题目内容
在地面上方的真空室内,存在匀强电场和匀强磁场。已知电场强度和磁感应强度的方向是相同的,电场强度的大小E ="4N/c,B=" 0.15T。今有一个带负电的质点以v = 20m/s的速度在此区域内沿垂直于电场强度方向做匀速直线运动,求此带电质点的电量与质量之比q/m以及磁场的所有可能方向。(g=9.8m/s2)
由题意可知质点受的重力,电场力和洛伦兹力的合力必为零。由此推知三力在同一竖直平面内,如图所示由平衡条件得
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241232401863203.jpg)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824123240202591.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824123240218628.gif)
解得
代入数据得:q/m
= 1.96C/kg,
,所以
,
即磁场是沿与重力方向成
,且斜向向下的一切方向。
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241232401863203.jpg)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824123240202591.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824123240218628.gif)
解得
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824123240233624.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/2014082412324024972.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824123240264547.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824123240280489.gif)
即磁场是沿与重力方向成
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824123240311519.gif)
略
![](http://thumb2018.1010pic.com/images/loading.gif)
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