题目内容
在半径为R的圆形区域内有垂直于纸面向内的匀强磁场,磁感应强度为B,一质量为
、电荷量为
的负电荷,以速率
从A点垂直于磁场方向正对着磁场的圆心射入后从C点射出,已知角AOC为1200,则该粒子在磁场中的运动时间为:( )
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241230201603871.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824123019911346.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824123020129314.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824123020145273.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241230201603871.png)
A.![]() | B.![]() |
C.![]() | D.![]() |
AB
分析:带电粒子在磁场中由洛伦兹力提供向心力,使粒子做匀速圆周运动.从而可推导出轨道半径公式与周期公式,由题中运动的时间与磁感应强度可求出粒子的比荷,由于圆磁场的半径未知,所以无法求出轨道半径,也不能算出粒子的初速度.
解答:解:带电粒子在洛伦兹力作用下做匀速圆周运动,则有:Bqv=m![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824123020348429.png)
由几何关系可得:R=rtan30°
再由周期公式可得:T=
,而运动时间为:t=
T=![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824123020457690.png)
或者t=
×![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824123020566553.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824123020207653.png)
故选:AB.
点评:带电粒子在磁场中运动的题目解题步骤为:定圆心、画轨迹、求半径,同时还利用圆弧的几何关系来帮助解题.值得注意是圆形磁场的半径与运动轨道的圆弧半径要区别开来.
解答:解:带电粒子在洛伦兹力作用下做匀速圆周运动,则有:Bqv=m
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824123020348429.png)
由几何关系可得:R=rtan30°
再由周期公式可得:T=
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824123020363682.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824123020379338.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824123020457690.png)
或者t=
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824123020535413.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824123020566553.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824123020207653.png)
故选:AB.
点评:带电粒子在磁场中运动的题目解题步骤为:定圆心、画轨迹、求半径,同时还利用圆弧的几何关系来帮助解题.值得注意是圆形磁场的半径与运动轨道的圆弧半径要区别开来.
![](http://thumb2018.1010pic.com/images/loading.gif)
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