第22届全国中学生物理竞赛预赛的最后一题,是一个有关电磁感应的问题,问题立足于教材所学知识,但对学生的解题能力提出了较高的要求,它着重考查了学生建立物理模型、应用数学方法处理物理问题的能力,是一道体现学生综合能力的竞赛题.原题如下:如图1所示,水平放置的金属细圆环半径为 a,竖直放置的金属细圆柱(其半径比 a小得多)的端面与金属圆环的上表面在同一平面内,圆柱的细轴通过圆环的中心 O.一质量为 m、电阻为 R 的均匀导体细棒被 The 22nd session of the National Physics Physics Contest final question, is a question about electromagnetic induction, the problem based on the knowledge of textbooks, but the ability to solve problems for students put forward higher requirements, which focuses on examining the students to establish physical Model, the ability to apply mathematical methods to deal with physical problems, is a competition that reflects the comprehensive ability of students. The original title is as follows: As shown in Figure 1, the horizontal metal ring with a radius of a, vertical metal column Its radius is much smaller than a) and the upper surface of the metal ring in the same plane, the thin cylinder axis through the center of the ring O. A mass of m, the resistance of the R uniform conductor thin rod is