题目内容
摆线长为L,摆球质量为m带正电量q的单摆从如图所示位置A摆下,在一个磁感应强度为B的匀强磁场中运动,摆动平面垂直磁场,若图中
,摆球从A开始运动,当它第一次到最低点时,摆线上的拉力大小为_____________.
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824124755003505.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824124755097813.png)
小球在下落过程中,只有重力做功,所以到最低点时,
,小球在最低点时,根据左手定则可得受到竖直向下的洛伦兹力,故小球受到竖直向下的重力,竖直向上的拉力,竖直向下的洛伦兹力,它们的合力充当向心力,所以![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824124755190987.png)
故两式联立可得![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824124755222716.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241247551591190.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824124755190987.png)
故两式联立可得
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824124755222716.png)
![](http://thumb2018.1010pic.com/images/loading.gif)
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