题目内容
如图(甲)所示,
、
为平行放置的水平金属轨道,
、
为相同半径,平行放置的竖直半圆形金属轨道,
为切点,P、Q为半圆轨道的最高点,轨道间距
,圆轨道半径
,整个装置左端接有阻值
的定值电阻。M1M2N2N1、M3M4N4N3为等大的长方形区域Ⅰ、Ⅱ,两区域宽度
,两区域之间的距离
;区域Ⅰ内分布着均匀的变化的磁场B1,变化规律如图(乙)所示,规定竖直向上为B1的正方向;区域Ⅱ内分布着匀强磁场B2,方向竖直向上。两磁场间的轨道与导体棒CD间的动摩擦因数为
,
右侧的直轨道及半圆形轨道均光滑。质量
,电阻
的导体棒CD在垂直于棒的水平恒力F拉动下,从
处由静止开始运动,到达
处撤去恒力F,CD棒可匀速地穿过匀强磁场区,并能通过半圆形轨道的最高点PQ处,最后下落在轨道上的位置离
的距离
。若轨道电阻、空气阻力不计,运动过程导棒与轨道接触良好且始终与轨道垂直,g取10 m/s2
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241722281488623.jpg)
求:
(1)水平恒力F的大小;
(2)CD棒在直轨道上运动过程中电阻R上产生的热量Q;
(3)磁感应强度B2的大小。
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824172223390529.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824172223624482.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824172223843518.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824172224061513.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824172224295604.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824172224529503.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824172224545531.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824172224794479.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824172225028488.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824172225231492.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824172225450505.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824172225715590.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824172225933566.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824172226276525.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824172226557594.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824172225715590.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824172227556556.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824172227883566.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241722281488623.jpg)
求:
(1)水平恒力F的大小;
(2)CD棒在直轨道上运动过程中电阻R上产生的热量Q;
(3)磁感应强度B2的大小。
(1)F=160N(2)
(3)B2=1.25T
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824172228164541.png)
试题分析:(1)CD棒在PQ处速度为
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824172228507322.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824172228804632.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824172229147456.png)
设CD棒在匀强磁场区Ⅱ速度为
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824172229615295.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824172229927987.png)
CD棒在恒力F作用下:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824172230317919.png)
由①②③得:F=160N (1分)
(2) 棒在直轨道上运动,产生感应电流时间
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824172230520530.png)
感应电动势
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241722307541049.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824172230956642.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824172231362615.png)
由⑤⑥⑦⑧得
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824172228164541.png)
(3)由于CD棒穿过匀强磁场区,此过程无感应电流,设CD棒进入M3N3界后的任一短时间Δt内
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824172232033925.png)
且由图乙可得
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824172232594753.png)
由⑩(11)(12)得 B2=1.25T (1分)
点评:本题综合运用了动能定理、牛顿第二定律和能量守恒定律,综合性较强,对学生能力要求较高,关键是理清运动过程,选择合适的定律进行求解.
![](http://thumb2018.1010pic.com/images/loading.gif)
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