题目内容
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241202352486144.jpg)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824120235279332.jpg)
(1)电子从板间射出的速度
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824120235279332.jpg)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824120235310332.jpg)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824120235310332.jpg)
(1)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824120235638700.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824120235653794.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824120235669989.gif)
(1)电子离开A点后在磁场中做匀速圆周运动,洛伦磁力提供电子做匀速圆周运动的向心力.设其运动半径为R,
由几何关系可知:R2=L2+(R-d)2 ①(2分)
由
② (3分)
由①②式得:
③(2分)
(2)电子由O运动到A处过程中洛伦磁力不做功,由动能定理可得:
④ (2分)
解得:电压
⑤ (2分)
(3)在A点,电场力和洛伦磁力的合力提供加速度,由牛顿第二定律得:
⑥ (3分)
得加速度![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824120235669989.gif)
由几何关系可知:R2=L2+(R-d)2 ①(2分)
由
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824120235669528.gif)
由①②式得:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824120235638700.gif)
(2)电子由O运动到A处过程中洛伦磁力不做功,由动能定理可得:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824120235731528.gif)
解得:电压
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824120235653794.gif)
(3)在A点,电场力和洛伦磁力的合力提供加速度,由牛顿第二定律得:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824120235762585.gif)
得加速度
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824120235669989.gif)
![](http://thumb2018.1010pic.com/images/loading.gif)
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