To determine the condition for the normal component of the flux density vector , we consider a small pill box P with vanishingly small thickness h and having an elementary area for the faces. Over the pill box, we can write

                                                 ....................................................(4.36)

Since h --> 0, we can neglect the flux through the sidewall of the pill box.                                              

                                                ...........................(4.37)

                                                 and ..................(4.38)

                                                
where
                                                and ..........................(4.39)