问题 计算题

如图16所示,足够长的光滑平行金属导轨MN、PQ竖直放置,一个磁感应强度B=0.50T的匀强磁场垂直穿过导轨平面,导轨的上端M与P间连接阻值为R=0.30Ω的电阻,导轨宽度L=0.40m。电阻为r=0.20Ω的金属棒ab紧贴在导轨上,导轨电阻不计,现使金属棒ab由静止开始下滑0.7 m后以5 m/s的速度匀速运动 。(g=10m/s2

求: (1)金属棒的质量m;

(2)在导体棒下落2.70m内,回路中产生的热量Q

答案

(1)(2)Q=0.58J

(1)由题可知,棒最终作匀速运动,且速度

;(1分)

;(1分)

;(1分)

(1分)

解得(1分)

(2)棒在下滑过程中,有重力和安培力做功,克服安培力做的功等于回路的焦耳热。

则:

   (2分)

解得Q=0.58J

本题考查法拉第电磁感应定律与能量、牛顿第二定律的结合问题,开始时重力大于向上的安培力,导体棒向下加速,随着速度的增大,安培力逐渐增大,加速度减小,当安培力增大到等于重力时加速度减小到零,此后导体棒向下匀速运动,由此时安培力等于重力和安培力,法拉第电磁感应定律、恒定电流可求得导体棒的质量,棒在下滑过程中,有重力和安培力做功,克服安培力做的功等于回路的焦耳热,有能量守恒定律可求得焦耳热的值

完形填空

E

任务型读写

Directions : Read the following text and choose the most suitable heading from A---E for each paragraph. 

A. Classroom participation

B. Differences in American university education

C. Examination system

D. Competition among students

E. Honest policy in students' school work

F. (AB) Students' responsibility for learning

56                

There is considerable variety in university classrooms in the United States. Because of diverse teaching methods and non-standardized curricula (课程), no two courses are the same.    Undergraduate courses are considerable different from graduate courses. The classroom atmosphere in expensive, private universities may differ from that in community colleges, which are free and open to everyone.

57                

Participation in the classroom is not only accepted but also expected of the student in many courses. Some professors base part of the final grade on the student's oral participation. Although there are formal lectures during which the student has a passive role (i.e., listening and taking notes), many courses are organized around classroom discussions, student questions, and informal lectures.

58                

Many teachers believe that the responsibility for learning lies with the student. (Courses are not designed merely for students to pass exams.) The ideal student is considered to be one who is motivated to learn for the sake of learning, not the one interested only in getting high grades.

59                

Ideally, the teacher-student relationship at universities is characterized by trust. Thus, cheating on tests, Plagiarizing (抄袭) in written work, presenting other's ideas as original, and turning in homework completed by someone else are all prohibited (禁止).

60                 

Relationships between students in the classroom can be cooperative or competitive. International students should not hesitate to ask for help if it is needed. in classes where such a grading curve is used, students may be reluctant to share lecture notes or information for fear that their own grades will suffer.

单项选择题 A1/A2型题