问题 选择题

如图所示,几个同学在只有电流表或电压表时,利用一个已知阻值的电阻R0,设计了下面四个测量未知电阻RX阻值的电路,其中不可行的是(    )

答案

答案:C

题目分析:A、Rx和定值电阻串联,电压表U1测量定值电阻两端的电压,根据I="U" /R ,求出通过定值电阻的电流.根据串联电路电流处处相等,通过Rx电流等于通过定值电阻电流Ix=I0.电压表U2测量Rx两端的电压Ux,根据Rx=Ux/ Ix求出电阻.不符合题意.

B、Rx和定值电阻并联,电流表A1测量定值电阻的电流,根据U=IR,求出定值电阻两端的电压.根据并联电路各支路电压相等,Rx两端电压等于定值电阻两端的电压Ux=U0.电流表A2测量Rx的电流Ix,根据Rx=Ux /Ix求出电阻.不符合题意.

C、Rx和定值电阻串联,电流表测量串联电路的电流,不能直接或间接测量Rx两端的电压,不能求出Rx电阻.符合题意.

D、开关S和S1都闭合时,电压表测量电源电压.开关S闭合S1断开时,电压表直接测量Rx两端的电压.根据串联电路总电压等于各串联导体电压之和,求出定值电阻两端的电压,根据I="U/" R ,求出通过定值电阻的电流.根据串联电路电流处处相等,通过Rx电流等于通过定值电阻电流Ix=I0.根据Rx=Ux /Ix求出电阻.不符合题意.故选C.

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     This Sunday, I want to go __1_in Wal-Mart. My T-shirt and my pants are too   2   . So I want     3  
a new T-shirt and    4   . Tomorrow is Christmas Day(圣诞节),I need     5     Christmas cards(卡片).
And I want to give my     6    some presents for Christmas. I want to buy a shirt for my father and a dress
for my mother. I want to hold a Christmas party(晚会)     7   , and I would like to ask a few friends to
come to my home for the party. So I must buy something for the party. I want to buy two__8__ of oranges, two bottle of    9   , a kilo of sweets(糖果), some cakes  10   a kilo of apples. I know they like Sichuan
food, so I want to buy some hot food.
( )1. A. shopping        
( )2. A. new        
( )3. A. buy        
( )4. A. two pairs of pants
( )5. A. any        
( )6. A. parent      
( )7. A. at school      
( )8. A. kilo        
( )9. A.milk        
( )10. A. but        
B. buying        
B. old          
B. buys          
B. two pair of pants
B. some          
B. mother        
B. at home        
B. kilos          
B. milks        
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C. swimming      
C. long          
C. to buy        
C. two pants    
C. few          
C. parents      
C. in my office  
C. kiloes        
C.milkes        
C. or            
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