问题 问答题

如图1所示,两根足够长的平行金属导轨MN、PQ相距为L,导轨平面与水平面夹角为α,金属棒ab垂直于MN、PQ放置在导轨上,且始终与导轨接触良好,金属棒的质量为m.导轨处于匀强磁场中,磁场的方向垂直于导轨平面斜向上,磁感应强度大小为B.金属导轨的上端与开关S、定值电阻R1和电阻箱R2相连.不计一切摩擦,不计导轨、金属棒的电阻,重力加速度为g.现在闭合开关S,将金属棒由静止释放.

(1)判断金属棒ab中电流的方向;

(2)若电阻箱R2接入电路的阻值为0,当金属棒下降高度为h时,速度为v,求此过程中定值电阻上产生的焦耳热Q;

(3)当B=0.40T,L=0.50m,α=37°时,金属棒能达到的最大速度vm随电阻箱R2阻值的变化关系,如图2所示.取g=10m/s2,sin37°=0.60,cos37°=0.80.求定值电阻的阻值R1和金属棒的质量m

答案

(1)由右手定则,金属棒ab中的电流方向为b到a.

(2)由能量守恒,金属棒减小的重力势能等于增加的动能和电路中产生的焦耳热:

mgh=

1
2
mv2+Q

解得:Q=mgh-

1
2
mv2

(3)设最大速度为v,切割磁感线产生的感应电动势:

E=BLv

由闭合电路的欧姆定律:

I=

E
R1+R2

从b端向a端看,金属棒受力如图:

金属棒达到最大速度时满足:

mgsinα-BIL=0

由以上三式得:v=

mgsinα
B2L2
R2+
mgsinα
B2L2
R1

由图象可知:斜率为:k=

60-30
2
m/s•Ω=15m/s•Ω,纵截距为v0=30m/s,得到:

v0=

mgsinα
B2L2
R1

mgsinα
B2L2
=k

解得:R1=2.0Ω,m=0.1kg.

答:值电阻的阻值R1=2.0Ω,金属棒的质量m=0.1kg.

阅读理解

B

Unfortunately, concerned about instability in the job market, many students are worried about facing an ever more uncertain future and consequently they have begun to feel lost, according to a survey. The survey, conducted by the Social Survey Institute of China, covers more than 2,000 college students from big cities, such as Beijing and Shanghai.

Tapping into (利用) this vast network of students, the survey revealed about 75 percent enjoy their lives on campus, feel free from restraints (限制) and are filled with enthusiasm. Nevertheless, 16.7 percent “lost ones” of the students refer to life as boring and aimless while 8.3 percent complain about the heavier study load they must now bear. Undoubtedly, pressure exists, and 75 percent of the students are worried about finding a job upon graduation. Around half of all participants have no clear career plan after graduation and a high 41.7 percent admit they have not thought about it. Only 8.3 percent have a clear idea of what they want to do and are confident of achieving their hopes and dreams.

The question “Are you a popular guy” seemed to provoke a mixed response from participants with 58.3 percent answering positively, leaving 41.7 percent as unsure, 91.7 percent of the students thought they got along very well with others, the final 8.3 percent finding it difficult to choose a particular answer. Independent answers such as “do things in your own way” and “forget other people’s opinions” seemed to be popular among modern college students with 75 percent conforming more or less to this attitude. 25 percent alone admitted to worrying about comments from other people.

The survey also discovered a strong feeling of confidence among students when dealing with interpersonal relationships and that personalities were slightly influenced by the surrounding environment. Most students criticized the phenomenon of suicide on campus. 58.3 percent urged authorities to discover hidden social reasons behind suicide, thus helping to prevent it. 33.3 percent thought students who try to commit suicide will not be able to face life’s bigger challenges in the future even if they survive. Surprisingly, 8.3 percent confessed that they once wanted to kill themselves, but finally decided against it.

45. What is college life like in the eyes of most students according to the passage?

A. Easy                       B. Interesting         C. Stressed             D. Aimless

46. How many students refer to life as boring in the survey?

A. More than 334       B. More than 166   C More than 1,500   D. More than 625.

47. The underlined phrased “provoke a mixed response” (in Paragraph 4) probably means “_____”.

A. answer in a mixed way                                   B. give a variety of answers

C. have sympathy for other students                     D. suddenly begin to affect someone

48. Which of the following can be the best title of the passage?

A. What should college life be like?                     B. Lost students in China

C. Do things in your own way.                            D. Frustrated college life

填空题