题目内容
把一线框从一匀强磁场中拉出,如图所示。第一次拉出的速率是v ,第二次拉出速率是 2 v,其它条件不变,则前后两次拉力大小之比是_________,线框产生的热量之比是_____________。![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824124412235278.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824124412235278.png)
1:2 、 1:2
解:由于线框是正方形,设每一边长为L,其电阻为R,磁场强度为B,
当线框以速率
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824124412250265.png)
拉力
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824124412562546.png)
电动势
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824124412672527.png)
拉出磁场的时间
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824124412703494.png)
线框产生的热量
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824124412718689.png)
而当线框以速率
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824124412750358.png)
拉力
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824124412765571.png)
电动势
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824124412781612.png)
拉出磁场的时间
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824124412812583.png)
线框产生的热量
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824124412874703.png)
①则前后两次拉力大小之比为
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824124412890863.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241244129211364.png)
所以,
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824124412937704.png)
②则线框产生的热量之比为
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241244129991969.png)
答案:前后两次线框拉力之比
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824124413015583.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824124413030619.png)
![](http://thumb2018.1010pic.com/images/loading.gif)
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