Question

define current density (J ) and write expression for current density in terms of drift velocity.​

Answer 1

Current density is the electric current per unit area of cross section.

Consider a conductor of length l and area of cross section A. Let the number of free electrons per unit volume be n. When a pd V is applied, let the drift velocity of electrons be v.

The total no. of free electrons in the piece of conductor = nAl (no. of free electrons per unit volume x volume)

Total charge due to free electrons; q  = nAle

The time taken by this charge to completely traverse the conductor, t =l/v

Therefore, the current in the conductor, I=q/t = nAle/(l/v) = nAev

i.e; I=nAev

Hope it helped.

But it is not mine.

Answer 2

Current Density

The electric current flowing per unit area of cross-section of conductor is called current density.

Current density (J) = I / A