问题 填空题

在如图所示的电路中,电源电压不变,当电键S由闭合到断开时,电流表A的示数将  (19)    ,电压表V1的示数将  (20)   。S断开后,将滑片向右移动,则电流表A的示数将  (21)    ,电压表V1示数与电压表V2示数的比值将  (22)    。(均选填“变大”、“变小”或“不变”)

答案

变大;变小;变小;变大。

分析:(1)当电键S闭合时,电路为定值电阻的简单电路,电流表被短路,电压表测电源的电压;当电键S断开时,定值电阻与滑动变阻器串联,电路中的电阻变大,电路中的电流变小,根据欧姆定律可知滑动变阻器两端的电压变小.

(2)根据滑动变阻器的移动可知接入电路电阻的变化和电路总电阻的变化,根据欧姆定律可知串联电路电流的变化和测定值电阻两端的电压V2示数的变化,根据串联电路电压特点可知测滑动变阻器两端电压V1示数的变化,进一步可知两电压表示数比值的变化.

解答:解:(1)当电键S闭合时电流表被短路,电压表测电源的电压;

当电键S断开时电流表测电路中的电流,电压表测串联电路的部分电压(滑动变阻器两端的电压);

故电流表A的示数将变大,电压表V1的示数将变小.

(2)滑片向右移动,接入电路的电阻变大、电路中的总电阻变大,

根据可知电路中的电流变小,即电流表的示数变小;

根据U=IR可知定值电阻两端的电压变小,即V2的示数变小;

根据串联电路总电压等于各分电压值和可知,滑动变阻器两端的电压变大,即V1的示数变大;

所以电压表V1示数与电压表V2示数的比值将变大.

故答案为:变大;变小;变小;变大.

选择题
阅读理解

FRIDAY, Aug. 3 (Health Day News) -- Middle school students who are physically fit are likely to score higher on standardized tests measuring reading and math abilities, a new study has found. And, the average scores went up in connection with levels of fitness, the findings showed.

“The more physically fit kids were, the higher their scores,” said the study’s lead author, Trent Petrie, director of the Center for Sport Psychology at the University of North Texas in Denton. “Parents should encourage their kids to be physically active. There are some real cognitive (认知的) and academic benefits that come from physical fitness”, Petrie said.

Results of the study are scheduled to be presented Friday at the annual meeting of the American Psychological Association in Orlando, Fla. The study included more than 1,200 middle school students from five schools in a suburban area of Texas, with 561 boys and 650 girls. About 57 percent of the children were white, and nearly one-quarter were Mexican American. Nine percent were black and about 2 percent were of Asian descent.

The school district provided the researchers with information on the children’s race, age, grade level and whether they qualified for the free school lunch program, which was an indicator of the family’s socioeconomic status. The schools also provided scores to the tests, which were given between one and four months after the researchers had assessed the children’s levels of fitness. Fitness tests were administered during physical education classes to determine the youngsters’ heart and lung health (cardiorespiratory fitness), as well as their body mass index (BMI), an indicator of how much body fat a person has. The children also filled out questionnaires that helped the researchers determine factors such as self-esteem and social support.

After accounting for factors such as age, sex, family income and self-esteem, the researchers found that for both boys and girls, higher levels of heart and lung health predicted better scores on both the math and reading tests.

For boys, perceived (感观的) social support also seemed to increase their reading scores, the investigators found. In girls, while being physically fit predicted higher reading scores, so too did a higher BMI-- which indicates more body fat. “We were a little surprised by this finding,” Petrie said.

“It was not as strong an association as the one with physical fitness,” he added. The authors suspect it may have something to do with girls this age entering adolescence, which may be related to a higher BMI and slightly higher brain development. He said he plans to make great effort to look for this relationship again in larger studies to see if it was a chance finding, or if the association holds up.

“While we can’t say 100 percent that physical fitness causes better academic performance, we can say that there is a strong and predictive relationship between physical fitness and academic performance,” Petrie said.

“It’s hard to tease apart (梳理) the exact reason for this association,” said Becky Hashim, an attending clinical psychologist and assistant professor in the departments of psychiatry and pediatrics at the Children’s Hospital of Montefiore, in New York City.

“It may be that the children are getting more oxygen. When the heart and lungs are working at a higher capacity, it may allow the brain to work at peak performance. Children who are less fit may be sleepier during school,” she noted. “I personally feel that there’s probably a strong relationship between the confidence you get from being able to do something physical well and academic performance.”

Whatever the reason behind this association may be, “there’s certainly no harm in pushing physical fitness,” Hashim added. “Physical fitness may make you feel better, give you more confidence and improve your performance across the board,” she said.

Petrie agreed. “Physically fit kids are happier, have higher self-esteem and, tend to have better relationships, and now we’re beginning to see that there also seem to be benefits cognitively and academically. Our study sends a strong warning to policymakers to reconsider the service program of physical education classes for kids,” he said.

小题1:How did the school district help the researchers carry out the study?

A.By analyzing the data of fitness tests.

B.By giving some basic information.

C.By completing some questionnaires.

D.By offering a certain amount of Money.小题2:According to the passage, the result of the study will be published ______.

A.in a book

B.in a journal

C.at a conference

D.on the Internet小题3:What’s the suitable title of the passage?

A.Parents should be aware of the benefit of fitness.

B.Fitter kids are likely to make better grades.

C.Fitness affects kids’ social behavior greatly.

D.Teachers should value physical education classes.小题4:What’s Becky Hashim’s attitude towards the relationship between physical fitness and academic performance?

A.Supportive

B.Indifferent

C.Critical

D.Doubtful小题5:What does the underlined word “one” in Paragraph refer to?

A.The BMI.

B.The body fat.

C.The brain development.

D.The test score.小题6:From the last paragraph we can infer that physically fit kids ______.

A.deserve to be treated well

B.live much longer than others

C.get along well with others

D.enjoy a peaceful life