题目内容
如图所示,电阻R1=12Ω,R2=8Ω,当开关S 断开时,电流表A的示数为0.5A;当S闭合时,A的示数为0.75A。试求:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241555271009536.png)
(1)电源的电动势和内电阻分别是多少?
(2)S闭合时,电源的效率多大?
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241555271009536.png)
(1)电源的电动势和内电阻分别是多少?
(2)S闭合时,电源的效率多大?
(1)12V、4Ω (2)![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824155527178440.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824155527178440.png)
试题分析:(1)当S断开时,由闭合电路欧姆定律有:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824155527303826.png)
当S闭合时,
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824155527506361.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824155527677779.png)
联立①、②并带入已知数据解得:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824155527771533.png)
(2)S闭合时,电源的效率为:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241555278332061.png)
点评:本题给出了一种测电源电动势和内电阻的方法;计算电源效率时要注意电源总功率与输出功率的关系。
![](http://thumb2018.1010pic.com/images/loading.gif)
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