If we now consider that the dielectric region containing charge density the total volume charge density becomes

....................(2.72)

Since we have taken into account the effect of the bound charge density, we can write

                                                             ....................(2.73)

Using the definition of we have

                                                             ....................(2.74)

Therefore the electric flux density

When the dielectric properties of the medium are linear and isotropic, polarisation is directly proportional to the applied field strength and

........................(2.75)