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Next: PowerUp: Chapter 27Current Previous: Current Current Density The current, I, is a coarse, average quantity that tells what is happening inan entire wire.If we want to describe in more detail how the charges move through aconductor, we use the concept of the current density : where n is the number of free charged particles per unit volume,q is their charge, and is their average drift velocity.Since it is a vector quantity, it can indicate the direction of thecurrent flow.Now for the bad news.This formula says that if free electrons are drifting to the right,then the resulting current flows to the left because the electronsare negative.It is difficult to keep this straight, so when we workwith circuits, we usually just think of current as moving positivecharges, even though we know that the electrons carry the current.The current density is an area density, I.e., is the current perunit area.In fact, the current through an area A is simply the fluxof the current density through that area: Or, if the current density is uniform and flows straight into the area, Ross Spencer Tue Apr 8 10:33:28 MDT 1997