题目内容
在竖直平面内固定一半径为
的金属细圆环,质量为
的金属小球(视为质点)通过长为
的绝缘细线悬挂在圆环的最高点.当圆环、小球都带有相同的电荷量
(未知)时,发现小球在垂直圆环平面的对称轴上处于平衡状态,如图所示.已知静电力常量为
,则有( )
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250010472272406.jpg)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001047133303.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001047149337.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001047165280.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001047180333.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001047196312.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250010472272406.jpg)
A.绳对小球的拉力![]() |
B.电荷量![]() |
C.绳对小球的拉力![]() |
D.电荷量![]() |
AB
试题分析:由于圆环不能看作点电荷,我们取圆环上一部分
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001047305383.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001047180333.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001047336740.png)
由库仑定律可得,该部分对小球的库仑力
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250010473521063.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001047367333.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250010473996810.jpg)
两力的合力应沿圆心与小球的连线向外,大小为:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250010474141936.png)
因圆环上各点对小球均有库仑力,故所有部分库仑力的合力
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250010474301397.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001047445685.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001047477695.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001047492891.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250010475081350.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825001047258904.png)
![](http://thumb2018.1010pic.com/images/loading.gif)
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