Define the term current density.

Explanation

Current density at a point in a conductor is defined as the amount of current flowing per unit area of the conductor aroung that point proiveded the area
`J = (I)/(A)`
Current density is a vector quantity. Its direction is the direction of motion of positive charge. The unit of current density is amper/`"meter"^(2), or [Am^(-2)]`
Relation between J and E:
`I = n A e v_(d)`
` = n Ae ((eE)/(m )tau )`
`= n (A e ^(2) rau E)/(m)`
`or I /A = ( n e ^(2) tau E)/( m )`
or `J = (1)/(rho) E`
`[ because J = I/A and rho = (m)/(n e ^(2) tau)]`
`J = sigma E [ because sigma = (1)/(rho)]`