题目内容
如图所示,电子自静止开始经从M、N板间的电场加速后从A点垂直于磁场边界射入宽度为d的匀强磁场中,两板间的电压为U,电子离开磁场时的位置P偏离入射方向的距离为L,在距离磁场边界S处有屏幕N,电子射出磁场后打在屏上。(已知d>L,电子的质量为m,电荷量为e)求:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250017331712992.jpg)
(1)电子进入磁场的速度大小。
(2)匀强磁场的磁感应强度
(3)电子打到屏幕上的点距中心O点的距离是多少?
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250017331712992.jpg)
(1)电子进入磁场的速度大小。
(2)匀强磁场的磁感应强度
(3)电子打到屏幕上的点距中心O点的距离是多少?
(1)
(2)
(3)![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001733218775.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001733187773.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250017332021045.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001733218775.png)
试题分析:(1)作电子经电场和磁场中的轨迹图,如图所示,
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250017332495260.jpg)
设电子在M、N两板间经电场加速后获得的速度为
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001733265265.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001733280764.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001733187773.png)
(2)电子进入磁场后做匀速圆周运动,设其半径为r,则:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001733311735.png)
由几何关系得:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001733327695.png)
联立求解①②③式得:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250017333431139.png)
(3)离开磁场后做匀速直线运动如下图
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250017333589532.jpg)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250017333742628.png)
![](http://thumb2018.1010pic.com/images/loading.gif)
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