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如图所示，宽度为L=0.20m的足够长的平行光滑金属导轨固定在绝缘水平桌面上，导轨的一端连接阻值为R=0.9Ω的电阻．导轨cd段右侧空间存在垂直桌面向上的匀强磁场，磁感应强度B=0.50T．一根质量为m=10g，电阻r=0.1Ω的导体棒ab垂直放在导轨上并与导轨接触良好．现用一平行于导轨的轻质细线将导体棒ab与一钩码相连，将重物从图示位置由静止释放．当导体棒ab到达cd时，钩码距地面的高度为h=0.3m．已知导体棒ab进入磁场时恰做v=10m/s的匀速直线运动，导轨电阻可忽略不计，取g=10m/s2．求：

（1）导体棒ab在磁场中匀速运动时，闭合回路中产生的感应电流的大小

（2）挂在细线上的钩码的重力大小

（3）求导体棒ab在磁场中运动的整个过程中电阻R上产生的热量．

答案

（1）感应电动努为：E=BLv=1.0V

感应电流：I=

BLv
R+r
=
0.50×0.20×10
0.9+0.1
A=1A

（2）导体棒匀速运动，安培力与拉力平衡，则有：

BIL=Mg    

所以：M=

BIL
g
=
0.50×1×0.20
10
kg=0.01kg

（3）导体棒移动30cm的时间为：t=

l
v
=0.03s

根据焦耳定律有：Q1=I2(R+r)t=0.03J（或Q1=Mgh=0.03J）   

根据能量守有：Q2=

1
2
mv2=0.5J

电阻R上产生的热量：Q=(Q1+Q2)

R
R+r
=0.477J

答：（1）导体棒ab在磁场中匀速运动时，闭合回路中产生的感应电流的大小1A；

（2）挂在细线上的钩码的重力大小0.01Kg；

（3）则导体棒ab在磁场中运动的整个过程中电阻R上产生的热量0.477J．

判断题

控制系统通常由被控对象、变送器、调节器、调节阀等四部分组成。
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    Do you know what kind of things the young people are reading? More and more   1   and parents have
noticed another kind of pollution, which comes from the printed papers   2   on streets. 　　
    These printed things   3   newspapers but have hardly anything to do with   4  . You can only find reading
materials badly made up there-some are too strange for anyone to   5  , others are frightening stories of
something   6  . However, many of the young readers are getting interested in such   7   reading, which   8   
them what they should pay for their breakfast and brings them nightmares (噩梦) and immoral (邪恶) ideas
in   9  . Homework is left  10 , and daily games are lost. 　　
    These sellers shout on streets selling their papers well. The writers, publishers and printers,  11  they are,
we never know, are  12  their silent money.
    The sheep-skinned wolf's story seems to have been forgotten once again. Why not  13  this kind of thing?
Yes, both teachers and parents have asked each other for more strict control of the young readers.  14 , the
more you want to forbid it, the more they want to have a look at it.  15  you may even find several children,
driven by the curious natures,  16  one patched paper, which has travelled from hand to hand. 　　
    It really does  17  to our society. It has already formed a sort of moral pollution. The  18  teachers and
parents need more powerful support in their protection of the young generation. At the same time the young
 19  need more interesting books to help them  20  those ugly papers.
( )1. A. teachers　　    
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( )14. A. Happily　　     
( )15. A. Seldom　　　    
( )16. A. take　　　　    
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( )19. A. writers　　　   
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C. break down　  		D. students        
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D. get rid of      
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