题目内容
在相互垂直的匀强电场和匀强磁场中,有一倾角为θ、足够长的光滑绝缘斜面,磁感应强度为B,方向垂直纸面向外,电场方向竖直向上.有一质量为m、带电荷量为+q的小球静止在斜面顶端,这时小球对斜面的正压力恰好为零,如下图所示,若迅速把电场方向反转为竖直向下,
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241454004552566.jpg)
问:(1)小球能在斜面上连续滑行多远?
(2)所用时间是多少?
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241454004552566.jpg)
问:(1)小球能在斜面上连续滑行多远?
(2)所用时间是多少?
(1)
(2)![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824145400907715.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241454008141048.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824145400907715.png)
试题分析:电场反转前:mg=qE①
电场反转后,小球先沿斜面向下做匀加速直线运动,到对斜面压力减为零时开始离开斜面,此时有
qvB=(mg+qE)cos θ②
小球在斜面上滑行距离为:x=
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824145401079338.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824145401079338.png)
a=2g sin θ④
联立①②③④得x=
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241454008141048.png)
所用时间为t=
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824145400907715.png)
点评:分析带电粒子在复合场中的运动要注意根据左手定则分析洛伦兹力的大小以及方向,本题中随着速度的增大洛伦兹力则逐渐增大,当小球与斜面脱离时明确洛伦兹力与重力的关系,这是求解本题的关键,本题难度中等
![](http://thumb2018.1010pic.com/images/loading.gif)
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