问题 选择题

一个定值电阻两端电压为3V时,通过它的电流为0.15A,当定值电阻两端电压降低为原来的一半时,则下列说法正确的是(  )

A.电流为原来的1/2

B.电阻为原来的2倍

C.电流为原来的2倍

D.电阻为原来的1/2

答案

导体的电阻是导体的本质属性,不会随电压的改变而改变.所以电阻不变,故B、D错误

电阻不变,电压降低为原来的一半,根据欧姆定律I=

U
R
可知电流为原来的一半,所以A正确,C错误.

故选A.

单项选择题
阅读理解

阅读理解

     The breathless pace of everyday life made many people sleepless, said our reporter Wang Ling,

when talking about the World Sleep Day (WSD)with sleep experts in order to find out how to get a

better sleep.

     Cui Shuliang, founder of 51sleep.com, which provides online advice for those yearning for a sound

sleep, found many of his workmates suffering badly from poor sleep quality, as an IT technician in

earlier times. "Each morning, I would hear complaints from someone that kept waking up during the

night, or someone else who had to spend all night fighting for sleep."

     Pressed by busy city life, more and more people are suffering from sleep problems ranging from

insomnia to narcolepsy (嗜眠症). Some people may be able to fall asleep, but spend the whole time in

fear. A survey conducted by the Sleep Research Society (SRS) shows that about 40% of the population

is troubled by sleep disorders, which can result in lowered awareness and less productive days. Serious

insomnia can even lead to psychiatric problems.

     Professor Zhang Xi, director of the organization, says the focus of this year's World Sleep Day will

be on scientific ways to sleep. Professor Zhang Xi explained that the causes of sleep disorders differ

among cases, but more than 80% of his patients had trouble in sleeping because they always have a

nervous worried feeling that makes it impossible for them to relax. "That's why we keep suggesting

people pay more attention to sleep problems, since there remains a hidden danger in the physical and

mental condition. On the other hand, people with sleep problems should learn how to ease their mind

and relax themselves. Staying mentally and physically healthy, plus having healthy habits, is the best

way to realize a sound sleep."

1. What do we know about Cui Shuliang according to the text?

A. He founded a website with the help of his friends.

B. He is experienced in using information technology.

C. He was suffering from serious sleep problems.

D. He often complained about being sleepy.

2. What can be inferred from the text?

A. Most people cannot have a good sleep.

B. Most sleep problems are caused by fear.

C. Sleep disorders can lead to serious results.

D. The SRS is an organization of 51sleep.com.

3. According to Professor Zhang Xi,we know that ________.

A. the WSD's focus is on the importance of sleep this year

B. people have paid enough attention to sleep problems

C. keeping in good health is necessary for a good sleep

D. different sleep problems have the same cause

4. Most of Professor Zhang Xi's patients have difficulty in sleeping because of ________.

A. stress  

B. excitement

C. noise  

D. fear

5. The underlined word "sound"in the second paragraph means"________".

A. light and long  

B. quick and dreamful

C. heavy and short  

D. deep and peaceful