This is Kirchhoff's law of the university and an extension of Ohm's law. There are two Kirchhoff's laws: one is the node current equation in the form of σ (I) = 0; Secondly, it should be the method used in the figure to change the loop voltage equation into σ (ε) = σ (IR). This kind of problem should be difficult to solve by applying the method of high school, and it will only appear in the physics competition of high school. If you don't have this consideration, don't worry about it. This kind of topic should be an extracurricular topic in the college entrance examination. If in doubt, you can ask.