题目内容
如图所示的电路中,电阻R1=R2=R3=10Ω,电源内阻r=5Ω,电压表可视为理想电表.当开关S1和S2均闭合时,电压表的示数为10V.求:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241526062162367.png)
(1) 电源的电动势;
(2)当开关S1闭合而S2断开时,电压表的示数和此时电源的效率。
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241526062162367.png)
(1) 电源的电动势;
(2)当开关S1闭合而S2断开时,电压表的示数和此时电源的效率。
(1)20V (2) 16V 80﹪
试题分析:(1)当开关
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824152606278318.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824152606450355.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824152606559304.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824152606715348.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824152606855349.png)
电压表的示数为10V,由欧姆定律可知干路电流为
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824152607074727.png)
外电路总电阻为
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824152607167979.png)
根据闭合电路欧姆定律
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824152607245616.png)
电源电动势
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824152607370877.png)
(2)当开关
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824152606278318.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824152606450355.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824152606559304.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824152606855349.png)
电路中的总电流为
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241526079161003.png)
电压表的读数等于路端电压即
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824152608025942.png)
此时电源的效率为
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241526081031078.png)
点评:对闭合电路的计算问题,首先要识别电路的结构,其次要明确电压表测量哪部分电路的电压。
![](http://thumb2018.1010pic.com/images/loading.gif)
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