Therefore we can write,

................(3.20)

The equation (3.20) is called the continuity equation, which relates the divergence of current density vector to the rate of change of charge density at a point.

For steady current flowing in a region, we have

......................(3.21)

Considering a region bounded by a closed surface,

..................(3.22)

which can be written as,

......................(3.23)