题目内容
(14分)如图所示的区域中,左边为垂直纸面向里的匀强磁场,磁感应强度为 B ,右边是一个电场强度大小未知的匀强电场,其方向平行于OC且垂直于磁场方向.一个质量为m 、电荷量为-q 的带电粒子从P孔以初速度v0沿垂直于磁场方向进人匀强磁场中,初速度方向与边界线的夹角θ=600,粒子恰好从C孔垂直于OC射入匀强电场,最后打在Q点且OQ间距是OC间距的2倍,不计粒子的重力,求:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250034294049387.jpg)
(1)电场强度E的大小
(2)粒子从P运动到Q所用的时间 t
(3)粒子到达Q点时的动能EKQ
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250034294049387.jpg)
(1)电场强度E的大小
(2)粒子从P运动到Q所用的时间 t
(3)粒子到达Q点时的动能EKQ
(1)
(2)
(3)![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825003429450738.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825003429419671.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250034294351007.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825003429450738.png)
试题分析:(1)粒子在电场中做类平抛运动
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250034294661170.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825003429482617.png)
由题意知
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825003429497690.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825003429419671.png)
(2)画出粒子运动的轨迹如图示的三分之一圆弧(O1为粒子在磁场中圆周运动的圆心):
∠PO1C=120°
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250034295285026.png)
设粒子在磁场中圆周运动的半径为r,
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250034295601085.png)
有几何关系得r+rcos 60°= OC=x O C="x=3r/2" 2分
粒子在磁场中圆周运动的时间
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250034295751008.png)
粒子在电场中类平抛运动 O Q=2x=3r 1分
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250034295911060.png)
粒子从P运动到Q所用时间
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250034296061174.png)
(3)由动能定理:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250034296221022.png)
解得
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825003429638730.png)
![](http://thumb2018.1010pic.com/images/loading.gif)
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