# Homework Help: Mean free path at low temperatures

1. Jul 22, 2009

### Armandito

1. The problem statement, all variables and given/known data

Hi, It's not a homework question, but as i am a beginner in physics, I prefer to post here first :

I try to evaluate thermal resistance at low temperature. I need to compute the mean free path of both electron and phonon in metals, but I do not find any formulas. Can any body tell me where I could find that?

By the way, Is there two different mean free path : far from the wall and near the wall?

Best thanks!

2. Relevant equations

3. The attempt at a solution

2. Jul 23, 2009

### kNYsJakE

I assume that you know what mean free path is so here is the basic formula for the mean free path:

$$l=\left(\sigma n\right)^{-1}$$

where $$l$$ is the mean free path, $$n$$ is the number of target particles per unit volume, and $$\sigma$$ is the effective cross sectional area for collision.

From this I guess you can derive whatever you want to get about mean free path.

If you are doing the particle physics, then the mean free path is replaced by the similar concept called 'attenuation length' or 'absorption length.' It is the distance $$\lambda$$ into a material when the probability (that particle has not been absorbed) has dropped to $$1/e$$.

Hope it helped.