# Time Scale problem

1. Feb 4, 2009

### ishmandoo

I am having a lot of trouble starting the following problem. Any ideas? Thanks a lot for helping!

Derive from first principles, the diffusion time for particles traveling at speed v in a two dimensional (x,y) space to reach a distance R from their release point if the scatterers have a density n(m^-2) and linear cross section for scatter $$\sigma$$(m) Let each particle scatter with equal probability into the +/- x, +/-y directions.

I don't know what some of these things mean and they aren't explained well in my book or anywhere i look. What is the density it is referring to? What is the linear cross section for scatter? Again, thanks for any help.

2. Feb 5, 2009

### mathman

Density n(m^-2) is number per square meter. The cross-section determines the probability of scatter - I am not sure how the normalization has been set up.