问题 计算题

如图所示,abcd是一边长为l的匀质正方形导线框,总电阻为R,今使线框以恒定速度v水平向右穿过方向垂直于纸面向里的匀强磁场区域.已知磁感应强度为B,磁场宽度为3l,求线框在进入磁区、完全进入磁区和穿出磁区三个过程中ab两点间电势差的大小.

答案

题目分析:导线框在进入磁区过程中,ab相当于电源,等效电路如下图甲所示.

EBlv rRRRI

Uab为端电压;所以UabIR.

导线框全部进入过程中,磁通量不变,感应电流

I=0,但UabEBlv

导线框在穿出磁区过程中,cd相当于电源,等效电路如下图乙所示.

EBlvrRRRI

UabIRab×R.

点评:本题由楞次定律判断电势的高低,确定电势差的正负.分析UAB与感应电动势关系是关键,要区分外电压和内电压.

阅读理解

Your peers are people your age or close to it. Besides close friends, your peers include other children you know who are the same age------like people in your grade, sports team, or neighborhood. Peers may have a positive influence on each other and play important roles in each other’s lives:

Positive examples

Peers set plenty of good examples for each other. Having peers who do well in school or do their best in a sport can influence you to work hard to realize your dream, too. Peers who are kind and helpful can influence you to build these qualities in yourself. Even peers you’ve never met can be role models(模范). For example, watching someone your age compete in the Olympics or give a piano concert might inspire you to go after a dream of your own.

Feedback(反馈) and advice

Your peers listen and give you feedback as you try out new ideas. They can help you make decisions, too: what courses to take, whether to get your hair cut or let it grow, or how to solve a problem. Peers often give each other good advice. Your peers will be quick to tell you when they think you’re making a mistake or doing something dangerous.

Socializing(交际)

Being with your peers----such as classmates or teammates----gives you a chance to learn how to make friends and deal with disagreements with people.

New experiences

Your peers might get you involved(参与) in clubs, sports, or volunteer groups. Your world would be far less rich without peers encouraging you to try some new food for the first time, or listen to a CD you have never heard before.

小题1:The first paragraph is mainly about     .

A.who your peers are

B.how to find peers whose interests are similar to yours

C.the negative influence of peers on each other

D.the difference between your close friends and your peers小题2:The author makes  points of how peers have a positive influence on each other.

A.three

B.four

C.five

D.six小题3:With which of the following may the author agree?

A.Peers you have never met cannot influence you.

B.You can ask your friends to make a decision for you.

C.You should not play with peers you don’t like.

D.Your peers may cause you to experience new things.小题4:The author mentions that good role models can influence you to      .

① work hard to realize your dream

② do something dangerous

③ build good qualities in yourself

④ have fewer friends

A.①③

B.②④

C.②③

D.③④小题5:From the fourth paragraph, we can learn that being with your peers gives you a chance to       .

A.influence other people

B.play an important role

C.learn how to make more friends

D.improve your English skills

判断题