题目内容
(11分)如图10所示,一质量为m、电荷量为q的带正电的小球以水平初速度v0从离地高为h的地方做平抛运动,落地点为N,不计空气阻力,求:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241414012802986.jpg)
(1)若在空间加一个竖直方向的匀强电场,使小球沿水平方向做匀速
直线运动,则场强E为多大?
(2)若在空间再加上一个垂直于纸面向外的匀强磁场,小球的落地点仍
为N,则磁感应强度B为多大?
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241414012802986.jpg)
(1)若在空间加一个竖直方向的匀强电场,使小球沿水平方向做匀速
直线运动,则场强E为多大?
(2)若在空间再加上一个垂直于纸面向外的匀强磁场,小球的落地点仍
为N,则磁感应强度B为多大?
(1)
(2) ![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824141401405926.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824141401358528.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824141401405926.png)
(1)由于小球受电场力和重力且做匀速直线运动,故qE=mg,所以E=
.
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241414015762660.jpg)
(2)再加上匀强磁场后,由于重力与电场力平衡,故小球在洛伦兹力作用下做匀速圆周
运动R=![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824141401592626.png)
由几何关系得:
R2-x2=(R-h)2
其中x=v0t=v0![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824141401966587.png)
由以上几式解得:B=
.
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824141401358528.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241414015762660.jpg)
(2)再加上匀强磁场后,由于重力与电场力平衡,故小球在洛伦兹力作用下做匀速圆周
运动R=
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824141401592626.png)
由几何关系得:
R2-x2=(R-h)2
其中x=v0t=v0
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824141401966587.png)
由以上几式解得:B=
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824141401405926.png)
![](http://thumb2018.1010pic.com/images/loading.gif)
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