The research object of electrodynamics is the basic properties of the electromagnetic field, the law of motion of the electromagnetic field and the interaction between it and charged matter. This paper explores a common problem in electrodynamics: solving the bulk density of bound charges with known polarization strength, so that students will have difficulties in the situation, find new problems in the difficulties, and stimulate students^, cognitive conflicts. In the teaching process, emphasis is placed on the application and transfer of knowledge and methods, arousing students^, positive thinking, and cultivating students^, scientific thinking and scientific inquiry abilities.