题目内容
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),求:
(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的拉力.![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241226335393718.jpg)
(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的拉力.
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241226335393718.jpg)
(1) 小球静止在电场中受力如图:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241226335542900.jpg)
显然小球带正电, 由平衡条件得:
-------------2分
故![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824122633586218.gif)
-------------1分
(2) 电场方向变成向下后,小球开始摆动做圆周运动,重力、电场力对小球做正功。
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241226337102957.jpg)
由动能定理:![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824122633726909.gif)
-------------2分
由圆周运动知识,在最低点时,
-------------2分
由③,④得:![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824122633773210.gif)
-------------1分
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241226335542900.jpg)
显然小球带正电, 由平衡条件得:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824122633570563.gif)
故
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824122633586218.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824122633710447.gif)
(2) 电场方向变成向下后,小球开始摆动做圆周运动,重力、电场力对小球做正功。
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241226337102957.jpg)
由动能定理:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824122633726909.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/2014082412263374273.gif)
由圆周运动知识,在最低点时,
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824122633757748.gif)
由③,④得:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824122633773210.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824122633788481.gif)
略
![](http://thumb2018.1010pic.com/images/loading.gif)
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