# Homework Help: Drude Model / scattering time

1. Sep 14, 2007

### tigigi

I got the answer of the hw, but still have question about it.
I need to calculate the scattering time μ = eτ / m. τ is the mean time.
the unit needs to be in cm^2 / V.S

I put the unit like this :

e = coulumb
τ = s
m = kg

then it's impossible to get the unit like that.

Then I'm wondering if I could use this to find τ and plug it in :
σ = ne^2 τ / m then

μ = σ / ne , but I got a problem that it's still not possible to solve it since we don't know the material then don't know σ.

Who could tell me what's going here. I appreciate it.

2. Sep 14, 2007

### Gokul43201

Staff Emeritus
No, it is possible. You need to convert from Coulombs to units involving volts (use the relation E =QV).

The Drude scattering time is a material property - it depends on the mean free path and the free electron density. We need to see the original question, exactly as it was given to you, to help with this.

3. Sep 17, 2007

### tigigi

Thanks !! I'm approaching it, but there's still sth missing.

coulumb x s / kg

coulumb = J / volt , J=kg.m/s^2 -> plug in, and get

( kg.m/s^2 ) . s (1/kg) = m / v.s -> cm / v.s -> still can't get cm^2 / v.s

4. Sep 19, 2007

### Gokul43201

Staff Emeritus
No, that's a Newton. 1J=1kg.m^2/s^2

5. Sep 20, 2007

### tigigi

I really really appreciate it. I forget too many things.