题目内容
如图所示,一束电子(电量为e)以速度v垂直射入磁感应强度为B、宽度为d的匀强磁场,穿透磁场时的速度与电子原来的入射方向的夹角为600。求:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241512521123261.png)
(1) 电子的质量m (2) 电子在磁场中的运动时间t
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241512521123261.png)
(1) 电子的质量m (2) 电子在磁场中的运动时间t
(1)
(2)![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151252284756.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151252221775.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151252284756.png)
试题分析:(1)电子垂直射入匀强磁场中,只受洛伦兹力作用做匀速圆周运动,画出轨迹如图所示,
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241512523307063.jpg)
由几何关系,得:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151252393781.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151252471763.png)
由牛顿第二定律,得
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151252533778.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241512525961109.png)
(2)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151252720717.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151252783658.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151252876862.png)
点评:中等难度。关键要画出轨迹,根据圆心角求时间,由几何知识求半径是常用方法。
![](http://thumb2018.1010pic.com/images/loading.gif)
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