题目内容
(19分)如图所示的平面直角坐标系中,在y>0的区域存在匀强电场,场强沿y轴负方向,在y<0的区域存在匀强磁场,磁场方向垂直于坐标平面向外。一电荷量为q、质量为m的带正电粒子,经过y轴上y=h处的点Pl时速率为vo,方向沿x轴正方向;然后经过x轴上x=2h处的P2点进入磁场。不计粒子重力。
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250039272244422.jpg)
(1)求电场强度的大小;
(2)若粒子进人磁场后,接着经过了y轴上y=-2h处的P3点,求磁感应强度的大小;
(3)若只改变磁场的大小(仍为匀强磁场),让粒子仍从Pl经P2沿原路径进入磁场后,为了使粒子能再次通过P2点,求磁感应强度的大小满足的条件。
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250039272244422.jpg)
(1)求电场强度的大小;
(2)若粒子进人磁场后,接着经过了y轴上y=-2h处的P3点,求磁感应强度的大小;
(3)若只改变磁场的大小(仍为匀强磁场),让粒子仍从Pl经P2沿原路径进入磁场后,为了使粒子能再次通过P2点,求磁感应强度的大小满足的条件。
(1)
(2)
(3)
或者![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250039273021214.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825003927255848.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825003927271921.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825003927271921.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250039273021214.png)
试题分析:(1)粒子运动轨迹如图甲所示
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250039273333796.jpg)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250039273493991.jpg)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250039273806179.jpg)
在电场中,
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825003927395651.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825003927427576.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825003927442652.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825003927255848.png)
(2)在电场中
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825003927629578.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825003927645817.png)
在磁场中,连接P2、P3两点,由几何关系知,
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825003927661439.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825003927676561.png)
又由
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825003927692778.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825003927271921.png)
(3)设满足条件的磁感应强度为B′,下面分两种情况进行讨论.
第一种情况:根据对称性,轨迹关于y轴对称,能过P2点的轨迹如图乙所示,由几何关系知
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825003927723597.png)
又由
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825003927739800.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250039277541010.png)
第二种情况:如果轨迹与y轴不对称,能过P2点的轨迹如图丙所示,设粒子此后在电场中偏转n次后再经过P2点,当n=1时,粒子在磁场中将偏转2次过P2点,
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250039277701163.png)
当n=2时,粒子在磁场中将偏转3次过P2点,
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250039277851225.png)
……
当粒子在电场中偏转n次时,粒子在磁场中将n+1次过P2点,
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250039278011418.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825003927817780.png)
又由
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825003927739800.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250039273021214.png)
所以过P2点的条件是
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825003927271921.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250039273021214.png)
![](http://thumb2018.1010pic.com/images/loading.gif)
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