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Homework Help: Number of conduction electrons question

  1. Jan 28, 2007 #1
    1. The problem statement, all variables and given/known data

    Calculate how many conduction electrons pass through a given area per second when a current of 1A flows.

    2. Relevant equations

    Current density, j = i/A (i=current, A=cross sectional area)
    Current desntiy, j = nevD (n=no of electrons per unit volume, e=charge on electron, vD=drift velocity)

    3. The attempt at a solution

    I've taken an element of volume of length vDt and cross-sect area, A. I've surmised then that the total number of electrons in that area will be N=nAvDt.

    All of those electrons will then cross a plane in 1 second, so i've equated j=i/A=nevD, and substituted n from the earlier (bold) equation - n=N/AvD. However, that produces a result of N=i/e=1/e ... doesn't look right! Help!!
  2. jcsd
  3. Jan 28, 2007 #2
    That looks right to me. How do you think it should look like?
  4. Jan 28, 2007 #3


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    Homework Helper

    Look at it a different way, forget about the current density etc.

    1A = 1 coulomb/sec

    If the charge on one electron is e coulombs, then one coulomb is the charge on 1/e electrons.
    Last edited: Jan 28, 2007
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