# Homework Help: Drift speed on copper cable

1. Sep 20, 2008

### goWlfpack

1. The problem statement, all variables and given/known data
A 150 km long high-voltage transmission line 2.0 cm in diameter carries a steady current of 1030 A. If the conductor is copper with a free charge density of 8.9 1028 electrons per cubic meter, how many years does it take one electron to travel the full length of the cable?

2. Relevant equations

I= nq(vd)A

where A is the area
vd is the drift speed
n is the charge density
and I is the current

3. The attempt at a solution

SO we are solving for the drift speed so i thought this equation would work. i found the area first using surface area of a cylinder 2pir^2+2pirh and i got 18840.0025 because i converted cm and km both to m.
then i plugged everything else in and the answer was wrong
:(

oh yea and i used -1 for the q which is the charge. i assumed -1 b/c they are electrons

2. Sep 20, 2008

### olgranpappy

wrong area. you want the cross-sectional area only.

3. Sep 20, 2008

### goWlfpack

ok so im not solving for drift speed, im slving for delta t... so now the equations im trying are Q=(nA(x)) q

and then Q=(nAvdt)q

4. Sep 20, 2008

### goWlfpack

ok so now i've got an A of the cross section which is pir^2.... i got .001256
so i multiplied that by 8.9*10^28 and then multiplie that by 150000m .. to get the charge. then i divide the charge by the current to get the time.. still wrong though.. any ideas

5. Sep 20, 2008

### olgranpappy

solve for the drift velocity (v). How long does if take for an electron moving at v to travel $\delta x=150000m$? I.e., what's $\delta t$ in terms of $\delta x$ and v.