State macroscopic form of Ohm's law.

Explanation

Current density J at a point in a conductor is the amount of current flowing per unit area of the conductor around that point provided the area is held in a direction normal to the current.
`J=(I)/(A)`
Drift velocity,
`V_(d)=(eE)/(m)tau`
`I=n eAV_(d)=n eA((eE)/(m))tau " " [J=(I)/(A), sigma =(n e^(2)tau)/(m)]`
`(I)/(A)=((n e^(2)tau)/(m))E`
`J=sigmaE`