一、真题再现2014年全国高考理综Ⅱ物理第25题是一道电磁感应的综合题,涉及电磁感应、电路、磁场和功能关系。题目如下:半径分别为r和2r的同心圆形导轨固定在同一水平面内,一长为r、质量为m且质量分布均匀的直导体棒AB置于圆导轨上面,BA的延长线通过圆导轨中心O,装置的俯视图如图所示。整个装置位于一匀强磁场中,磁感应强度的大小为B,方向竖直向下,在内圆导轨的C点和 First, the real question 2014 national college entrance examination science II physical 25 questions is a comprehensive induction of electromagnetic induction, involving electromagnetic induction, circuit, magnetic field and functional relationship. The topics are as follows: concentric circular rails of radius r and 2r, respectively, are fixed in the same horizontal plane, a length of r, mass m and mass distribution of the straight conductor bar AB is placed above the circular guide, BA extension through circular guide Center O, the top view of the device as shown. The whole device is located in a uniform magnetic field, the size of the magnetic flux density B, the direction of vertical down in the inner circular guide point C and