题目内容
(8分)如图所示电路中,电源电动势E=12V,内阻r=1
,电阻R1=9
,R2=5
,R3是一只滑动变阻器,其阻值变化范围为0~20
。求:
(1)电流表的示数为0.4A时,R3的阻值为多大?
(2)当R3为多大时电源的总功率最大?最大功率是多少?![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241515002972810.jpg)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151459939337.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151459939337.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151459939337.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151459939337.png)
(1)电流表的示数为0.4A时,R3的阻值为多大?
(2)当R3为多大时电源的总功率最大?最大功率是多少?
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241515002972810.jpg)
(1)3.3Ω(2)0 14.4W
试题分析:(1)U并=I2R2=0.4×5V=2V…………………………………………………………1分
据闭合电路欧姆定律得E=U并+I(R1+r)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241515003601141.png)
据分流规律得
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151500594522.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241515007811102.png)
(2)据P=IE得,要使P最大,必需使I最大,即R最小 ∴R3的阻值为零…………1分
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151501031985.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241515010931037.png)
点评:关键是对电路图以及欧姆定律的正确理解
![](http://thumb2018.1010pic.com/images/loading.gif)
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