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如图所示电路，电源电压一定．电阻R1的阻值为R2的2倍．当开关S1、S2都闭合时，电流表A1的示数为I1，电流表A2的示数为5I1．当开关S1、S2都断开时，电流表A1的示数为I1′．则I1′与I1之比为______．

答案

设电源的电压为U，

（1）当开关S1、S2都闭合时，R1、R2、R3并联，三电阻两端的电压相等；

∵并联电路中干路电流等于各支路电流之和，

∴根据欧姆定律可得：

I1=

U
R1
，I2=
U
R2
+
U
R3
，

∵I2=5I1，R2=

1
2
R1，

∴

1
1
2
R1
+
1
R3
=
5
R1
，

解得R3=

1
3
R1，

（2）当开关S1、S2都断开时，R1、R2、R3串联，

则I1′=

U
R1+R2+R3
=
U
R1+
1
2
R1+
1
3
R1
=
U
11
6
R1
，

所以I1′：I1=

U
11
6
R1
：
U
R1
=6：11．

故答案为：6：11．

单项选择题

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A.肠道扩大膀胱术

B.左肾造瘘术

C.右肾切除术

D.右肾切除+左肾造瘘术

E.抗结核治疗的同时，左肾造瘘术
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2（）

A.but

B.and

C.or

D.yet
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