Let us consider any point P , shown in Fig.1 located by a cartesian coordinate system. The value of D at the point P may be expressed in cartesian components, $\mathbf{D}_{0}=D_{x 0} \mathbf{a}_{x}+D_{y 0} \mathbf{a}_{y}+D_{z 0} \mathbf{a}_{z}$ . We choose as our closed surface the small rectangular box, centered at …