如图所示电路,电源电压不变,电阻r1=2Ω,当开关s断开时,电压表v1和v2的.
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①(R1+R2)/(R1+R3)=5/7 代入R1=2Ω (2+R2)/(2+R3)=5/7

② I²*R2+I²R3=4I²R1 R2+R3=4R1=4*2=8Ω 所以R2=8-R3代入①得

③ (2+8-R3)/(2+R3)=5/7 解得R3=5Ω R2=8-R3=8-5=3Ω

④ S闭合后,V1+V2=7V 根据串联电路元件的电压比等于电阻比,即(R1+R2)+R1=7

(2+3)+2=7 电源电压=V1=2+3=5V