# Homework Help: Finding electron mobility

1. Oct 2, 2011

### S_Flaherty

The electrical conductivity of copper is 5.96 x 10^7 S/m; its atomic weight is
65.546 AMU; the charge on the electron is 1.06 x 10-19 coulombs; the density of
copper is 8.92 g/cm^3. Assume there is one free electron per copper atom. Calculate
the mobility in cm^2/volt-sec.

This is a question from my electronics lab course and i am not sure how to even start with it.
This problem probably isn't that difficult but i have no idea what equations to use since my prof
thinks that everyone knows much more than they do and rarely tells us what equations to use.
Any help with this would be really appreciated.

2. Oct 2, 2011

### S_Flaherty

Anyone willing to help?

3. Oct 2, 2011

### rude man

I'm learning this alongside with you:

<< There is a simple relation between mobility and electrical conductivity. Let σ = conductivity, n = number density of electrons, and μ = mobility. Then

σ = nqμ.

This formula is valid when the conductivity is due entirely to electrons. >>

http://en.wikipedia.org/wiki/Electron_mobility

4. Oct 2, 2011

### S_Flaherty

So i did the calculations and got 68.61 cm^2/V s, is that correct?

5. Oct 2, 2011

### rude man

Didn't do them myself. What were your σ, n and q?

6. Oct 2, 2011

### S_Flaherty

σ and q are in the question and i calculated n by converting the density of copper to the density of electrons and got 8.195 x 10^22 electrons/cm^3

7. Oct 2, 2011

### rude man

We got problems.

Starting with the charge of the electron not being 1.06e-19 but 1.60e-19 C.

Then, you're mixing units. You must express all quantities in one consistent system of units. Personally I'm an SI man but there are others, e.g. the cgs system the wacko theoretical types prefer :-). Grams and cm are cgs, but the conductivity is given in S/m which is SI. I suggest you convert to SI for everything. Mainly because if I am to check your calculations I want to do it in SI. Cgs makes me nervous.

n is number of Cu atoms per m^3. Convert amu of 1 Cu atom to kg, then knowing the density of Cu you can compute the number of Cu atoms in 1 m^3 volume = n.