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问题 问答题

如图所示，已知某电源电动势E=20V，内阻r=1Ω，定值电阻R=7Ω，当开关S闭合时，电路中标有“6V 3W”的灯泡L和内阻为1Ω的小型直流电动机M恰能正常工作，求4：

（1）电动机的额定工作电压；

（2）电动机的额定输出功率．

答案

（1）依题，“6V 3W”的灯泡L正常工作，则电路中电流为

    I=

P
UL
=
50
10
=0.5 A          

电动机的额定工作电压U=E-I（R+r）-UL=10V   

（2）根据能量转化与守恒定律得

   电动机的额定输出功率P=UI-I2r=4.75W         

答：

（1）电动机的额定工作电压为10V；

（2）电动机的额定输出功率4.75W．

完形填空

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单项选择题

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A、高度

B、水平

C、宽度

D、斜边
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