题目内容
一束电子流在经U =5000V的加速电压加速后,在距两极板等距处垂直进入平行板间的匀强电场,如图所示.若两板间距d ="1.0" cm,板长l ="5.0" cm,那么,要使电子能 从平行板间飞出,两个极板上最多能加多大电压?
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250016393353743.jpg)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250016393353743.jpg)
400V
试题分析:在加速电压一定时,偏转电压U′越大,电子在极板间的偏转距离就越大。当偏转电压大到使电子刚好擦着极板的边缘飞出,此时的偏转电压,即为题目要求的最大电压。加速过程,由动能定理得:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001639351802.png)
进入偏转电场,电子在平行于板面的方向上作匀速运动:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001639507459.png)
在垂直于板面的方向上作匀加速直线运动,加速度:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001639523873.png)
偏转距离:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001639538648.png)
能飞出的条件为:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001639554588.png)
解①~⑤式得:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250016395691961.png)
即要使电子能飞出,所加电压最大为400V
![](http://thumb2018.1010pic.com/images/loading.gif)
练习册系列答案
相关题目