问题 选择题

边长为L的正方形金属框在水平恒力F作用下运动,穿过方向如图的有界匀强磁场区域,磁场区域的宽度为d(d>L).已知ab边进入磁场时,线框的加速度恰好为零.则线框进入磁场和穿出磁场的过程相比较,有

A.产生的感应电流方向相反

B.受的安培力大小相等方向相反

C.进入磁场过程的时间等于穿出磁场过程的时间

D.进入磁场过程中产生的热量少于穿出磁场过程产生的热量

答案

AD

题目分析:线框进入磁场时,磁通量增大.穿出磁场时,磁通量减小,由楞次定律得知,感应电流方向相反.故A正确.安培力阻碍导体与磁场的相对运动,则安培力均向左.故B错误.进入磁场时,线框做匀速运动.进入磁场后,线框匀加速通过位移d-L,穿出磁场时,速度增大,安培力增大,则线框做减速运动,穿出磁场时,线框速度大于等于进入磁场时的速度.则进入磁场过程的时间大于穿出磁场过程的时间.故C错误.根据功能关系:进入磁场过程,F做功等于线框的发热量.穿出磁场过程,F做功与动能的减小量之和等于线框的发热量,而拉力做功相等,则进入磁场过程的发热量少于穿出磁场过程的发热量.故D正确.

点评:关键在于运用牛顿定律对线框运动过程的分析.

阅读理解

阅读理解。

     Bad days happen to us all. Often a day that starts out bad just continues to get worse. But why?

     Sometimes there's a "domino effect (多米诺骨牌效应)" with bad events. For example, you sleep

through the alarm, so you're stressed. And you will sit in the bad traffic, which will also make you late to

work, etc.

     How can we turn a bad day to a good day? Here are some ideas that have worked for me:

     Smile (微笑) a lot

     Smiling a lot will help you feel relaxed. Research has shown that smiling produces three useful

chemicals (化学物质) which make us feel good.

     Talk to a good friend

     Talk to someone who cares and listens really does help us feel better, too.

     Concentrate on your good things

     It's hard to concentrate on how bad things are when you're concentrating on how good things are!

Think of 10 or more things you're thankful for, and you'll achieve a whole new outlook (人生观) on

your day.

     Get some exercise

     If you can, go for a walk. Exercising will help you feel better. Exercising can get your day back to be

normal again.

     Make the day a challenge

     If you make yourself think of the day as a challenge, you'll start to do all the things you can to make

your day better.

1. According to the passage, the "domino effect" is an effect that                                  .

A. turns a good day to a bad day

B. makes one person affect another

C. turns bad things into good things

D. gets one event to cause other similar events to happen

2. Study shows that smiling produces three useful chemicals which make us feel                                  .

A. terrible    

B. relaxed  

C. stressed    

D. worried

3. According to the writer, if we consider a bad day as a challenge, we'll                                  .

A. try our best to do things    

B. give up doing things

C. lose all our confidence      

D. have a terrible day

4. How many ways does the writer give us to help us not to make a bad day stay bad?

A. 3.    

B. 4.    

C. 5.        

D. 6.

5. Which of the following can be the BEST title for the passage?

A. When we should smile a lot

B. How to deal with problems

C. How to turn a bad day around

D. What things we should concentrate on

单项选择题