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Homework Help: Finding the electron drift speed

  1. Nov 12, 2015 #1


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    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

    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?

  2. jcsd
  3. Nov 12, 2015 #2


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    Science Advisor

    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.
  4. Nov 12, 2015 #3


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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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