问题 问答题

如图所示,横截面为矩形的管道中,充满了水银,管道的上下两壁为绝缘板,左右两壁为导体板,(图中斜线部分),两导体板被一无电阻的导线短接.管道的高度为a,宽度为b,长度为c.加在管道两端截面上的压强差恒为p,水银以速度v沿管道方向流动时,水银受到管道的阻力f与速度成正比,即f=kv(k为已知量).求:

(1)水银的稳定速度v1为多大?

(2)如果将管道置于一匀强磁场中,磁场与绝缘壁垂直,磁感应强度的大小为B,方向向上,此时水银的稳定流速v2又是多大?(已知水银的电阻率为ρ,磁场只存在于管道所在的区域,不考虑管道两端之外的水银对电路的影响)

答案

(1)水银匀速运动时达到稳定状态,压力与阻力二力平衡,则有

   pab=kv1

解得,水银稳定速度  v1=

pab
k

(2)将管道置于一匀强磁场中,水银切割磁感线产生的感应电动势为  E=Bbv2

  回路中电阻为 R=ρ

b
ac

由欧姆定律得感应电流为I=

E
R
=
Bbv2
ρ
b
ac
=
Bacv2
ρ

由平衡条件可得 p?ab=BIb+kv2

所以  v2=

p?abρ
kρ+B2abc

答:(1)水银的稳定速度v1

pab
k

(2)将管道置于一匀强磁场中,水银的稳定流速v2

pabρ
kρ+B2abc

完形填空
Many teenagers have hobbies.But sometimes these hobbies can get       way of schoolwork. and Parents might worry about their       at school.Teenagers often think they should be allowed to practice their hobbies as much as they want.Do you agree?
Liu Yu, a       boy from Shandong.is a running star.He is on his school running team and has always wanted to be a professional athlete.However,his parents won’t allow him to tram as much as he would like. “Of course we want to see Liu Yu       his dream.”says Mr. Liu.“and we know       he loves running. That's great.and my wife and I have watched him in every one of his races.We have nothing     running. But we do think that our son needs to be realistic.Now that he is getting older. he needs to think about what will happen       he doesn’t become a professional runner in the end.
Liu Yu really       .“Well.I think I should be allowed to make decisions for myself.”he says.“My parents have always taught me the       of working hard and not just to do what I enjoy. I understand this. But I'm serious about running.      a professional runner is the only thing I have ever wanted to do.
Mr. and Mrs. Liu believe that Liu Yu should study hard in the evenings. and so they don’t allow him to practice       on school nights.“I know this       seem strict,”says Mrs. Liu“but we think we’re doing the right thing.He has to understand very      people can become professional athletes. It's a very difficult dream to achieve. so we think he needs to spend more time     his homework.
But Liu Yu still doesn’t agree.I know my parents care about me,but they are always talking about what         if I don't succeed in the future.But I will succeed! I think I should be allowed to make this decision for myself.Only then will I have a chance of achieving my dream.
小题1:
A.in theB.on theC.by theD.in this
小题2:
A.studyB.successC.failureD.health
小题3:
A.fifteen-years-oldB.fifteen years oldC.fifteen-year-old D.fifteen-year-olds
小题4:
A.achieveB.come trueC.realizesD.to achieve
小题5:
A.how manyB.how muchC.how oftenD.how long
小题6:
A.forB.againstC.inD.of
小题7:
A.whetherB.ifC.thatD.unless
小题8:
A.agreesB.disagreesC.think soD.supports
小题9:
A.importanceB.importantC.necessaryD.useful
小题10:
A.To beingB.BeingC.BecomeD.To do
小题11:
A. runningB.runC.to runD.runing
小题12:
A.shouldB.mightC.mustD.need
小题13:
A.manyB.a littleC.fewD.a few
小题14:
A.onB.inC.withD.for
小题15:
A.happensB.has happenedC.will happenD.happened
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