题目内容
用电压恒为45V的电源给一个变光灯供电,变光灯中的灯泡的额定电压也是45V,其电阻恒为20Ω。变光灯中的调光电路如右图所示。欲使在调节灯泡亮度的过程中灯泡两端的电压U降低到30V时变光灯的效率仍不低
。
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241225337912882.jpg)
(1)滑动变阻器R的阻值至少应取多大?此时这个滑动变阻器至少应当允许通过多大的电流?
(2)这个电路的最高效率可达多少?
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824122533775262.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241225337912882.jpg)
(1)滑动变阻器R的阻值至少应取多大?此时这个滑动变阻器至少应当允许通过多大的电流?
(2)这个电路的最高效率可达多少?
(1)设当
时,滑动变阻器滑动端以上那一段的电阻为R1,则滑动端以下包括灯泡的电阻一定为2R1(因为下段电压为
30V,上段电压必为
15V),于是有![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824122533947794.gif)
将![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824122533900371.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824122533978373.gif)
代入上式得
。
设滑动变阻器下半端电阻为R2,再由
得
。则滑动变阻器的总电阻至少应为 ![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824122534118521.gif)
滑动变阻器允许通过的最小电流应等于滑动变阻器与灯泡均处于U=45V电压下的电流之和,即![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241225341341056.gif)
(2)当滑动端滑到最上端时,该电路的效率最高,此时的效率应等于通过灯泡的电流与总电流的比值,即
%
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824122533900371.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824122533916202.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824122533931217.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824122533947794.gif)
将
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824122533900371.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824122533978373.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824122533994414.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824122534009299.gif)
设滑动变阻器下半端电阻为R2,再由
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824122534025662.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824122534040439.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824122534118521.gif)
滑动变阻器允许通过的最小电流应等于滑动变阻器与灯泡均处于U=45V电压下的电流之和,即
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241225341341056.gif)
(2)当滑动端滑到最上端时,该电路的效率最高,此时的效率应等于通过灯泡的电流与总电流的比值,即
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824122534150767.gif)
略
![](http://thumb2018.1010pic.com/images/loading.gif)
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