# Homework Help: Current and Drift Speed

1. Mar 2, 2007

### AznBoi

1. The problem statement, all variables and given/known data
A 200-km-long high-voltage transmission line 2 cm in diameter carries a steady current of 1000 A. If the conductor is copper with a free charge density of 8.5 x 10^28 electrons per cubic meter, how many years does it take one electron to travel th full length of the cable?

3. The attempt at a solution

I know that you need to use this equation: $$R=p\frac {l}{A}$$ and I think I will also need this equation: $$I= \frac {\Delta {Q}}{\Delta t}$$, but I'm not entirely sure.

The thing I most confused about is the "density of 8.5 x 10^28 electrons per cubic meter. Is that a volume? Are you suppose to use: $$I= \frac {\Delta {Q}}{\Delta t}$$ to find the time (years)?? I'm stuck from here on out. It'd be great if you could give me some hints as to how to continue. Thanks in advance!

2. Mar 2, 2007

### Staff: Mentor

I don't think you need the resistivity equation, just use the charge density equation and the equation for current expressed in terms of charge flux.