问题 选择题

如图所示,在磁感应强度为B0、方向竖直向上的匀强磁场中,水平放置一根长通电直导线,电流的方向垂直于纸面向里. a、b、c、d是以直导线为圆心的同一圆周上的四点,在这四点中(  )

A.a点的磁感应强度最大

B.b点的磁感应强度最大

C.c点的磁感应强度最大

D.d点的磁感应强度最大

答案

用右手螺旋定则判断通电直导线在abcd四个点上所产生的磁场方向,如图所示:

A、a点有向上的磁场,还有电流产生的向上的磁场,电流产生的磁感应强度和原磁感应强度方向相同,叠加变大.故A正确;

B、b点有向上的磁场,还有电流产生的水平向左的磁场,磁感应强度叠加变大,方向向左上.故B错误;

C、c点电流产生的磁感应强度和原磁感应强度方向相反,叠加变小.故C错误;

D、d点有向上的磁场,还有电流产生的水平向右的磁场,叠加后磁感应强度的方向向右上.d点与b点叠加后的磁场大小相等,但是方向不同.故D错误;

故选:A

阅读理解

Mark was walking home from school one day when he noticed a boy ahead of him. He had tripped (绊倒) and dropped all of the books he was carrying, along with some clothes, a baseball bat and a glove.

Mark knelt (跪) down and helped the boy pick up the things. Since they were going the same way, he offered to carry some things for the boy.

As they walked, Mark discovered the boy’s name was Bill. He loved video games, baseball and history, but he was having trouble with some of his subjects.

They arrived at Bill’s home first. Mark was invited in for a Coke and to watch some television. The afternoon passed pleasantly with some laughs and talk. Then Mark went home.

They continued to see each other around school, had lunch together once or twice, then both graduated from junior school.

Three days before graduation, Bill asked Mark if they could talk.

Bill reminded (提醒) Mark of the day years ago when they had first met. “Did you ever wonder why I was carrying so many things home that day?” asked Bill.

“You see, I took away all my things from school. I had stored away some of my mother’s sleeping pills (安眠药). I was going home to kill myself. But after we spent some time talking and laughing, I realized that if I had died, I would have missed so many good times. So you see, Mark, when you picked up those books that day, you did a lot more._____________________”

小题1:When Bill fell down on his way home, Mark ________.

A.didn’t notice that

B.went away quickly

C.carried all the things for him

D.helped him pick up the things小题2:Which of the following is NOT true?

A.Mark was a warm-hearted boy.

B.Bill and Mark were in the same school.

C.Bill was good at all of his subjects.

D.Bill loved video games, baseball and history.小题3:At Bill’s home, they didn’t ________.

A.drink some Coke

B.play video games

C.have a pleasant talk

D.watch TV小题4:Three days before graduation, Bill asked Mark to talk because he ________.

A.hoped to go on holiday together

B.wanted to kill himself again

C.hoped to have lunch together

D.wanted to tell him his story小题5:What would Bill probably say at last?

A.You saved my life.

B.I should die.

C.You did a good job.

D.You helped me a lot.

判断题