# I Is collsional frequency actually frequency per an electron?

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1. Jun 12, 2016

### goodphy

Hello.

The collisional frequency is generally expressed by ν = nt×average of (σv), where nt, σ and v are density of target (atoms or molecules), collisional cross section of target and kinetic velocity of incident particles. σ depends on kinetic energy of incident particles

I thought collisional frequency must also be replied on density of incident particles but typically expression doesn't have this factor. Let's imagine thermal collision of electrons to other particles. With the same electron temperature and target density, collision frequency should be proportional to the electron density. That is why this is werld for me.

So I guess collision frequency calculated from the formula above is actually collisional frequency per an indicent particle. I'm wondering my interpretation is right.

2. Jun 13, 2016

### sunrah

It is just the frequency of collisions bbetween incident and target particles. The mean free path can be defined as l = 1/(σ*nt). You can think of this as the average distance travelled between collisions. So your equation can be rewritten as nu = v/l = speed/distance. This is obviously in units of Hz, e.g. it is a frequency. Perhaps easier to understand if you want time between collisions: t = l/v. And of course nu = 1/t.
The collision frequency just depends on the material properties of the target and not the beam (obviously velocity of beamed particles but that's not a material property)

There is something calle dthe luminosity of the beam and that depends on the number of beam particles per unit time.

3. Jun 13, 2016

### sophiecentaur

This value of frequency will be an 'average' value because the particles will not be arriving at regular intervals.
1/(mean interval between collisions) would be a mean frequency.

4. Jun 13, 2016