问题 选择题

(2009年高考福建理综卷改编)如图9-3-14所示,固定放置在同一水平面内的两根平行长直金属导轨的间距为d,其右端接有阻值为R的电阻,整个装置处在竖直向上磁感应强度大小为B的匀强磁场中.一质量为m(质量分布均匀)的导体杆ab垂直于导轨放置,且与两导轨保持良好接触,杆与导轨之间的动摩擦因数为μ.现杆在水平向左、垂直于杆的恒力F作用下从静止开始沿导轨运动距离l时,速度恰好达到最大(运动过程中杆始终与导轨保持垂直).设杆接入电路的电阻为r,导轨电阻不计,重力加速度大小为g.则此过程(  )

图9-3-14

A.杆的速度最大值为

B.流过电阻R的电量为

C.恒力F做的功与摩擦力做的功之和等于杆动能的变化量

D.恒力F做的功与安培力做的功之和等于杆动能的变化量

答案

答案:B

选B.A选项中,当杆达到最大速度v时,其受力情况如图所示,在水平方向受拉力F、安培力F、滑动摩擦力f=μmg,三个力的合力为零:F--μmg=0,解得v=;B选项中,平均电动势为,平均电流为,通过的电量q=·Δt=,而ΔΦ=B·ΔS=Bdl,则q=;C选项中,由动能定理得WF-Wf-W=ΔEk;D选项中,由前式可得WF-W=ΔEk+Wf>ΔEk.本题正确选项为B.

完形填空
Liberal (自由) education is designed not only to teach and learn knowledge, but also to develop a person’s character. Its aim is to   1  each student to think for himself and make wise decisions. Now liberal education has a great    2 on the world.
Much work has been done to  3  a better way of teaching. In a liberal class, students are  4 to have their own thoughts and own ideas when they have  5  which are often held in the form of group discussion. They are properly guided to   6  the knowledge learned. Punishment no longer exists (存在).  7 and students are just like friends.
The idea is of course a perfect one if it can really be carried out. However, it  8 exists in the imagination and will never be realized. We must admit (承认) that  9  should at least be guided properly. Just close your eyes and think how many students will   10  the duties of a student if they are not pushed.   11 is the use of advice only if it is not supported by a reasonable   12  of  control and punishment?
It is   13  for us to judge the success and failure of the kind of education now, but   14  the bad behavior of children, which is really the   15  of liberal education, we can see that something must be done to help these children.
小题1:
A.teachB.orderC.learnD.push
小题2:
A.organization B.figureC.effectD.fact
小题3:
A.holdB.findC.makeD.answer
小题4:
A.encouragedB.toldC.usedD.driven
小题5:
A.shoppingB.lunchC.sportsD.lessons
小题6:
A.rememberB.useC.reviewD.act
小题7:
A.EnemiesB.FathersC.TeachersD.Classmates
小题8:
A.alsoB.onlyC.evenD.never
小题9:
A.childrenB.familiesC.adultsD.schools
小题10:
A.arrangeB.makeC.performD.return
小题11:
A.WhyB.WhenC.WhatD.Where
小题12:
A.degreeB.restC.personD.master
小题13:
A.easyB.possible C.hardD.helpful
小题14:
A.withB.againstC.aboutD.from
小题15:
A.progressB.goalC.resultD.purpose
单项选择题