问题 选择题

如图所示,环形金属软弹簧,套在条形磁铁的中心位置。若将弹簧沿半径方向向外拉,使其面积增大,则穿过弹簧所包围面积的磁通量将如何变化 (  )

A.增大

B.不变

C.减小

D.因弹簧所在处的磁场不是匀强场,故无法确定磁通量如何变化

答案

答案:C

题目分析:磁感线在条形磁铁的内外形成闭合曲线,磁铁外部的磁感线总数等于内部磁感线的总数,而且磁铁内外磁感线方向相反.而磁铁外部的磁感线分布在无穷大空间,所以图中线圈中磁铁内部的磁感线多于外部的磁感线,由于方向相反,外部的磁感线要将内部的磁感线抵消一些,将弹簧沿半径向外拉,使其面积增大,内部磁感线总数不变,而抵消增大,剩余减小,则磁通量将减小.故C正确.

填空题
阅读理解

阅读理解。

     Have you ever seen any students whose trousers hang so low you can see their underwear? What do you

think of that? Fashionable? Some of today's teenagers are big fans of such a look. But recently this trend has

been at the center of an argument in Italian middle schools. The headmaster of a school in central Italy has

asked students to stop wearing low-rise jeans that expose underwear and parts of the body. His request came

after a class trip,when he saw one boy's baggy trousers slide to his feet. He pointed out that this way of

dressing is not suitable for school. But in Italy, a nation that takes fashion very seriously, the suggestion caused

a debate among parents, teachers and students. The issue is whether the headmaster's request will limit students'

freedom-or whether dress in Italian schools is too casual. A parents' group praised the move in favor of good

taste, while others advised schools to stop worrying about fashion and fix up old school buildings. "We do not

want to kick fashion out," the headmaster explained, "but extremes of fashion like this are not right in school."

Many other schools have now requested that their students also stop wearing such trousers. Most students

have simply ignored the request. Ludovica Gaudio, 14, wore extremely low trousers exposing orange underwear

in class. It was cold, so she wore a matching orange scarf. Another 14-year-old said she would probably respect

the request, simply for practical reasons. "I don't really feel comfortable in those sort of jeans," said Sarah

Lattanzi, "in winter,when dressed like that, it's quite cold and I am afraid my stomach will ache."

1. What led to the argument in Italian middle schools? [ ]

A. Students' craze for fashions.

B. Clothes that are too exposing.

C. Students' ignoring dressing .

D. Students' underwear.

2. Which of the following supports the headmaster's request? [ ]

A. Fashion should be taken seriously.

B. Fashion should not be followed in school.

C. Students should have their freedom in choosing what they wear.

D. Students should be encouraged to have good taste in clothes.

3. The argument against the headmaster's request is that _____. [ ]

A. schools should pay attention to things more important than students' clothes

B. fashion should not be followed in school

C. dress in Italian schools is too casual

D. low-rise jeans can do harm to youngster's health

4. The purpose of this story is to _____. [ ]

A. show Chinese students that wearing fashionable clothes in school is under attack in other countries, too

B. show that dress code is necessary even in a country like Italy

C. tell us that a debate started in Italian middle schools over the way students dress in school

D. let us see that Italian students react differently to schools' requests