[问题由来]解物理习题,我们的一般习惯是根据物理问题所描绘的物理情景利用相应的物理公式建立方程,然后解出结果,这里方程的个数和未知量的个数一般都是相等的。但有些问题虽然未知量的个数比所建立的方程多但仍能求出一些物理量,这在直流电路问题中比较多见。 [Problem Origin] Solving Physics Exercises. Our general practice is to create an equation based on the physical situation depicted by the physical problem using the corresponding physical equations and then solve the result. The number of equations and the number of unknowns are generally equal . However, some problems, although the number of unknowns than the established equation but still able to find some physical quantities, which is more common in DC circuit problems.