**What is Current Density?**

By definition, the current density, denoted by the symbol J, is the current per unit area of the conducting medium i.e.,

where

- current I is the rate of flow of charge.

If N is the free electron density moving in the conducting medium, e is the charge of electron, then the rate of flow of charge is given by

Thus the current density J is given by

But the time

Therefore,

Current density,

where

*l*A is the volume of conductor

where

- ρ = ne, the charge density, in coulombs per cubic metre and
- ν = is the velocity in metres per second.

The above derivation is independent of the form of conducting medium. Consequently, it is not necessary that the conducting medium is a wire conductor. It may be equally well a portion of gaseous-discharge tube or a volume element in the space-charge cloud of a vacuum tube or semiconductor. Furthermore, neither ρ nor ν need be constant, but may vary from point to point in space or may vary with time.