题目内容
如图所示.离子源从小孔发射出带电量为e的正离子(初速可忽略),在加速电压U的作用下,沿MO方向进入匀强磁场中,磁场限制在以O为圆心,半径为r的区域内,磁感应强度为B,方向垂直纸面向外,离子从N点射出,已知
(不计重力),则正离子质量为多少?正离子通过磁场所需的时间为多少?
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241247575776642.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824124757546819.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241247575776642.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241247576081022.png)
:(1)粒子在匀强电场中被加速,由动能定理可得:qU=
mv2
由洛伦兹力提供向心力,则有:qvB=
根据几何知识,由图可见:R=R0cot30°
解得m=![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824124757686611.png)
(2)由图所示,离子飞出磁场,偏转60°角,故在磁场中飞行时间为
,故有:t=![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824124757733593.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824124757640338.png)
由洛伦兹力提供向心力,则有:qvB=
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824124757671524.png)
解得m=
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824124757686611.png)
(2)由图所示,离子飞出磁场,偏转60°角,故在磁场中飞行时间为
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824124757702410.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824124757733593.png)
![](http://thumb2018.1010pic.com/images/loading.gif)
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