诗歌赏析
                                                                                                 次北固山下
                                                                                                     王湾
                                                                                      客路青山外，行舟绿水前。
                                                                                      潮平两岸阔，风正一帆悬。
                                                                                      海日生残夜，江春入旧年。
                                                                                      乡书何处达? 归雁洛阳边。

如图所示，水平向右、磁感应强度为B的匀强磁场中，一边长为L的正方形单匝线圈abcd绕水平中以轴OO’沿逆时针方向以角速度 匀速转动，OO’与磁场方向垂直。线圈的两端与磁场外的电阻相连组成闭合电路，则(   )