题目内容
如图所示是质谱仪工作原理的示意图.带电粒子a、b经电压U加速(在A点的初速度为零)后,进入磁感应强度为B的匀强磁场做匀速圆周运动,最后分别打在感光板S上的x1、x2处.图中半圆形的虚线分别表示带电粒子a、b所通过的路径,则( )
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241256494912051.png)
A.a的质量一定小于b的质量 |
B.a的电荷量一定大于b的电荷量 |
C.在磁场中a运动的时间大于b运动的时间 |
D.a的比荷大于b的比荷 |
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241256494912051.png)
D
设粒子经电场加速后的速度大小为v,磁场中圆周运动的半径为r,电荷量和质量分别为q、m,打在感光板上的距离为S.
根据动能定理,得
qU=
mv2,v=
由qvB=m
,r=
=![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824125649616693.png)
则S=2r=![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824125649632708.png)
得到
由图,Sa<Sb,U、B相同,则![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824125649756604.png)
故选D
根据动能定理,得
qU=
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824125649522373.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824125649538545.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824125649569396.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824125649600520.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824125649616693.png)
则S=2r=
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824125649632708.png)
得到
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824125649663829.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824125649756604.png)
故选D
![](http://thumb2018.1010pic.com/images/loading.gif)
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