题目内容
(15分)如图所示,在直角坐标系xoy的第一、四象限区域内存在两个有界的匀强磁场:垂直纸面向外的匀强磁场Ⅰ、垂直纸面向里的匀强磁场Ⅱ,O、M、P、Q为磁场边界和x轴的交点,OM=MP=L.在第三象限存在沿y轴正向的匀强电场. 一质量为
带电量为
的带电粒子从电场中坐标为(-2L,-L)的点以速度v0沿+x方向射出,恰好经过原点O处射入区域Ⅰ又从M点射出区域Ⅰ(粒子的重力忽略不计)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250018336066502.png)
(1)求第三象限匀强电场场强E的大小;
(2)求区域Ⅰ内匀强磁场磁感应强度B的大小;
(3)如带电粒子能再次回到原点O,问区域Ⅱ内磁场的宽度至少为多少?粒子两次经过原点O的时间间隔为多少?
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001833574333.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001833590330.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250018336066502.png)
(1)求第三象限匀强电场场强E的大小;
(2)求区域Ⅰ内匀强磁场磁感应强度B的大小;
(3)如带电粒子能再次回到原点O,问区域Ⅱ内磁场的宽度至少为多少?粒子两次经过原点O的时间间隔为多少?
(1)
(2)
(3)![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001833668481.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250018336211516.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001833637389.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001833668481.png)
试题分析:(1)带电粒子在匀强电场中做类平抛运动:粒子沿x轴正方向的位移:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001833684258.png)
粒子沿电场方向的位移:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001833699406.png)
联立①②式得:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250018336211516.png)
(2)设到原点时带电粒子的竖直分速度为
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001833715215.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001833746436.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001833746430.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001833777392.png)
则合速度:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001833793275.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001833793200.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001833824226.png)
粒子进入区域Ⅰ做匀速圆周运动,由几何知识可得:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001833840319.png)
由洛伦兹力充当向心力:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001833855326.png)
可解得:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001833637389.png)
(3)运动轨迹如图,在区域Ⅱ做匀速圆周的半径为:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001833886281.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250018339021535.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001833933383.png)
运动时间:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001833949466.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001833964393.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001833996481.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001833668481.png)
![](http://thumb2018.1010pic.com/images/loading.gif)
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