（2011?房山区二模）在如图所示的电路中，闭合电键S1，同时断开电键S2，灯泡L1和L2均不亮，电压表有示数 （2008?静安区二模）在如图所示的电路中，电源电压为4.5...

\u5982\u56fe\u6240\u793a\uff0c\u5408\u4e0a\u7535\u952eS1\uff0e\u5f53\u7535\u952eS2\u95ed\u5408\u65f6\uff0c\u7535\u6d41\u8868\u7684\u793a\u6570\u4e3a0.75A\uff1b\u5f53\u7535\u952eS2\u65ad\u5f00\u65f6\uff0c\u7535\u6d41\u8868\u7684\u793a\u6570\u4e3a0.5A\uff0cR1=

\u5f53\u7535\u952eS2\u65ad\u5f00\u65f6\uff0c\u6839\u636e\u95ed\u5408\u7535\u8def\u6b27\u59c6\u5b9a\u5f8b\uff1aE=I1\uff08R1+r\uff09\u5f53\u7535\u952eS2\u95ed\u5408\u65f6\uff0c\u6839\u636e\u95ed\u5408\u7535\u8def\u6b27\u59c6\u5b9a\u5f8b\uff1aE=I2\uff08R1R2R1+R2+r\uff09\u4ee3\u5165\u6570\u636e\uff1aE=0.5\uff08r+2\uff09E=0.75\uff08r+1\uff09\u89e3\u5f97\uff1aE=1.5V\uff0cr=1\u03a9\uff0e\u7b54\uff1a\u7535\u6e90\u7535\u52a8\u52bfE\u4e3a1.5V\uff0c\u5185\u7535\u963br\u4e3a1\u03a9\uff0e

\uff081\uff09\u2235I=UR\uff0c\u2234\u7535\u963bR1=UI=4.5V0.3A=15\u03a9\uff1b\uff082\uff09\u2235I=UR\uff0c\u2234\u603b\u7535\u963bR=UI\u2032=4.5V0.1A=45\u03a9\uff0cR2=R-R1=45\u03a9-15=30\u03a9\uff0c\u2235I=UR\uff0c\u2234\u7535\u538bU2=I\u2032R2=0.1A\u00d730\u03a9=3V\uff1b\u7b54\uff1a1\uff09\u7535\u963bR1\u7684\u7535\u963b\u4e3a15\u03a9\uff1b\uff082\uff09\u5f53\u95ed\u5408\u5f00\u5173S1\u3001\u65ad\u5f00\u5f00\u5173S2\u65f6\uff0c\u7535\u963bR2\u4e24\u7aef\u7684\u7535\u538b\u4e3a3V\uff1b\u7535\u963bR2\u7684\u963b\u503c\u4e3a30\u03a9\uff0e

闭合S₁，断开S₂，L₁和L₂串联．同时闭合S₁、S₂，L₂短路
A 如果L₁断路，电压表测量电源电压，所以有示数；电路断路，所以电流表无示数．此选项符合题意；
B 如果L₂断路，只闭合S₁时，电流表、电压表都无示数．此选项不符合题意；
C 如果L₁短路，同时闭合S₁、S₂，整个电路短路，会烧坏电源，电流表、电压表都无示数．此选项不符合题意；
D 如果L₂短路，电流表、电压表都有示数．此选项不符合题意．
故选A

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