题目内容
如图8所示,直线MN的上侧有垂直于纸面向里的有界匀强磁场,磁场的上边界与MN平行,磁感应强度为B. 质量为m,带电量为-q的粒子以速度v从MN上的p点射入磁场,入射方向与MN间夹角为θ.沿MN方向磁场范围足够大,要使粒子能从MN上的某点射出磁场,磁场上边界到下边界MN的距离至少应为多大?![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241159491701234.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241159491701234.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241159492011512.jpg)
粒子在磁场中的运动轨迹如图所示,设轨道半径为R,则有
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241159492322781.gif)
(4分)
(1分)
设上、下边界的最小距离为d,由图可知
(5分)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241159492322781.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115949248540.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115949263504.gif)
设上、下边界的最小距离为d,由图可知
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115949279879.gif)
![](http://thumb2018.1010pic.com/images/loading.gif)
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