题目内容
如图所示,两平行的光滑金属导轨安装在一光滑绝缘斜面上,导轨间距为
、足够长且电阻忽略不计,导轨平面的倾角为
,条形匀强磁场的宽度为d,磁感应强度大小为B、方向与导轨平面垂直。长度为2d的绝缘杆将导体棒和正方形的单匝线框连接在一起组成“
”型装置,总质量为m,置于导轨上。导体棒中通以大小恒为I的电流(由外接恒流源产生,图中未图出)。线框的边长为d(d <
),电阻为R,下边与磁场区域上边界重合。将装置由静止释放,导体棒恰好运动到磁场区域下边界处返回,导体棒在整个运动过程中始终与导轨垂直。重力加速度为g。
求:(1)装置从释放到开始返回的过程中,线框中产生的焦耳热Q;
(2)线框第一次穿越磁场区域所需的时间t1;
(3)经过足够长时间后,线框上边与磁场区域下边界的最大距离
。
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/2014082411370583615166.jpg)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113705758185.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113705774201.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113705790915.jpg)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113705758185.gif)
求:(1)装置从释放到开始返回的过程中,线框中产生的焦耳热Q;
(2)线框第一次穿越磁场区域所需的时间t1;
(3)经过足够长时间后,线框上边与磁场区域下边界的最大距离
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113705821208.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/2014082411370583615166.jpg)
见解析
(1)设装置由静止释放到导体棒运动到磁场下边界的过程中,作用在线框上的安培力做功为W
由动能定理![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113705852975.gif)
且
解得![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113705961606.gif)
(2)设线框刚离开磁场下边界时的速度为
,则接着向下运动
由动能定理![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113706055916.gif)
装置在磁场中运动时受到的合力
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113706070546.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/2014082411370614873.gif)
感应电动势
=Bd![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113706211193.gif)
感应电流
=![](http://thumb.1010pic.com/pic2/upload/papers/20140824/2014082411370614873.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113706273236.gif)
安培力![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113706304423.gif)
由牛顿第二定律,在t到t+
时间内,有![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113706554437.gif)
则![](http://thumb.1010pic.com/pic2/upload/papers/20140824/2014082411370614873.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241137066011121.gif)
有![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113706632739.gif)
解得![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241137066481295.gif)
(3)经过足够长时间后,线框在磁场下边界与最大距离
之间往复运动
由动能定理![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113706726788.gif)
解得![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113706757748.gif)
由动能定理
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113705852975.gif)
且
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113705852400.gif)
解得
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113705961606.gif)
(2)设线框刚离开磁场下边界时的速度为
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113706008200.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113706055225.gif)
由动能定理
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113706055916.gif)
装置在磁场中运动时受到的合力
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113706070546.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/2014082411370614873.gif)
感应电动势
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113706164189.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113706211193.gif)
感应电流
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113706226199.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/2014082411370614873.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113706273236.gif)
安培力
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113706304423.gif)
由牛顿第二定律,在t到t+
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113706320200.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113706554437.gif)
则
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/2014082411370614873.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241137066011121.gif)
有
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113706632739.gif)
解得
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241137066481295.gif)
(3)经过足够长时间后,线框在磁场下边界与最大距离
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113706694312.gif)
由动能定理
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113706726788.gif)
解得
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113706757748.gif)
![](http://thumb2018.1010pic.com/images/loading.gif)
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