题目内容
一个质量m=0.1g的小滑块,带有q=5×10-4C的电荷放置在倾角 α=30°光滑斜面上(绝缘),斜面置于B=0.5T的匀强磁场中,磁场方向垂直纸面向里,如图所示,小滑块由静止开始沿斜面滑下,其斜面足够长,小滑块滑至某一位置时,要离开斜面。(g=10m/s2)求:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241509274553002.jpg)
(1)小滑块带何种电荷?
(2)小滑块离开斜面的瞬时速度多大?
(3)该斜面的长度至少多长?
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241509274553002.jpg)
(1)小滑块带何种电荷?
(2)小滑块离开斜面的瞬时速度多大?
(3)该斜面的长度至少多长?
(1)负电荷 (2)3.4m/s (3)
试题分析:(1)小滑块沿斜面下滑过程中,受重力mg、斜面支持力
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824150927471412.png)
(2)小滑块沿斜面下滑时,垂直斜面方向的合外力为零,有
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824150927549375.png)
当
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824150927471412.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824150927767309.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824150927861337.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824150927954466.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824150928142249.png)
(3)下滑过程中,只有重力做功,由动能定理得
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824150928266357.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824150928485332.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824150928594374.png)
斜面的长度至少应是s ="1.2" m。
点评:本题学生能熟练运用左手定则、动能定理去分析问题,可画出小滑块的受力示意图,据图求解问题。
![](http://thumb2018.1010pic.com/images/loading.gif)
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