问题 选择题

下列说法中,错误的是(  )

A.电工钳的手柄外套用塑料制成,是利用塑料的绝缘性

B.在连接电路时,应使开关处于断开状态

C.电压表和电流表都不能直接连接在电源的两端

D.某变阻器上标有“50Ω 1.2A”字样,则允许通过它的最大电流为1.2A

答案

A、电工钳的手柄外套之所以用塑料制成,是因为塑料是绝缘体,能起绝缘作用,这样人在用电工钳检修电路时,就不会触电了,所以选项A的说法是正确的;

B、在连接电路时,开关应处于断开状态,这是为了防止由于电路接错出现短路而造成电路电流过大烧坏电子元件,当然对于家庭电路而言,开关断开时,还能起到安全的作用,防止人触电,所以选项B的说法是正确的;

C、电流表的内阻很小,电流表直接接到电源上时,根据I=

U
R
可知,电路中电流会很大,会烧坏电源和电流表,所以电流表不能直接接在电源两极上;电压表的内阻很大,电路中的电流很小,不会损坏电源和电压表,所以在电源电压不超过电压表所选量程的情况下,电压表是可以直接接在电源两极上测电源电压,所以选项C的说法是错误的;

D、某变阻器上标有“50Ω 1.2A”字样,其中1.2A是该变阻器允许通过它的最大电流,所以选项D的说法是正确的.

故选 C.

阅读理解

阅读理解

         There is a one-day holiday in most countries on New Year's Eve. There are several interesting customs in the West, and there are many differences from country to country.  

         Though Christmas is a family get-together. New Year for some is a time to be with friends too. Parties are most often held to welcome New Year. At such arties, there is food and drink and dancing until the time nears for the coming of New Year.  

         At midnight, people often say “Happy New Year ”to each other. Champagne(香槟酒)is a favorite drink for New Year. In many western countries fireworks(烟火)are let off at midnight ,and many people like to make a noise, possibly to drive away bad spirits(幽灵).  

        One important song is often sung at New year-the Scottish(苏格兰)song of Auld Lang syne (good times long ago.)

1. People usually have great celebrations for New Year on         .   

A. New Year's Day                    

B. the night before New Year's Eve  

C. the night of New Year's Day      

D. the night before New Year's Day

2. In the West, New Year is also a time when.         .   

A. friends get together              

B. friends write to each other  

C. people can have a week's holiday  

D. people came from country to country

3. The word “customs ”in the passage means          .   

A. 习俗                    

B.  聚会            

C. 饮食                  

D. 活动

4.At midnight on New Year's Eve western people often          .  

① hold parties to welcome New Year        

② make a noise to drive away bad spirits  

③ drink with friends or families          

④ say “Happy New Year ”to each other  

⑤ sing an important French song          

⑥ let off fireworks  

A. ①②③④             B. ③④⑤⑥           C. ②③④⑥         D. ①②⑤⑥        

5. This passage mainly talks about

A. western people drink together on New Year's Eve  

B. what western people do on New Year's Day  

C. how western people spend their New Year's Eve

D. western people have a good time on New Year's Day

单项选择题