题目内容
(15分)如图所示,一正方形线圈从某一高度自由下落,恰好匀速进入其下方的匀强磁场区域.已知正方形线圈质量为m,边长为L,电阻为R,匀强磁场的磁感应强度为B,高度为2L,求:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250035053932364.png)
(1)线圈进入磁场时回路产生的感应电流I1的大小和方向;
(2)线圈离开磁场过程中通过横截面的电荷量q;
(3)线圈下边缘刚离开磁场时线圈的速度v的大小.
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250035053932364.png)
(1)线圈进入磁场时回路产生的感应电流I1的大小和方向;
(2)线圈离开磁场过程中通过横截面的电荷量q;
(3)线圈下边缘刚离开磁场时线圈的速度v的大小.
(1)
逆时针 (2)
(3)![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250035054551164.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825003505408673.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825003505440892.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250035054551164.png)
试题分析:(1)线圈进入磁场时匀速,有
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825003505471594.png)
且
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825003505486610.png)
所以
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825003505408673.png)
方向:逆时针 (1分)
(2)在线圈离开磁场的过程中:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825003505533543.png)
又
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825003505549918.png)
所以:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825003505440892.png)
(3)线圈刚进入磁场时:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825003505596601.png)
而:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825003505611552.png)
所以,线圈刚进入磁场时的速度
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825003505642825.png)
从线圈完全进入磁场到线圈下边缘刚离开磁场的过程中,线圈做匀加速运动
所以:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825003505658647.png)
所以:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250035054551164.png)
![](http://thumb2018.1010pic.com/images/loading.gif)
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