题目内容
(12分) 如图所示,匀强电场场强的方向与水平方向成
角,有一带电量为
,质量为
的小球,用长为
的绝缘细线悬挂于O点,当静止时,细线恰被拉成水平方向,重力加速为g,求:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250007570463701.jpg)
(1)小球的电性及匀强电场场强E的大小;
(2)若突然改变匀强电场方向,使其竖直向下,求小球摆到最低点时的速度及绳子的拉力大小。
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000756984474.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000757000310.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000757015337.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000757031280.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250007570463701.jpg)
(1)小球的电性及匀强电场场强E的大小;
(2)若突然改变匀强电场方向,使其竖直向下,求小球摆到最低点时的速度及绳子的拉力大小。
(1)
(2) ![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000757078484.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000757062599.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000757078484.png)
试题分析: (1)以小球为研究对象,根据平衡条件及电场力方向与场强方向关系知,受力情况如图所示,由于电场力方向与场强方向相同,所以小球带正电.在三角形中,有
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000757093757.png)
解得:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250007571241017.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250007571402828.jpg)
(2) 若突然改变匀强电场方向,使其竖直向下,则小球由水平位置开始做竖直平面内圆弧运动,设小球摆到最低点时的速度为v,在下摆过程由动能定理得:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000757156973.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000757171570.png)
在最低点处,根据牛顿第二定律得:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000757187949.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000757202565.png)
![](http://thumb2018.1010pic.com/images/loading.gif)
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