this ball? My ...
2
votes
1answer
109 views

Derivation of relativistic mean free path

I am working through a short derivation found in Abramowicz 1991 regarding the mean free path of a photon. We have a fluid moving in a particular direction with velocity $v$ and in an inertial rest ...
0
votes
1answer
52 views

How photons get distributed in a system?

Let's say I'm in a room, and there are plenty of things in my room like carpet,mirrors,glass and other stuff.And I see there are shadow regions and fully bright regions, and I want to know how photons ...
2
votes
0answers
73 views

Mean free path in spherical coordinate

I am thinking about a distribution with spatially inhomogeneous cross section in spherical coordinate. Starting with 1-dimensional case, a change of the flux passing through a slab with length $dx$ is ...
6
votes
1answer
308 views

Does it make sense to define the mean free path in quantum mechanics?

The mean free path defined in classical molecule dynamics has a strong classical flavor. Is it sensible to generalize the idea to quantum mechanics?
2
votes
0answers
491 views

Copper mean free path dependence on temperature

I'm doing simulations of copper, where the temperature can reach up to ~1300 K. Some calculations depend on the mean free path (MFP) of copper. The only value I've found for it is 39nm and it's ...
4
votes
2answers
4k views

RMS Free Path vs Mean Free Path

I am trying to determine the mathematical difference between mean free path and root-mean-square free path. For an ideal gas, the relaxation time is $$\tau=\frac{1}{\sqrt2 \pi nd^2 \bar v}$$ and the ...
0
votes
2answers
835 views

Molecular mean free path probability

Let's let $p(\xi)$ be the probability a molecule travels at least $\xi$ between collisions, lets say $\xi=0.01$. When I think of this statement, I think it it is the probability the molecule is able ...
1
vote
2answers
600 views

Energy of particle in electric field

I'm taking a physics class and the professor teaches us really basic things in lecture and then gives homework way beyond what he taught in lecture. Obviously I need to find some resource other than ...
0
votes
1answer
104 views

Do particles in the solar core travel at the same speed as they would in vacuum?

I just read the Wikipedia article on the Sun. The section on Solar Core states It has a density of up to 150 g/cm3[47][48] (about 150 times the density of water) Do the particles (atomic, and sub-...
2
votes
1answer
149 views

The number of mean free paths which a particle traverses is independent of the material traversed

This is a manual from a Monte Carlo physics manual: Computing the occurrence of a process Can anyone tell me why the number of mean free paths of a particle is independent of the material which it ...
3
votes
1answer
9k views

formula for mean free path in two dimensions

I'm running some simulations of particle collisions in two dimensions with discretised time and space. In essence, particles only collide if they occupy the same location (cell) at the same time step. ...
2
votes
3answers
2k views

Mean free path of UV photon

I was wondering if there's a simple way to compute the mean free path of UV photons in a optically thick medium with density n. I've looked up at the literature ...