# Help with electrical resistivity equation

1. May 1, 2010

### niqolas619

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

I'm just trying to understand what the variables in this equation represent. This is part of a study of electrical resistivity lab that I am doing. I don't have my Kittel book with me, and the lab manual for some reason doesn't define these variables.

2. Relevant equations

$$\rho = \frac{m}{ne^2} \frac{1}{\tau}$$

3. The attempt at a solution

I could kind of guess at some of them - m = atomic mass, e = elementary charge, $$\tau$$ = relaxation time...but what about n? And I'm not sure about m, either.

Thanks.

2. May 2, 2010

### ehild

n is the number density of the free charge carriers, electrons in the conduction band or holes in the valence band, and m is their effective mass, which is not the same as the electron mass, as it is connected to the band structure. For free electrons in silicon, the effective mass is about 0.35 times the electron mass.

ehild