Trouble understanding derivation of current density

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SUMMARY

The derivation of current density is defined by the equation Current Density = (I/A), where I represents current and A is the cross-sectional area. The current I is expressed as I = (Q/t) = (nVq)/t, indicating that total charge Q is the product of the number of charges n, the volume V, and the average charge q, divided by time t. The inclusion of the volume term V clarifies that n is the number of charges per unit volume, thus nV represents the total number of charges in the volume, leading to the final expression for current density as nvq.

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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?
 
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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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