题目内容
如图所示,电源内阻r=1Ω,R1=2Ω,R2=6Ω,灯L上标有“3V、1.5W”的字样,当滑动变阻器R3的滑片P移到最右端时,电流表示数为1A,灯L恰能正常发光。
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241325218442660.jpg)
(1)求电源的电动势;
(2)求当P移到最左端时,电流表的示数;
(3)当滑动阻器的Pb段电阻多大时,变阻器R3上消耗的功率最大?最大值多大?
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241325218442660.jpg)
(1)求电源的电动势;
(2)求当P移到最左端时,电流表的示数;
(3)当滑动阻器的Pb段电阻多大时,变阻器R3上消耗的功率最大?最大值多大?
(1)6V(2)2A(3)2W
(1)
——————————3分
(2)
———————————————————3分
(3)
,
,
,将已知量代入,化简得,
,又有
————————————2分
所以
——————————————2分
可算得,当
时变阻器R3上消耗的功率最大,且最大值为8W————2分
也可用等效电源的方法求解:等效电动势
,等效内电阻
,
所以当
时变阻器R3上消耗的功率最大,且最大值为![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241325223901032.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241325218591151.png)
(2)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824132521891996.png)
(3)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824132521937815.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241325219841007.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824132522015780.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824132522031960.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824132522156990.png)
所以
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241325221871512.png)
可算得,当
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824132522234558.png)
也可用等效电源的方法求解:等效电动势
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824132522265512.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824132522296514.png)
所以当
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824132522343643.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241325223901032.png)
![](http://thumb2018.1010pic.com/images/loading.gif)
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