题目内容
如图所示,在x<0且y<0的区域内存在匀强磁场,磁场方向垂直于xy平面向里.磁感应强度大小为B,在x>0且y<0的区域内存在沿y轴正方向的匀强电场. 一质量为m、电荷量为q的带电粒子从x轴上的M点沿y轴负方向垂直射入磁场,结果带电粒子从y轴的N点射出磁场而进入匀强电场,经电场偏转后打到x轴上的P点,已知
=
=
=l。
不计带电粒子所受重力,
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241549109834764.jpg)
求:(1)带电粒子进入匀强磁场时速度的大小;
(2)带电粒子从射入匀强磁场到射出匀强电场所用的时间;
(3)匀强电场的场强大小.
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824154910765440.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824154910812419.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824154910890341.png)
不计带电粒子所受重力,
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241549109834764.jpg)
求:(1)带电粒子进入匀强磁场时速度的大小;
(2)带电粒子从射入匀强磁场到射出匀强电场所用的时间;
(3)匀强电场的场强大小.
(1)
(2)
(3)![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824154911217700.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824154910999587.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824154911124874.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824154911217700.png)
试题分析:(1)设带电粒子射入磁场时的速度大小为v,由带电粒子射入匀强磁场的方向和几何关系可知,带电粒子在磁场中做圆周运动,圆心位于坐标原点,半径为l。
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824154911342762.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824154911483673.png)
(2)设带电粒子在磁场中运动时间为
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824154911639301.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824154911685336.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824154911748941.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824154911888946.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824154911951786.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241549120751142.png)
(3)带电粒子在电场中做类平抛运动
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824154912185654.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824154912247525.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824154912309652.png)
联立解得
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824154912481811.png)
点评:这类问题的解决思路一般是在电场中做类平抛运动,在磁场中洛伦兹力做向心力,做匀速圆周运动,根据几何关系找出圆心角来求周期,找到半径来求磁场。
![](http://thumb2018.1010pic.com/images/loading.gif)
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