题目内容
如图所示,固定在水平桌面上的光滑金属框架cdef处于竖直向下磁感应强度为B0的匀强磁场中.金属杆ab与金属框架接触良好.此时abed构成一个边长为l的正方形,金属杆的电阻为r,其余部分电阻不计.
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241915351432245.png)
⑴若从t=0时刻起,磁场的磁感应强度均匀增加,每秒钟增量为k,施加一水平拉力保持金属杆静止不动,求金属杆中的感应电流.
⑵在情况⑴中金属杆始终保持不动,当t= t1秒末时,求水平拉力的大小.
⑶若从t=0时刻起,磁感应强度逐渐减小,当金属杆在框架上以恒定速度v向右做匀速运动时,可使回路中不产生感应电流.写出磁感应强度B与时间t的函数关系式.
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241915351432245.png)
⑴若从t=0时刻起,磁场的磁感应强度均匀增加,每秒钟增量为k,施加一水平拉力保持金属杆静止不动,求金属杆中的感应电流.
⑵在情况⑴中金属杆始终保持不动,当t= t1秒末时,求水平拉力的大小.
⑶若从t=0时刻起,磁感应强度逐渐减小,当金属杆在框架上以恒定速度v向右做匀速运动时,可使回路中不产生感应电流.写出磁感应强度B与时间t的函数关系式.
⑴
⑵
⑶![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824191536204716.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824191535471589.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241915358141000.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824191536204716.png)
试题分析:(1)由题得:磁感应强度B的变化率
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824191536594809.png)
由法拉第电磁感应定律知:回路中感应电动势
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241915368751016.png)
感应电流
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824191537124591.png)
(2)当
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824191537421348.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824191537811526.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824191538076587.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241915384351137.png)
(3)为了使棒中不产生感应电流,则回路中总磁通量不变.
t=0时刻,回路中磁通量为
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824191538778474.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824191539059586.png)
应有
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824191539418760.png)
则
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824191539823694.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824191539823694.png)
点评:本题根据法拉第电磁感应定律求解感应电动势,由欧姆定律和安培力公式推导安培力的表达式,是常用的方法和思路.当回路中没有感应电流产生时,回路总的磁通量应保持不变.
![](http://thumb2018.1010pic.com/images/loading.gif)
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