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Trouble understanding derivation of current density

  1. Sep 14, 2013 #1
    Current Density = (I/A)
    I = (Q/t) =
    where n is the number of charges, V is the volume, q is the average charge and t is time.

    nVq/t = n(Ax)q/t = nAvq
    where v is velocity.

    Current Density = (I/A)
    (nAvq)/A = nvq

    My question is on the step where it goes Q/t = nVq/t
    Why does Q (total charge) not just equal nq? Why is there a volume term thrown in there?
  2. jcsd
  3. Sep 14, 2013 #2
    n must be the number of charges per unit volume, nV is the total number of charges, multiply by q and you get the total charge in the volume.
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