题目内容
一束初速度不计的电子流在经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,若板间距离d =1.0cm,板长L =5.0cm,则
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241504351263174.png)
(1)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?
(2)若在偏转电场右侧距极板右边缘X ="2.5" cm处放置一半径0.5 cm的光屏(中线过光屏中心且与光屏垂直),要使电子能从平行板间飞出,且打到光屏上,则两个极板上最多能加多大电压?
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241504351263174.png)
(1)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?
(2)若在偏转电场右侧距极板右边缘X ="2.5" cm处放置一半径0.5 cm的光屏(中线过光屏中心且与光屏垂直),要使电子能从平行板间飞出,且打到光屏上,则两个极板上最多能加多大电压?
(1)400V(2)两个极板最多能加200V的电压
试题分析:(1)加速电场:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824150435204785.png)
偏转电场:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824150435298168.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824150435501443.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824150435579603.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824150435641497.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824150435782477.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824150435984589.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824150436187891.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824150436296883.png)
两个极板最多能加400V的电压
(2)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824150436390736.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824150436452407.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824150436499972.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241504366081056.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824150436655714.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824150436718544.png)
两个极板最多能加200V的电压
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241504369837105.jpg)
点评:在列式计算时应注意不要提前代入数值,应将公式简化后再计算,这样可以减少计算量.
![](http://thumb2018.1010pic.com/images/loading.gif)
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