问题 选择题

在匀强磁场中有一个半径为r的n匝圆形线圈,总电阻为R,线圈与一个电荷量计串联.线圈平面与磁感线垂直.当线圈由原位置迅速翻转180°过程中,电荷量计显示通过线圈的电荷量为q.由此可知该匀强磁场的磁感应强度B的大小为(  )

A.

qR
πnr2

B.

qR
2πnr2

C.

qR
r2

D.

πqr2
2nR

答案

由法拉第电磁感应定律:线圈产生的感应电动势 E=n

△Φ
△t

由闭合电路欧姆定律得感应电流 I=

E
R

根据电量的公式q=It,可得q=n

△Φ
R

由于开始线圈平面与磁场垂直,现把探测圈翻转180°,则磁能量的变化△Φ=2BS=2B?πr2

所以由上公式可得:q=n

2Bπr2
R

则得 B=

qR
2πnr2

故选B

阅读理解

I will never forget the lesson about my window. Four years ago, I moved to a house in a large town. One of my new neighbors’ house was only a few feet away from mine. There lived a woman. Through one of my windows, I could see her reading by her window every afternoon.

Several months later, I found I couldn’t see the woman clearly. I thought her window was too dirty. I said to myself, “Why doesn’t she clean her window? It looks terrible!”

One afternoon, I decided to clean my house including(包括) the window. I felt tired after three hours of hard work. So I sat down by the window for a rest. What a surprise! I could see the woman reading there clearly again! By that time, I realized that my own window was too dirty, not hers! I really felt ashamed for myself. I had been watching her through my dirty window in the past days!

The experience is very important for me. So I try to clean the window of my heart before judging(评判) others.

小题1:How long has the writer lived in the house according to the passage?

A. Two years.                B. Three years.          C. Four years.    

小题2:The writer couldn’t see the woman clearly because ______________.

A. the writer’s window was dirty              

B. the woman’s house was too far

C. the woman didn’t open her window

小题3:The underlined word “ashamed” means _________________.

A. 高兴的                    B. 放松的                C. 羞愧的

小题4:According to the passage, the writer has learnt how to __________________.

A. clean the house           B. judge others          C. help neighbors

问答题 简答题