题目内容
以0点为圆心的圆形空间内,存在着垂直于纸面向里的匀强磁场,磁感应强度为B.一带电量为e的电子从A点以速度V垂直于磁场方向射人磁场中,并从B点射出,∠AOB=120°,如图所示,已知电子质量为m,则电子在磁场中运动的时间为( )![](http://thumb.1010pic.com/pic6/res/gzwl/web/STSource/20131028203103864640338/SYS201310282031038646403003_ST/images0.png)
A.
![](http://thumb.1010pic.com/pic6/res/gzwl/web/STSource/20131028203103864640338/SYS201310282031038646403003_ST/0.png)
B.
![](http://thumb.1010pic.com/pic6/res/gzwl/web/STSource/20131028203103864640338/SYS201310282031038646403003_ST/1.png)
C.
![](http://thumb.1010pic.com/pic6/res/gzwl/web/STSource/20131028203103864640338/SYS201310282031038646403003_ST/2.png)
D.
![](http://thumb.1010pic.com/pic6/res/gzwl/web/STSource/20131028203103864640338/SYS201310282031038646403003_ST/3.png)
【答案】分析:带电粒子在磁场中做匀速圆周运动,由几何关系可求出圆心角,根据
的关系可求得运动的时间.
解答:解:由图可知,粒子转过的圆心角为60°,
带电粒子在磁场中做匀速圆周运动,洛伦兹力提供向心力其运动的周期:
![](http://thumb.1010pic.com/pic6/res/gzwl/web/STSource/20131028203103864640338/SYS201310282031038646403003_DA/1.png)
又:![](http://thumb.1010pic.com/pic6/res/gzwl/web/STSource/20131028203103864640338/SYS201310282031038646403003_DA/2.png)
所以:
;故B选项正确,ACD选项都错误.
故选:B.
点评:本题用周期来求时间,根据
的关系是解决该题的关键.属于简单题.
![](http://thumb.1010pic.com/pic6/res/gzwl/web/STSource/20131028203103864640338/SYS201310282031038646403003_DA/0.png)
解答:解:由图可知,粒子转过的圆心角为60°,
带电粒子在磁场中做匀速圆周运动,洛伦兹力提供向心力其运动的周期:
![](http://thumb.1010pic.com/pic6/res/gzwl/web/STSource/20131028203103864640338/SYS201310282031038646403003_DA/1.png)
又:
![](http://thumb.1010pic.com/pic6/res/gzwl/web/STSource/20131028203103864640338/SYS201310282031038646403003_DA/2.png)
所以:
![](http://thumb.1010pic.com/pic6/res/gzwl/web/STSource/20131028203103864640338/SYS201310282031038646403003_DA/3.png)
故选:B.
点评:本题用周期来求时间,根据
![](http://thumb.1010pic.com/pic6/res/gzwl/web/STSource/20131028203103864640338/SYS201310282031038646403003_DA/4.png)
![](http://thumb2018.1010pic.com/images/loading.gif)
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