题目

如图所示，直径为R的绝缘筒中为匀强磁场区域，磁感应强度为B、磁感线垂直纸面向里，某一正离子，以速度v从圆筒上C孔处沿直径方向射入筒内，如果离子与圆筒碰撞三次（碰撞时不损失能量，且时间不计），又从C孔飞出，则离子在磁场中运动的时间为                                                                                                ．                                                                                                                                                                                                                                                              答案：．

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