题目内容
如图14所示,在坐标系xoy的第一象限内存在匀强磁场,磁场方向垂直于xoy面向里;第四象限内有沿y轴正方向的匀强电场,电场强度大小为E。一质量为m、带电荷量为
的粒子自y轴的P点沿x轴正方向射入第四象限,经x轴上的Q点进入第一象限。已知P点坐标为(0,-2),Q点坐标为(4,0),不计粒子重力。求:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250029328472380.jpg)
(1)求粒子过Q点时速度的大小。
(2)若磁感应强度的大小为一定值B,粒子将以垂直y轴的方向经H点进入第二象限,求B的大小及H点的坐标值;
(3)求粒子在第一象限内运动的时间t。
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002932816236.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250029328472380.jpg)
(1)求粒子过Q点时速度的大小。
(2)若磁感应强度的大小为一定值B,粒子将以垂直y轴的方向经H点进入第二象限,求B的大小及H点的坐标值;
(3)求粒子在第一象限内运动的时间t。
(1)
(2)
(3)![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002932941962.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002932878834.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002932894801.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002932909569.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002932941962.png)
试题分析:(1)设粒子在电场中运动的时间为t0,加速度的大小为a,粒子的初速度为v0,过Q点时速度的大小为v,沿y轴方向分速度的大小为vy,在第四象限,粒子受电场力与初速度方向垂直而且为恒力,为雷平抛运动
竖直方向由牛顿第二定律得
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002932956407.png)
由运动学公式得
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002933112742.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002933128366.png)
水平方向
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002933143581.png)
合速度
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002933175444.png)
解得:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002932878834.png)
(2)粒子在第一象限内做匀速圆周运动,设粒子做圆周运动的半径为R,
第四象限类平抛运动的末速度与水平夹角为
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002933206297.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002933237746.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002933253471.png)
根据初末速度的洛伦兹力均指向圆心可得半径如下图所示,由几何关系可得
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002933268500.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250029332843479.jpg)
洛伦兹力提供向心力
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002933315778.png)
解得:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250029333311118.png)
由几何关系可得,
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002933346456.png)
H点的坐标为
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002932909569.png)
(3)粒子在第一象限内做匀速圆周运动的圆心角为
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002933377409.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002933409543.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250029334241133.png)
解得:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002932941962.png)
![](http://thumb2018.1010pic.com/images/loading.gif)
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