题目内容
如图所示,电源的电动势E="7.5" V,内阻r="1.0" Ω,定值电阻R2=12Ω,电动机M的线圈的电阻R=0.50Ω。开关S闭合,电动机转动稳定后,理想电压表的示数U1=4V,电阻R2消耗的电功率P2="3.0" W。不计电动机的摩擦损耗等,求:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241600590692880.jpg)
(1)电路的路端电压;
(2)电动机输出的机械功率。
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241600590692880.jpg)
(1)电路的路端电压;
(2)电动机输出的机械功率。
(1)6.0 V(2)3.5 W
试题分析:(1)路端电压等于R2两端的电压,由P=
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824160059069503.png)
得U=U2=
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824160059271497.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824160059381561.png)
(2)电源的电动势E=U2+Ir, (2分)
由此得干路电流I=
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824160059490586.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824160059677607.png)
通过R2的电流I2=
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824160059927525.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824160100161501.png)
通过电动机的电流I1=I-I2="1.0" A (1分)
电动机的输出功率P=I1U1-
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824160100301355.png)
由此得P="1.0×4" W-1.02×0.50W="3.5" W (2分)
点评:题要求学生能通过电路中开关的通断得出正确的电路图,并能灵活应用串并联电路的规律及欧姆定律求解.
![](http://thumb2018.1010pic.com/images/loading.gif)
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