electric po- larization density by deriving the electric field force on a medium in two

approaches. One is based on the polarized

charge distribution and the other is based on the electric polarization density. This article

also connects the magneti- zation density and the magnetizing current density by deriving the

magnetic field force on a medium in two approa-ches based on the magnetization density and the magnetizing current density

respectively. The methods used in this article not only show the aforementioned connections， but also directly tell the

relationship between the polarized charge surface density and the electric polarization as

well as the relation between the magnetization density and the magnetizing current surface

density at the boundary of the medium， which is powerful and easy to learn.