题目内容
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241206206061523.gif)
如图所示,正方形线框边长L,质量M,电阻R,从高H处自由下落,当线框的下边进入高h =" L" 的有界匀强磁场时,恰好匀速下降,设线框面总与磁场垂直,求
(1) 磁感应强度B
(2) 线框经过磁场过程中所产生热Q
(3) 线框穿过该区域共需时间T
(1)
(2) mg2L (3)![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824120620637385.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824120620621683.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824120620637385.gif)
(1)mgH=
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824120620652225.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824120620668142.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824120620668315.gif)
由于线圈保持匀速离开磁场,所以重力的功率与线圈电功率相等,即
mgv=
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824120620684520.gif)
∴mgR=B2L2V
联立解B=
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824120620621683.gif)
(2)通过磁场过程,重力做功与线框生热相同
mg2L=Q
(3)T=
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824120620715263.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824120620637385.gif)
若通过力学平衡解,也可以。
![](http://thumb2018.1010pic.com/images/loading.gif)
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