题目内容
如图1所示,A、B两块金属板水平放置,相距
,两板间加有一周期性变化的电压,当B板接地时,A板电势
随时间t变化的情况如图2所示。在两板间的电场中,将一带负电的粒子从B板中央处由静止释放,若该带电粒子受到的电场力为重力的两倍,要使该粒子能够达到A板,交变电压的周期至少为多大。(g取
)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241152158642527.gif)
图1 图2
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115215833446.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115215833225.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115215848411.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241152158642527.gif)
图1 图2
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115215880459.gif)
设电场力为F,则
,得
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115215911679.gif)
前半周期上升高度:
,后半周期先减速上升,后加速下降,其加速度:
得![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115215958570.gif)
减速时间为
则
,![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115216004295.gif)
此段时间内上升高度:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115216020809.gif)
则上升的总高度:![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115216036561.gif)
后半周期的
时间内,粒子向下加速运动,下降的高度:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115216067779.gif)
上述计算说明,在一个周期内上升
,再回落
,且具有向下的速度。
如果周期小,粒子不能到达A板。设周期为T,上升的高度
则:
,
。
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115215895482.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115215911679.gif)
前半周期上升高度:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115215926706.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115215942492.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115215958570.gif)
减速时间为
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115215958199.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115215989492.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115216004295.gif)
此段时间内上升高度:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115216020809.gif)
则上升的总高度:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115216036561.gif)
后半周期的
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115216051477.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115216067779.gif)
上述计算说明,在一个周期内上升
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115216082298.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115216082298.gif)
如果周期小,粒子不能到达A板。设周期为T,上升的高度
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115216114576.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115216129619.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115215880459.gif)
![](http://thumb2018.1010pic.com/images/loading.gif)
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