[案例]这是一堂高三复习课,物理教师正在进行试题分析指导,例题选自上海市高考2000年物理卷中的一道计算题.例.如图1所示,固定于水平桌面上的金属框架cdef,处在竖直向下的匀强磁场中,金属棒ab搁在框架上,可以无摩擦滑动,此时adeb构成一个边长为l的正方形,棒的电阻为r,其余部分 [Case] ​​This is a third-grade review class. The physics teacher is conducting analysis of the test questions. The example is selected from a calculation problem in the physical volume of the Shanghai College Entrance Examination 2000. Example: As shown in Figure 1, the metal frame fixed on the horizontal desktop Cdef, in a vertically downward uniform magnetic field, the metal rod ab rests on the frame and can slide without friction. At this time, the adeb forms a square with a side of l, the resistance of the rod is r, and the rest is