题目内容
( 12分)如图所示,电源电动势
内阻
,电阻
。间距
的两平行金属板水平放置,板间分布有垂直于纸面向里、磁感应强度
的匀强磁场。闭合开关
,板间电场视为匀强电场,将一带正电的小球以初速度v=0.1m/s沿两板间中线水平射入板间。设滑动变阻器接入电路的阻值为Rx,忽略空气对小球的作用,取
。
(1)当Rx=29Ω时,电阻
消耗的电功率是多大?
(2)若小球进入板间做匀速圆周运动并与板相碰,碰时速度与初速度的夹角为
,则Rx是多少?![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241514451815069.jpg)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151444151562.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151444229503.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151444401798.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151444463584.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151444541435.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151444666321.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151444729664.png)
(1)当Rx=29Ω时,电阻
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151444838361.png)
(2)若小球进入板间做匀速圆周运动并与板相碰,碰时速度与初速度的夹角为
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151444885397.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241514451815069.jpg)
(1)0.6W(2)54Ω
试题分析:(1)闭合电路的外电阻为
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241514452121528.png)
根据闭合电路的欧姆定律
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241514453841107.png)
R2两端的电压为
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241514454461246.png)
R2消耗的功率为
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241514456021153.png)
(2)小球进入电磁场做匀速圆周运动,说明重力和电场力等大反向,洛仑兹力提供向心力,
根据牛顿第二定律
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151445680782.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151445774772.png)
连立⑤⑥化简得
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151445836788.png)
小球做匀速圆周运动的初末速的夹角等于圆心角为60°,根据几何关系得
R=d ⑧
连立⑦⑧带入数据
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241514459451329.png)
干路电流为
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241514461641026.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241514462891288.png)
点评:本题的难点在于粒子在复合场中的运动的过程,需要明白,粒子做匀速圆周运动的条件为:重力等于电场力,
![](http://thumb2018.1010pic.com/images/loading.gif)
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