Trouble understanding derivation of current density

[PsychonautQQ]

Current Density = (I/A)
I = (Q/t) =
(nVq)/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?

 

[mikeph]

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.