问题 填空题

直流电动机是利用______的原理制成的,其中,有一个周期性改变线圈中电流方向的装置,叫______.

答案

直流电动机是根据通电线圈在磁场中受力转动原理制成的,在线圈转过平衡位置时,如果不改变电流的方向,线圈中两根导体的受力方向不变,在力的作用下线圈将在平衡位置附近来回摆动,为了使线圈能够持续转动,在输入电流时采用换向器,使线圈在刚过平衡位置时及时改变线圈中的电流方向,改变线圈的受力方向,从而使线圈能够实现连续转动.

故答案是:通电线圈在磁场中受力转动、换向器.

单项选择题
阅读理解

阅读理解。

     It was my first day at school in London and I was half-excited and half-frightened. On my way to

school I wondered what sort of questions the other boys would ask me and rehearsed (practiced) all

the answers: "I am nine years old. I was born here but I haven't lived here since I was two. I was living

in Farley. It's about thirty miles away. I came back to London two months ago." I also wondered if it

was the custom for boys to fight strangers like me, but I was tall for my age. I hoped they would decide

not to risk it.

     No one took any notice of me before school. I stood in the center of the playground, expecting someone

to say "hello", but no one spoke to me. When a teacher called my name and told me where my classroom

was, one or two boys looked at me but that was all the curiosity my arrival aroused.

     My teacher was called Mr. Jones. There were 42 boys in the class, so I didn't stand out there, either,

until the first lesson of the afternoon. Mr. Jones was very fond of Charles Dickens and he had decided to

read aloud to us from David Copperfield, but first he asked several boys if they knew Dickens' birthplace,

but no one guessed right. A boy called Brian, the biggest in the class, said: "Timbuktu", and Mr. Jones went

red in the face. Then he asked me. I said:"Portsmouth", and everyone stared at me because Mr. Jones said

I was right. This didn't make me very popular, of course.

     "He thinks he's clever," I heard Brian say.

     After that, we went out to the playground to play football. I was in Brian's team, and he obviously had

Dickens in mind because he told me to go in goal. No one ever wanted to be the goalkeeper.

     "He's big enough and useless enough," Brian said when someone asked him why he had chosen me.

     I suppose Mr. Jones remembered Dickens, too, because when the game was nearly over, Brian pushed

one of the players on the other team, and he gave them a penalty. As the boy kicked the ball hard along the

ground to my right, I threw myself down instinctively and saved it. All my team crowded round me. My

bare knees were grazed and bleeding. Brian took out a handkerchief and offered it to me.

     "Do you want to join my gang (team)?" he said.

     At the end of the day, I was no longer a stranger.

1. The writer prepared to answer all of the following questions EXCEPT "_______".

A. How old are you?

B. where are you from?

C. Do you want to join my gang?

D. When did you come back to London?

2. We can learn from the passage that _______.

A. boys were usually unfriendly to new students

B. the writer was not greeted as he expected

C. Brian praised the writer for his cleverness

D. the writer was glad to be a goalkeeper

3. The underlined part "I didn't stand out" in paragraph 3 means that the writer was not _______.

A. noticeable

B. welcome

C. important

D. outstanding

4. The writer was offered a handkerchief because _______.

A. he threw himself down and saved the goal

B. he pushed a player on the other team

C. he was beginning to be accepted

D. he was no longer a new comer