Questions tagged [mean-free-path]

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

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11
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4answers
2k views

Do photons slow down this much in the Sun's gravitational field?

I just heard someone mention that photons take 40 thousand years to travel from the centre of the Sun to its surface which is roughly 700,000 kilometres. How is that possible if the speed of light/...
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0answers
13 views

The question is about kinetic theory of gases regarding mean free path

In the derivation we have stated that the molecule moves only in a particular cylinder ( due to assumption that molecules of gas’s travel in straight line ) and travels through the cylinder even after ...
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1answer
49 views

How to calculate the mean free time in Drude model?

According to Drude model, the average electron takes some time to go from one ion to another: this time is called mean free time. The problem is there isn't a satisfactory definition of this mean free ...
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1answer
32 views

Why does thermal conductivity decrease from solids to gases?

I am learning in my heat transfer class that the thermal conductivity decreases from solids to liquids to gases i.e. $$K_{solid} > K_{liquid}>K_{gas}$$ The reason cited is that (based on my ...
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0answers
109 views

Mean free path of electron in air?

I'm interested in approximating the mean free path of an electron in air. I think I'm going to need to add something more into my approximation because currently I calculate $400m$ for the mean free ...
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0answers
25 views

Proportional chamber and electron-ion pair production

I have the following question that I need to solve: A cylindrical proportional chamber has a central wire of radius $r_a = 0.02 mm$ and an outer (cathode) of radius $r_c = 10 mm$, with a voltage of $...
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1answer
55 views

Two versions of Diffusion coefficient

I found two versions of the Diffusion coefficient, first: $$D=\frac{\pi \lambda }{8}\overline{c}$$ Where $ \overline{c}$ ist the particles mean thermal velocity and $\lambda$ the particles mean free ...
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33 views

Most Probably Path Length - Statistical Mechanics & Thermodynamics

I’m currently attempting to solve a question set by my lecturer to find the most probable path length of molecules of a substance. The question does not give any values, but I’m assuming it is asking ...
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0answers
29 views

Mean free path for charged molecule inside air

Question I would like to compute the mean free path ($\lambda$) for a "heavy" charged molecule ($M$amu, $q$ charge) in a low pressure gaz (air, but also ions such as Ca$^+$). I think I figured out ...
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1answer
119 views

The mean free path of electrons in high vacuum is 26 billion kilometres?

I used this formula to calculate the mean free path length of an electron in high vacuum. $$\lambda = \frac{k T}{\sqrt{2}\cdot4\pi r^2 \cdot p}$$ where k is the Boltzmann konstant, T the temperature ...
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1answer
265 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 ...
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0answers
240 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 ...
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3answers
1k 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}{\...
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0answers
46 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 ...
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2answers
542 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 ...
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2answers
50 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 ...
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0answers
65 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 ...
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1answer
57 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 ...
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1answer
904 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 ...
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2answers
135 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 ...
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0answers
276 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 ...
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1answer
4k 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, $\...
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1answer
569 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 ...
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1answer
676 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 ...
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1answer
181 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 ...
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1answer
53 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 ...
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0answers
103 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 ...
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1answer
388 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?
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0answers
630 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 ...
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2answers
5k 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 ...
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2answers
962 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 ...
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2answers
680 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 ...
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1answer
111 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-...
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1answer
150 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
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1answer
11k 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. ...
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3answers
3k 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 ...