题目内容
(16分)在如图所示的xoy坐标系中,y>0的区域内存在着沿y轴正方向、场强为E的匀强电场,y<0的区域内存在着垂直纸面向里、磁感应强度为B的匀强磁场.一带电粒子从y轴上的P(0,h)点以沿x轴正方向的初速度射出,恰好能通过x轴上的D(d,0)点.己知带电粒子的质量为m,带电量为-q.h、d、q均大于0.不计重力的影响.
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250012392985211.jpg)
(1)若粒子只在电场作用下直接到达D点,求粒子初速度的大小
;
(2)若粒子在第二次经过x轴时到达D点,求粒子初速度的大小![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001239313323.png)
(3)若粒子在从电场进入磁场时到达D点,求粒子初速度的大小
;
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250012392985211.jpg)
(1)若粒子只在电场作用下直接到达D点,求粒子初速度的大小
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001239313323.png)
(2)若粒子在第二次经过x轴时到达D点,求粒子初速度的大小
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001239313323.png)
(3)若粒子在从电场进入磁场时到达D点,求粒子初速度的大小
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001239313323.png)
(1)
(2)
(3)![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250012394071335.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001239360877.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250012393761081.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250012394071335.png)
试题分析:(1)若粒子只在电场作用下直接到达D点,设粒子在电场中运动的时间为t,
粒子沿x方向做匀速直线运动,则
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001239485492.png)
沿y方向做初速度为零的匀加速直线运动,则
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001239516652.png)
加速度
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001239532653.png)
粒子只在电场作用下直接到达D点的条件为
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001239547414.png)
联立以上各式,解得:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001239578990.png)
(2)若粒子在第二次经过x轴时到达D点,其轨迹如图3所示。
设粒子进入磁场的速度大小为v,v与x轴的夹角为θ,轨迹半径为R,则:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001239594589.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001239719778.png)
粒子第二次经过x轴时到达D点的条件为:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001239734731.png)
联立以上各式,解得:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250012397661187.png)
(3)若粒子在从电场进入磁场时到达D点,其轨迹如图4所示。
根据运动对称性可知:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001239797825.png)
粒子在从电场进入磁场时到达D点的条件为:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001239937974.png)
联立以上各式,解得:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250012399841460.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250012400003224.jpg)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250012400157467.jpg)
![](http://thumb2018.1010pic.com/images/loading.gif)
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