# Homework Help: Electron mobility in copper

1. Dec 11, 2007

### Staudinger

Hey all,

I have a pretty straightforward question about electron mobility in metals. We are given the length of a copper wire, it's radius, the current flowing through it, voltage drop across it, and the proportion of free electrons to copper atoms. I've found the current density, conductivity, resistance and resistivity of the wire, but I can't figure out how to calculate the electron mobility. We were given a few equations, one of which was specifically for metals,

$$\sigma$$ = $$n_{e}\mu_{e}$$

in which
$$\sigma$$ is conductivity
q is the charge of an electron
$$\mu_{e}$$ is the the mobility of electrons
$$n_{e}$$ is the density of free electrons

My problem is that I don't know how to translate the proportion of free electrons to copper atoms correctly. I tried using the ratio, which came out much too high, something like 10^24. We are not given the density of copper atoms in the wire, but I can find it in an appendix. Would it make sense to figure out how many copper electrons are in a cubic cm, and use that to find the density of free electrons?

Any help would be greatly appreciated, I feel as though I'm missing out on the aha! moment that would pull this all together.

Thanks!
Staudy

2. Dec 13, 2007

### Shooting Star

There is one free electron to one copper atom, if that's what's stopping you.

Why don't you just write down your calculations. It can't be very long.