题目内容
(10分)两块水平金属极板A、B正对放置,每块极板长均为l、极板间距为d.B板接地(电势为零)、A板电势为+U,重力加速度为g.两个比荷(电荷量与质量的比值)均为
的带正电质点以相同的初速沿A、B板的中心线相继射入,如图所示.第一个质点射入后恰好落在B板的中点处.接着,第二个质点射入极板间,运动一段时间
后, A板电势突然变为
并且不再改变,结果第二个质点恰好没有碰到极板.求:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241519496622882.jpg)
(1)带电质点射入时的初速
.
(2)在A板电势改变之前,第二个质点在板间运动的时间
.
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151949382752.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151949428374.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151949584338.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241519496622882.jpg)
(1)带电质点射入时的初速
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151949725325.png)
(2)在A板电势改变之前,第二个质点在板间运动的时间
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151949428374.png)
(1)
(2)![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151950177996.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151949912827.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151950177996.png)
试题分析:(1)第一个质点在极板间做类平抛运动
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151950286869.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151950380609.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151950552741.png)
解得质点的初速
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151949912827.png)
(2)第二个质点射入极板后,在时间
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151949428374.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151950801605.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151950864763.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151951004585.png)
A板电势突然变为
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151949584338.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151951254292.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151951363656.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151951488850.png)
由以上各式解得
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151950177996.png)
点评::带电粒子在电场中的运动,综合了静电场和力学的知识,分析方法和力学的分析方法基本相同.先分析受力情况再分析运动状态和运动过程(平衡、加速、减速,直 线或曲线),然后选用恰当的规律解题.解决这类问题的基本方法有两种,第一种利用力和运动的观点,选用牛顿第二定律和运动学公式求解;第二种利用能量转化 的观点,选用动能定理和功能关系求解.
![](http://thumb2018.1010pic.com/images/loading.gif)
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