问题 选择题

如图所示,足够长的U型光滑金属导轨平面与水平面成角(0<<90°),其中MN平行且间距为L,导轨平面与磁感应强度为B的匀强磁场垂直,导轨电阻不计。金属棒由静止开始沿导轨下滑,并与两导轨始终保持垂直且良好接触,棒的电阻为R,当流过棒某一横截面的电量为q时,它的速度大小为,则金属棒在这一过程中

A.运动的平均速度大小为

B.平滑位移大小为

C.产生的焦耳热为

D.受到的最大安培力大小为

答案

答案:B

题目分析:A、金属棒ab开始做加速度逐渐减小的变加速运动,不是匀变速直线运动,平均速度不等于,而是大于;错误

B、由电量计算公式可得,下滑的位移大小为;正确

C、产生的焦耳热,而这里的电流I比棒的速度大小为v时的电流小,故这一过程产生的焦耳热小于;错误

D、金属棒ab受到的最大安培力大小为;错误

故选B

点评:电磁感应综合题中,常常用到这个经验公式:感应电量,常用来求位移.但在计算题中,不能直接作为公式用,要推导。

阅读理解

In a time of low academic (学术的) achievement by children in the United States, many Americans are turning to Japan, a country of high academic achievement and economic success, for possible answers. However, the answers provided by Japanese preschools are not the ones Americans expected to find. In most Japanese preschools, surprisingly little emphasis is put on academic instruction. In one investigation, 300 Japanese and 210 American preschool teachers, child development specialists, and parents were asked about various aspects of early childhood education. Only 2 percent of the Japanese respondents (答问卷者) listed "to give children a good start academically" as one of their top three reasons for a society to have preschools. In contrast, over half the American respondents chose this as one of their top three choices. To prepare children for successful careers in first grade and beyond, Japanese schools do not teach reading, writing, and mathematics, but rather skills such as persistence, concentration, and the ability to function as a member of a group. The majority of young Japanese children are taught to read at home by their parents.

In the recent comparison of Japanese and American pre­school education, 91 percent of Japanese respondents chose providing children with a group experience as one of their top three reasons for a society to have preschools. 62 percent of the more individually oriented (强调个性发展的) Americans listed group experience as one of their top three choices. An em­phasis on the importance of the group seen in Japanese early childhood education continues into elementary school education.

Like in America, there is diversity (多样性) in Japanese early childhood education. Some Japanese kindergartens have specific aims, such as early musical training or potential (潜力) development. In large cities, some kindergartens are attached to universities that have elementary and secondary schools.

Some Japanese parents believe that if their young children attend a university-based program, it will increase the children’s chances of eventually being admitted to top-rated schools and universities. Several more progressive programs have introduced free play as a way out for the heavy intellectualizing in some Japanese kindergartens.

5. We learn from the first paragraph that many Americans believe ________.

A. Japanese parents pay more attention to preschool education than American parents

B. Japan’s economic success is a result of its scientific achievements

C. Japanese preschool education emphasizes academic in­struction

D. Japan’s higher education is better than theirs

6. Most American respondents believe that preschools should also attach

importance to ________.

A. problem solvingw。       B. group experience

C. parental guidancew。      D. individually oriented development

7. In Japan’s preschool education, the focus is on ________.

A. preparing children academically    B. developing children’s artistic interests

C. tapping children’s potentialw。     D. shaping children’s character

8. Why do some Japanese parents send their children to university-based kindergartens?

A. They can do better in their future studies.

B. They can gain more group experience there.

C. They can be individually oriented when they grow up.

D. They can have better chances of getting a first-rate edu­cation.

单项选择题