# Finding the electron drift speed

Tags:
1. Nov 12, 2015

### MBBphys

1. The problem statement, all variables and given/known data
A piece of n-type germanium is 2.0mm wide and 0.25 mm thick. At a certain temperature, the number of conduction electrons per cubic metre is 6 x 1020 m-3. What is the average drift speed of the electrons when a current of 1.5 mA flows? (e=1.6 x 10-19

2. Relevant equations
I=nAve

3. The attempt at a solution
So I converted all the units to SI base.

1.5/1000 = 6 x 1020 x (0.125/1000)2 x pi x 1.6 x 10-19 x v

v = 318.309...

v=318 metres per second (3sf)
.

But the answer booklet gives 31 metres per second. Am I wrong or is it wrong?

Thanks!

2. Nov 12, 2015

### Khashishi

You are wrong. I got 31m. Where did 0.125 and pi come from? I don't see any circles in the problem statement. It has a thickness and a width --- that sounds like a rectangular prism.

3. Nov 12, 2015

### MBBphys

Ah, right, so I have to do [(2/1000)*(0.25/1000)] rather than use pi, because it is a cuboid and not a circular piece of wire? I think because I usually do questions based on copper wires that are circular, I assumed the same here, but clearly that is wrong!

Thank you

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted