题目内容
如图,宽度为d的有界匀强磁场,磁感应强度为B。MM/和NN/是它的两条边界。现有质量为m,电量为e的电子沿图示方向垂直磁场射入,要使粒子不从边界NN’射出,电子入射速率的最大值是多少?![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241536175031891.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241536175031891.png)
V=2dqB/(
+2)m
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824153617565344.png)
试题分析:恰好没有从NN’射出的临界情况如图所示
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241536176597788.jpg)
有(d-R)/R=cos45° (3分)
得出 R=2d/(
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824153617565344.png)
又 R=MV/qB (3分)
代入有速度最大值
V=2dqB/(
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824153617565344.png)
点评:难度中等,粒子在有界磁场中的运动问题,首先应画出运动轨迹,巧妙应用对称性和几何关系,先找圆心,后求半径
![](http://thumb2018.1010pic.com/images/loading.gif)
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