# Homework Help: Drift velocity

1. Mar 21, 2012

### Johnahh

1. The problem statement, all variables and given/known data
a wire carrying a current of 3.2A has a radius of 1.2mm. the material of the wire has 2.5x10^25 free electrons per cubic metre. the elementary charge is 1.6x10^-19

2. Relevant equations
I=Anev
cross section = pi*r^2

3. The attempt at a solution
pi*0.0012^2 = 4.5x10^6

3.2/4.5x10^6*2.5x10^25*1.16x10^-19 = 0.1777 recurring
I am sure this is not correct as it seems to fast, I have seen some examples and they are more like 2.8x10^-4 m/s
Am i supposed to make 2.5x10^25 into 2.5x10^19 for mm^3 instead of m^3?

2. Mar 21, 2012

### ehild

2.5 x10^25 electrons per cubic metre is very low value for a metal wire. It should be in the range 10^28-10^29 electron/m^3.

ehild

3. Mar 21, 2012

### technician

you have the cross sec area as 4.5x10^6 ....it should be 4.5x10^-6.... this is just a typing error and does not affect your answer.
For the figures you have given I also get 0.177
I agree with ehild regarding electron concentration for a metal. 10^28 to 10^29

4. Mar 21, 2012

### Johnahh

Yeah it was a typo sorry. OK so the electron drift is 0.177 m/s. What happens if the cross section doubles in size is it v*2 or v/2? Thanks