WHY ELECTRIC CURRENT IS MACROSCOPIC WHILE CURRENT DENSITY IS MICROSCOPIC QUANTITY?
Answers
Answer:
Current is flow of charges across a point along a given point along the length of conductor / a point in circuit.
This current may or may not be uniform across the cross section of conductor. If we consider the whole cross section of conductor, and look at every point across it, actual charge flow can be (and will be in most cases) varying and different at different places.
Convert this into charge flow per unit area -- this is current density.
In solid conductor rods or wires current (particularly so in AC) always has tendency to concentrate near the outer surface. So charge density will be zero at centre of conductor, and will be maximum near outer surface.
This is the difference in microscopic (taken at minute sections of conductor) and macroscopic (taken as a whole of conductor ) quantities