题目内容
(12分)一根长为L的丝线吊着一质量为m的带电量为q的小球静止在水平向右的匀强电场中,如图13所示,丝线与竖直方向成37o角,现突然将该电场方向变为向下且大小不变,不考虑因电场的改变而带来的其他影响,重力加速度为g,sin370=0.6,cos370=0.8,求:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241445328611387.png)
(1)匀强电场的电场强度的大小;
(2)求小球经过最低点时丝线拉力的大小。
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241445328611387.png)
(1)匀强电场的电场强度的大小;
(2)求小球经过最低点时丝线拉力的大小。
(1)由
(2) ![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824144532970782.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824144532908766.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824144532970782.png)
试题分析:(1)由平衡条件得:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824144533048846.png)
解得
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824144532908766.png)
(2) 电场方向变成向下后,小球开始摆动做圆周运动
由动能定理:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241445331891362.png)
在最低点时,
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241445332511043.png)
解得:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824144532970782.png)
点评:求解电场与重力场的复合场问题时可转化为等效重力场,明确最低点沿半径方向的合力提供向心力是本题的关键
![](http://thumb2018.1010pic.com/images/loading.gif)
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