# Homework Help: Time between collisions

1. Jan 21, 2009

### jromega3

1. The problem statement, all variables and given/known data
compute the average time between collisions T for conduction electrons in copper.

2. Relevant equations
Vd=-e*E*T/(Me)
Where e is electron's charge
E is electric Field
T is what i'm solving for
And Me is mass of the electron

Vd=I/PA

3. The attempt at a solution

so it was given that a copper wire with a 1.0mm diameter had a current of 1A...and it's mass density is 8900kg/m^3.
So m= 1 electron(8900kg/m^3)(6.02X10^23 atoms/0.0635kg) = 8.4 X 10^28/m^3
So
Vd=1A/(8.4X10^28 m^-3)(1.6X10^-19C)(7.9x10^-7m^2) = 9.4 x 10^-5 m/s

So now...
Vd=-e*E*T/(Me)

So T=(Vd)(Me)/(-e)(E)
I have Vd, Me is mass of the electron, and -e is charge of the electron. I need the electric force...and am stumped. How do I get it from the given info? Thanks for the help.

2. Jan 21, 2009

### rl.bhat

Current density J = I/A, And resistivity of the material rho = E/J.
E = rho*I/A. For copper rho = 1.72x10^-8ohm.m
Vd = J/nq, where n is electron density in copper = 8.5x10^28 m^-3.
See whether these hints helpful to solve the problem.

3. Jan 21, 2009

### jromega3

Ok, so I get E=rho*J..................or since I don't know J J is also Vd*nq....so I get E=rho*Vd*nq.....
so my equation becomes T=(Vd)(Me)/(-e)(rho)(Vd)(Nq)
stuff cancels and I get Me/(-e)(rho)(Nq) and I get a really wrong number. hmm
Edit. Nevermind...my bad. We Have I and A can be easily solved giving J. Thanks again, always appreciated.