题目内容
如图所示,在水平正交的匀强电场和匀强磁场区域内,有一个带电小球A,已知电场强度为E,磁感应强度为B,小球在复合场区域中受到的电场力大小恰与它的重力大小相等,要使小球在电磁场中匀速运动,则小球的速度大小等于________,方向为____________.![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241425059197086.jpg)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241425059197086.jpg)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824142505935573.png)
小球带正电时,磁场力的大小等于重力与电场力的合力,方向如图(甲)所示.
则![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241425060441040.png)
根据左手定则,小球速度的方向与磁场力垂直,与E的正向成45°角;同理,小球带负电时,亦有
方向与E正向成135°角(如图乙所示).
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/2014082414250634018843.png)
则
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241425060441040.png)
根据左手定则,小球速度的方向与磁场力垂直,与E的正向成45°角;同理,小球带负电时,亦有
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824142506278686.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/2014082414250634018843.png)
![](http://thumb2018.1010pic.com/images/loading.gif)
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