Finding the electron drift speed

  • Thread starter MBBphys
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  • #1
MBBphys
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Homework Statement


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

Homework Equations


I=nAve

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!
 

Answers and Replies

  • #2
Khashishi
Science Advisor
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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
MBBphys
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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
 

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