Question

1. How are current density J and current i related?
2. When is it convenient to use J?

Answer 1

Current (I) - It is flow of electron or the time rate of charge and its unit is ampere.

               I = (dq/dt) columb/second or A

Current Density (J) = It is a vector quantity which is defined as the current flowing per unit area measured in a direction normal to the direction of flow of charge.

|J| = |I|/ A

As I is scaler alothough it have a magnitude and a direction. But a vector always obey the law of addition of vector but I doesn't obey it and it follows algebraic addition therefore current(I) is a scalar. So whenever we have to deal with vector quanties the we use current density (J)