Abstract:  By taking a problem in a textbook of electromagnetism as a typical example, we firstly identify that the definition of the area S in the general formula for resistor is not clearly specified.In comparison with three popular textbooks of the electromagnetism written by Liang Canbin, Zhao Kaihua, and Hu Youqiu, we notice that the teaching of Ohm’s Law in existing electromagnetism textbooks mostly follows the road of “the experiment→the formula→the microscopic mechanism.” Moreover, in a traditional classroom, educators often ignore the importance of the knowledge framework and theoretical derivation, so that it is difficult for students to understand the essential element of the physical conceptions and the basic principle.In order to improve the presentation of the Ohm’s Law in electromagnetism, we offer a novel constructive way to teach the Ohm’s Law by focusing on its differential form.It has been proved more convenient and effective for students to learn the background mechanism and physical principle of the law if we start with the differential form rather than the traditional form.Moreover, through the differential form, we can easily derive the general formula for the calculation of the resistor and the result of Joule’s Law.Therefore, our approach established in this work is an upgrade to the existing instruction with a renewal teaching logic from “the fundamental principle→physical laws.” At last, after clarifying the definition of the area S in the general formula for resistor, we can also prove that the solution for current I is independent of the shape of the surface.

Key words: comparative textbook, the differential form of the Ohm’s law, teaching representation, microscopic mechanism, general formula for resistor