题目内容
(18 分)如图所示,在平面直角坐标系第Ⅲ象限内充满+y 方向的匀强电场, 在第Ⅰ象限的某个圆形区域内有垂直于纸面的匀强磁场(电场、磁场均未画出);一个比荷为
的带电粒子以大小为 v 0的初速度自点
沿+x 方向运动,恰经原点O进入第Ⅰ象限,粒子穿过匀强磁场后,最终从 x轴上的点 Q(9 d,0 )沿-y 方向进入第Ⅳ象限;已知该匀强磁场的磁感应强度为
,不计粒子重力。
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250047341794843.png)
(1)求第Ⅲ象限内匀强电场的场强E的大小;
(2) 求粒子在匀强磁场中运动的半径R及时间t B;
(3) 求圆形磁场区的最小半径rm。
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825004734101608.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825004734147777.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825004734163673.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250047341794843.png)
(1)求第Ⅲ象限内匀强电场的场强E的大小;
(2) 求粒子在匀强磁场中运动的半径R及时间t B;
(3) 求圆形磁场区的最小半径rm。
(1)
(2)
(3)d
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825004734225670.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825004734241679.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825004734444801.png)
试题分析:⑴粒子在第Ⅲ象限做类平抛运动:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825004734459684.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825004734491653.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825004734506668.png)
解得:场强
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250047345221086.png)
(2)设粒子到达O点瞬间,速度大小为
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825004734553265.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825004734569266.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825004734584310.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250047346004903.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825004734615756.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825004734647943.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825004734662896.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825004734678553.png)
粒子在磁场中,洛伦兹力提供向心力:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825004734693761.png)
解得,粒子在匀强磁场中运动的半径
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250047347091000.png)
在磁场时运动角度:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825004734725812.png)
在磁场时运动时间
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825004734740996.png)
(3)如图,若粒子进入磁场和离开磁场的位置恰位于磁场区的某条直径两端,可求得磁场区的最小半径
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825004734756846.png)
解得:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825004734787887.png)
![](http://thumb2018.1010pic.com/images/loading.gif)
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