题目内容
如图所示,一电子从a点以速度v垂直进入长为d、宽为h的矩形磁场区域沿曲线ab运动,且通过b点离开磁场。已知电子的质量为m,电量为e,磁场的磁感应强度为B,ab的弧长为s,不计重力,则该电子在磁场中运动的时间为 ( )
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241519294763593.jpg)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241519294763593.jpg)
A.t=![]() | B.t=![]() |
C.t=![]() ![]() | D.t=![]() ![]() |
BC
试题分析:画出粒子的轨迹,由几何知识可得
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151930724731.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151930833758.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241519309741035.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151931145946.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241519312231701.png)
因为粒子做匀速圆周运动,所以时间可用弧长与速度的比值求的,即
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151931348451.png)
点评:做此类型的问题时,需要先画出粒子的运动轨迹然后根据几何知识求解
![](http://thumb2018.1010pic.com/images/loading.gif)
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