题目内容
(18分)如图所示,在xOy平面中第一象限内有一点P(4,3),OP所在直线下方有垂直于纸面向里的匀强磁场,OP上方有平行于OP向上的匀强电场,电场强度E=100V/m。一质量m=1×10-6kg,电荷量q=2×10-3C带正电的粒子,从坐标原点O以初速度v=1×103m/s垂直于磁场方向射入磁场,经过P点时速度方向与OP垂直并进入电场,在经过电场中的M点(图中未标出)时的动能为P点时动能的2倍,不计粒子重力。求:
(1)磁感应强度的大小;
(2)O、M两点间的电势差;
(3)M点的坐标及粒子从O运动到M点的时间。
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250003469782296.png)
(1)磁感应强度的大小;
(2)O、M两点间的电势差;
(3)M点的坐标及粒子从O运动到M点的时间。
(1)0.2T (2)7.5×102V (3) (3,8.5) 1.285×10-2s
试题分析:(1)粒子的运动轨迹如下图,因为粒子过P点时垂直于OP,所以OP为粒子做圆周运动的直径为5m,
由
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000347009761.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000347025500.png)
(2)进入电场后,沿电场线方向有:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000347040840.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000347056787.png)
垂直于电场方向有:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000347072623.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000347134665.png)
因为
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000347150696.png)
即
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250003471651085.png)
解得
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000347181461.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000347196559.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000347212620.png)
OM两点间的电势差为:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250003472281074.png)
(3)子在磁场中从O到P的运动时间为:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250003472431234.png)
粒子在电场中P到M的运动时间为:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000347212620.png)
所以,从O到M的总时间为:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000347290889.png)
M点坐标为:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250003473061222.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250003473211303.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250003473374335.png)
![](http://thumb2018.1010pic.com/images/loading.gif)
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