题目内容
如图9-5所示,足够长的水平绝缘杆MN,置于足够大的垂直纸面向内的匀强磁场中,磁场的磁感强度为B,一个绝缘环P套在杆上,环的质量为
,带电量为q的正电荷,与杆间的动摩擦因数为
,若使环以初速度
向右运动,试分析绝缘环克服摩擦力所做的功。
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241137413421960.jpg)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113741217204.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113741249204.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113741264205.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241137413421960.jpg)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241137413731335.gif)
当绝缘环以初速度
向右运动时,环受重力
、洛仑兹力
及杆的弹力N。由于N的大小、方向与重力和洛仑兹力大小有关,会约束水平方向的摩擦力变化,从而使绝缘环的最终运动可能有三种情况:
(1)若开始时
,即
,由于N=0,绝缘环不受摩擦力作用,做匀速直线运动。绝缘环克服摩擦力所做的功
。
(2)若开始时
,即
,N方向向上,绝缘环受杆摩擦力作用,做加速度变小的减速运动,直至静止。绝缘环克服摩擦力所做的功
。
(3)若开始时
,即
, N方向向下,绝缘环受杆摩擦力作用,做减速直线运动,洛仑兹力
不断减小,当
时,N=0,绝缘环不受摩擦力作用,做匀速直线运动,即最终速度
。绝缘环克服摩擦力所做的功:
。
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113741264205.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113741420238.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113741420428.gif)
(1)若开始时
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113741436475.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113741451506.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113741467419.gif)
(2)若开始时
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113741483483.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113741498514.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113741514558.gif)
(3)若开始时
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113741529483.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113741545520.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113741576199.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113741436475.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113741607482.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241137413731335.gif)
![](http://thumb2018.1010pic.com/images/loading.gif)
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