题目内容
如图所示的电路中已知电源电动势ε=36V,内电阻r=2Ω,R1=20Ω,每盏灯额定功率都是2W,额定电压也相同。当K闭合调到R2=14Ω时,两灯都正常发光;当K断开后为使L2仍正常发光,求R2应调到何值?![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241523174701249.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241523174701249.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824152317595588.png)
【错解分析】错解:设所求电阻R′2,当灯L1和L2都正常发光时,即通过灯的电流达额定电流I。
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824152317658611.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241523177821052.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241523177821052.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824152317923990.png)
解得:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241523180321479.png)
又当K闭合时,
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241523181571016.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824152318266974.png)
当K断开时
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824152318328861.png)
①②
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824152318453883.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824152318531971.png)
代入
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824152318640473.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241523187501667.png)
【错解原因】
分析电路时应注意哪些是恒量,哪些是变量。图9-10电路中电源电动势ε是恒量,灯L1和L2正常发光时,加在灯两端电压和通过每个灯的电流是额定的。错解中对电键K闭合和断开两种情况,电路结构差异没有具体分析,此时随灯所在支路电流强度不变,两种情况干路电流强度是不同的,错误地将干路电流强度认为不变,导致了错误的结果。
【正解】解法一:设所求阻值R′2,当灯L1和L2正常发光时,加在灯两端电压力额定电压UL。
当K闭合时,ε1=UL+I1(R1+r+R2)
当K断开时,ε2=UL+I2(R1+r+R′2),
又 ∵ε1=ε2=ε I1=2I2=2I,(I为额定电流)
得ε= UL+2I(R1+r+R2) ①
ε=USL+I(R1+r+R′2) ②
①-②I(R1+r+2R2-R2′)=0 但I≠0,∴R1+r+2R2=R′2
即R′2=20+2+2×14=50Ω
解法二:设所求阻值R′2,当灯L1和L2正常发光时,加在灯两端电压为额定电压UL,由串联电路电压分析可得:
K闭合时:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241523188281313.png)
K断开时
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241523188901053.png)
由①②得
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241523189521449.png)
代入数据
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241523191551181.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824152317595588.png)
【点评】电路中的局部电路(开关的通断、变阻器的阻值变化等)发生变化必然会引起干路电流的变化,进而引起局部电流电压的变化。应当牢记当电路发生变化后要对电路重新进行分析。
![](http://thumb2018.1010pic.com/images/loading.gif)
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