问题 多选题

一个矩形金属框MNPQ置于xOy平面内,平行于x轴的边NP的长为d,如图(a)所示.空间存在磁场,该磁场的方向垂直于金属框平面,磁感应强度B沿x轴方向按图(b)所示正弦规律分布,x坐标相同各点的磁感应强度相同.当金属框以大小为v的速度沿x轴正方向匀速运动时,下列判断正确的是(  )

A.若d=l,则线框中始终没有感应电流

B.若d=

1
2
,则当线框的MN边位于x=l处时,线框中的感应电流最大

C.若d=

1
2
,则当线框的MN边位于x=
1
4
处时,线框受到的安培力的合力最大

D.若d=

31
2,则线框中感应电流周期性变化的周期为
1
v

答案

当d=l时,线框移动时的磁通量始终不变,故无感应电流,故A正确;

d=

l
2
时,线框的MN边位于x=l处瞬间,MN、PQ所在处磁场为零,感应电动势为零,故B错误;

d=

l
2
时,线框的MN边位于
l
4
处时,两边所处的磁场最大,两边磁场方向相反,所以感应电动势最大,安培力也最大,故C正确;

d=

3
2
l时,线框每前进l,磁通量的变化将重复原来的情况,故线框中感应电流周期性变化的周期为
l
v
,故D正确.

故选ACD.

阅读理解

The meaning of the word “volunteer” may be a little different in different countries, but it usually means “one who offers his or her services.” There are many different ways in which people can volunteer, such as taking care of sick people, working in homes for homeless children, and picking up garbage from beaches and parks.Volunteers may work within their own countries or in other countries.They are often people with a strong wish to help those who are less fortunate than themselves.Volunteers don’t expect any kind of pay.

At the root of volunteering is the idea that one person may have the ability to offer services that can help other people.Tracy, a good friend of mine, however, recently came back from India with a new idea of what being a volunteer means.She worked for two and a half weeks in one of Mother Teresa’s homes in Calcutta.The following is her story.

“I first heard about Mother Teresa in my high school.We watched a video about her work in India and all over the world.I was so moved by her spirit to help others and her endless love for every human being that after I graduated from high school, I too wanted to try her kind of work.So with two friends I flew to Calcutta for a few weeks.”

“I was asked to work in a home for sick people.I helped wash clothes and sheets, and pass out lunch.I also fed the people who were too weak to feed themselves and tried to cheer them up.I felt it was better to share with them than to think that I have helped them.To be honest, I don’t think I was helping very much.It was then that I realized that I had not really come to help, but to learn about and experience another culture that helped improve my own understanding of life and the world.”

66.According to the text, a volunteer refers to a person who ______.

A.is willing to help those in need without pay  

B.can afford to travel to different places

C.has a strong wish to be successful          

D.has made a big fortune in life

67.Tracy started her work as a volunteer _______.

A.after she met Mother Teresa              

B.after she finished high school

C.when she was touring Calcutta            

D.when she was working in a hospital

68.Why did Tracy choose to be a volunteer?

A.She liked to work with Mother Teresa.     

B.She had already had some experience.

C.She was asked by Mother Teresa’s example.

D.She wanted to follow Mother Teresa’s example.

69.What is Tracy’s “new idea”(Paragraph 2) of being a volunteer?

A.Going abroad to help the sick.           

B.Working in Mother Teresa’s home.

C.Doing simple things to help the poor.      

D.Improving oneself through helping others.

70.What is the best title for the passage?

A.How To Be A Volunteer.             B.Voluntary Life In India.

C.A Different Meaning Of Volunteer.   D.Inspiration From Mother Teresa.

单项选择题 A1/A2型题