讲完电磁感应后 ,一位学生向笔者提出一个问题 .题目　如图 1所示 ,虚线两侧分别为磁感图 1强度 B1 、 B2 的匀强磁场 ,方向相反 .当闭合矩形金属线圈垂直于磁场方向向下运动时 ,金属线圈中是否有感应电流 ?分析　这一问题看似简单 ,其实并非容易回答 .当闭合矩形金属线圈垂直 After speaking about electromagnetic induction, a student asked a question to the author. The topic is shown in Fig. 1. The two sides of the dashed line are magnetically intensified magnetic fields with intensity B1 and B2, respectively. The directions are opposite. When the closed rectangular metal coil is perpendicular to When the direction of the magnetic field moves downwards, is there an induced current in the metal coil? The analysis of this problem seems simple, but it is not easy to answer. When the rectangular metal coil is closed vertically