问题 选择题

如图甲所示,有两个相邻的有界匀强磁场区域,磁感应强度的大小均为B,磁场方向相反,且与纸面垂直,磁场区域在x轴方向宽度均为a,在y轴方向足够宽。现有一高为a的正三角形导线框从图示位置开始向右匀速穿过磁场区域。若以逆时针方向为电流的正方向,在图乙中,线框中感应电流i与线框移动距离x的关系图象正确的是 (      )

答案

C

题目分析:本题导体的运动可分为三段进行分析,根据楞次定律可判断电路中感应电流的方向;由导体切割磁感线时的感应电动势公式可求得感应电动势的大小.线框从开始进入到全部进入第一个磁场时,磁通量向里增大,则由楞次定律可知,电流方向为逆时针,故B一定错误;因切割的有效长度均匀增大,故由E=BLV可知,电动势也均匀增加;而在全部进入第一部分磁场时,磁通量达最大,该瞬间变化率为零,故电动势也会零,故A错误;当线圈开始进入第二段磁场后,线圈中磁通量向里减小,则可知电流为顺时针,故D错误,C正确;

故选:C

阅读理解

More than 30 million kids buy school lunch each day. This year, many schools raised lunch prices to fight raising costs. Students pay about $2 a meal now. That is 27 cents more than last year. But they still get a good deal. The average cost for schools to produce one meal has gone up 30 cents, and is now $3. 

So, how do schools neither make money nor lose money? To start, the government pays for a small part of the difference. Schools have to make up the rest. Peggy Eller, a leader of school nutrition (营养) service in Hudson, Wisconsin, said her district is cutting costs by using fewer paper products and serving smaller portion(份额). Well, portion control is one key to healthful eating! 

The push for more healthful food has grown in recent years. One in five kids aged 6 to 19 is overweight. Being overweight can cause health problems. Many states passed laws that require schools to serve nutritious meals. 

Since 2004, more than 90% of all U.S. Schools have removed the fat from lunches. Foods high in fat, salt and sugar have been replaced by low-fat milk, fresh fruits and vegetables. These new items on lunch menus have encouraged kids to change their eating habits. “It makes easier to try new things at home, “said Savanna Mackey, a Florida fifth-grader.  

Students need healthful meals to grow strong and do well in school. “Rising prices won’t stand in the way,”says school nutritionist Jane Thornton. “We’ll just be more clever in how we do things.”

小题1:In the first paragraph, the author aims to tell us        .

A.the large number of kids buying school lunch

B.the average price of school lunch

C.the school's ways to deal with school lunch

D.the fact of school lunch's getting more expensive小题2:Many school districts raised lunch prices because        .

A.there is less healthful food on the menus

B.the cost of food has gone up

C.kids often forget to bring lunch money

D.some school dining-halls are serving smaller portions小题3:According to Paragraph 4, Savanna Mackey      .

A.doesn't like the new items on the lunch menus

B.always enjoys foods like fresh and vegetables

C.becomes interested in eating healthy food at home

D.prefers eating at school to eating at home小题4:What can be inferred from the passage?

A.American schools solve the problem of high food prices by themselves.

B.American schools are very concerned about their students’ health.

C.Almost all schools are forced by the government to serve healthy foods.

D.American childhood overweight is mainly caused by the present lunch foods.

名词解释