题目内容
如图所示,在直角坐标系xoy的原点O处有一放射源S,放射源S在xOy平面内均匀发射速度大小相等的正电粒子,位于y轴的右侧垂直于x轴有一长度为L的很薄的荧光屏MN,荧光屏正反两侧均涂有荧光粉,MN与x轴交于O'点。已知三角形MNO为正三角形,放射源S射出的粒子质量为m,带电荷量为q,速度大小为v,不计粒子的重力。
(1)若只在y轴右侧加一平行于x轴的匀强电场,要使y轴右侧射出的所有粒子都能打到荧光屏MN上,试求电场强度的最小值Emin及此条件下打到荧光屏M点的粒子的动能;
(2)若在xOy平面内只加一方向垂直纸面向里的匀强磁场,要使粒子能打到荧光屏MN的反面 O'点,试求磁场的磁感应强度的最大值Bmax;
(3)若在xOy平面内只加一方向垂直纸面向里的匀强磁场,磁感应强度与(2)题中所求Bmax相同,试求粒子打在荧光屏MN的正面O'点所需的时间t1和打在荧光屏MN的反面O'点所需的时间t2之比。![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241356590573562.jpg)
(1)若只在y轴右侧加一平行于x轴的匀强电场,要使y轴右侧射出的所有粒子都能打到荧光屏MN上,试求电场强度的最小值Emin及此条件下打到荧光屏M点的粒子的动能;
(2)若在xOy平面内只加一方向垂直纸面向里的匀强磁场,要使粒子能打到荧光屏MN的反面 O'点,试求磁场的磁感应强度的最大值Bmax;
(3)若在xOy平面内只加一方向垂直纸面向里的匀强磁场,磁感应强度与(2)题中所求Bmax相同,试求粒子打在荧光屏MN的正面O'点所需的时间t1和打在荧光屏MN的反面O'点所需的时间t2之比。
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241356590573562.jpg)
(1)
(2)
(3)
∶
=1∶2
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824135659088649.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824135659306869.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824135659416292.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824135659696327.png)
(1)所加电场电场强度的最小值
对应沿着
轴正方向射出的带电粒子正好打在荧光屏的端点
这一临界状态。对该临界态的粒子有
,
(3分)
其中
为该粒子运动的时间, 解得
(2分)
对此时从
射出能打到荧光屏上的任一粒子(包括打到荧光屏
点的粒子),设它到达屏时的动能为
,由动能定理有
-
(2分)
解得
=
(2分)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241357006485064.jpg)
(2)由题意,所加磁场的最大磁感应强度
对应来自
的粒子恰好经过荧光屏下端点N后打到
这一临界状态,如图所示(圆心在
)。从图中的几何关系得,粒子在磁场中做圆周运动的半径
满足
(2分)
粒子在磁场中做匀速圆周运动,洛伦兹力提供向心力,有
(2分)
联立解得
(1分)
(3)打在荧光屏正面O点的粒子的圆弧轨迹见图(圆心在
),根据匀速圆周运动规律有
,
(2分)
由图中的几何关系得
(2分)
联立解得
∶
=1∶2 (1分)
本题考查了带电粒子在磁场电场中运动,选好临界状态最关键,当加电场时所加电场电场强度的最小值
对应沿着
轴正方向射出的带电粒子正好打在荧光屏的端点
这一临界状态,当加磁场时所加磁场的最大磁感应强度
对应来自
的粒子恰好经过荧光屏下端点N后打到
这一临界状态
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824135659759458.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824135659821310.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824135659852399.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824135659915536.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241356599621116.png)
其中
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824135700024267.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241357000711059.png)
对此时从
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824135700102321.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824135659852399.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824135700274387.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824135700274387.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241357005081115.png)
解得
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824135700274387.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824135659088649.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241357006485064.jpg)
(2)由题意,所加磁场的最大磁感应强度
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824135700695451.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824135700102321.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824135701038334.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824135701132337.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824135701225303.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824135701334508.png)
粒子在磁场中做匀速圆周运动,洛伦兹力提供向心力,有
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824135701662917.png)
联立解得
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824135659306869.png)
(3)打在荧光屏正面O点的粒子的圆弧轨迹见图(圆心在
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824135701896371.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824135701943534.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824135702021710.png)
由图中的几何关系得
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824135702395576.png)
联立解得
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824135659416292.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824135659696327.png)
本题考查了带电粒子在磁场电场中运动,选好临界状态最关键,当加电场时所加电场电场强度的最小值
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824135659759458.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824135659821310.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824135659852399.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824135700695451.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824135700102321.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824135701038334.png)
![](http://thumb2018.1010pic.com/images/loading.gif)
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