Current density J is a vector quantity that is defined as the current flowing per unit area measured in a direction normal to the direction of flow of charge.
|J| = |I|/ A
Substituting the value of I and Vd in this equation |J| = |I|/ A ;
|J|= nAqVd / A
= nq. (qt|E|/ m)
|J| = (nq2t/ m).|E|
The vector J is parallel to the electric field vector.
In the above equation, the quantity nq2t/m is called conductivity σ.
|J| = (nq2t/ m).|E| = σ |E|