# Current and Drift Speed

## Homework Statement

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?

## 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!

## Answers and Replies

berkeman
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.