题目内容
竖直放置的两块足够长的平行金属板间有匀强电场.其电场强度为E,在该匀强电场中,用绝缘丝线悬挂质量为m的带电小球,丝线跟竖直方向成θ=30o角时小球恰好平衡,且此时与右板的距离为b,如图所示.已知重力加速度为g,求:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250016273703676.jpg)
(1)小球带电荷量是多少?
(2)若剪断丝线,小球碰到金属板需多长时间?
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250016273703676.jpg)
(1)小球带电荷量是多少?
(2)若剪断丝线,小球碰到金属板需多长时间?
(1)
(2)![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001627417641.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001627401669.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001627417641.png)
试题分析:(1)由于小球处于平衡状态知小球带正电,对小球受力分析如图所示
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250016274321811.jpg)
则 FTsin θ=qE ①
FTcos θ=mg ②
由
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001627448236.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001627479609.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001627495757.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001627401669.png)
(2)由第(1)问中的方程②知FT=
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001627526643.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001627526643.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001627573544.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001627588541.png)
小球由静止开始沿着丝线拉力的反方向做匀加速直线运动,当碰到金属板上时,它的位移为x=
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001627604535.png)
又由x=
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001627620338.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001627635552.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001627651899.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001627682771.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001627417641.png)
![](http://thumb2018.1010pic.com/images/loading.gif)
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