问题 多选题

如图所示,M是一小型理想变压器,接线柱a、b接在电压μ=311sin314t(V)的正弦交流电源上,变压器右侧部分为一火警报警系统原理图,其中R2为用半导体热敏材料(电阻率随温度升高而减小)制成的传感器,所有电表均为理想电表,电流表A2为值班室的显示器,显示通过R1的电流,电压表V2显示加在报警器上的电压(报警器未画出),R3为一定值电阻.当传感器R2所在处出现火情时,以下说法中正确的是(  )

A.A1的示数增大,A2的示数减小

B.A1的示数增大,A2的示数增大

C.V1的示数增大,V2的示数增大

D.V1的示数不变,V2的示数减小

答案

当传感器R2所在处出现火情时,R2的电阻减小,导致电路的总的电阻减小,所以电路中的总电流将会增加,A1测量的是原线圈中的总的电流,由于副线圈的电流增大了,所以原线圈的电流A1示数也要增加;由于电源的电压不变,原副线圈的电压也不变,所以V1的示数不变,由于副线圈中电流增大,R3的电压变压变大,所以V2的示数要减小,即R1的电压也要减小,所以A2的示数要减小,所以AD正确.

故选AD.

填空题
阅读理解

阅读理解。

     At this moment in different places of the world people are doing different things

     In Beijing it's early morning. People are sleeping.

     In London people are leaving work to go home. They are waiting for buses or trains. Some people

are driving their cars. Some are having afternoon tea at home or walking to the pubs (酒吧) to have a

drink.

     In Moscow it's evening and people are having supper at home or in restaurants. Some are going to

watch a ballet (芭蕾舞). Some are drinking in the pubs.

     In Los Angeles (洛杉矾) it's morning. People are working in their offices. Children are starting their

lessons.

     New York is noon now. People aren't working but having lunch. They are eating hamburgers or hot

dogs and drinking coffee or Coke. Some people are seeing friends or shopping.

1. According to the passage we can see that at night some people in Moscow like to _____.

A. go to watch a ballet  

B. do shopping  

C. wait for buses  

D. go to the library

2. When children in Los Angeles are starting their lesson, people in Beijing are _____.

A. playing basketball      

B. seeing films    

C. having lessons    

D. sleeping

3. From the passage, we can see the time in New York now is _____.

A.  6:00          

B. 9:00          

C. 10:00          

D. 12:00

4. We know that London is 3 hours _____.

A. earlier than Moscow            

B. later than Moscow

C. earlier than Los Angeles          

D. later than Los Angeles

5. This passage mainly talks about _____.

A. science      

B. holiday      

C. time difference (区别)    

D. life in western countries