问题 选择题

如图所示,电阻为r的矩形线圈在匀强磁场中绕垂直于磁场的轴以角速度ω=100πrad/s匀速转动。t=0时刻线圈平面与磁场垂直,各电表均为理想交流电表,则

A.t=0时刻线圈中的感应电动势最大

B.1s内电路中的电流方向改变100次

C.滑片P下滑时,电压表的读数变大

D.开关K处于断开状态和闭合状态时,电流表的读数相同

答案

BC

题目分析:t=0时刻,线圈与磁场垂直,位于中性面,感应电动势为零,A选项错误;由ω=2πf,得交变电流的频率为50Hz,电流方向每秒改变100次,B选项正确;当滑片P下滑时,滑动变阻器电阻增大,则电压表示数增大,C选项正确;开关K闭合时,交变电流流过电容器,则电流表示数改变,D选项错误。

阅读理解

阅读理解。

     My parents were in a huge argument, and I was really upset about it. I didn't know who I should talk

with about how I was feeling. So I asked Mom to allow me to stay the night at my best friend's house.

Though I knew I wouldn't tell her about my parents' situation, I was looking forward to getting out of the

house. I was in the middle of packing up my things when suddenly the power went out in the neighborhood.

Mom came to tell me that I should stay with my grandpa until the power came back on.

      I was really disappointed because I felt that we did not have much to talk about. But I knew he would

be frightened alone in the dark. I went to his room and told him that I'd stay with him until the power was

restored. He was quite happy and said, "Great opportunity."

      "What is?" I asked.

     "To talk, you and I," he said. "To hold a private little meeting about what we're going to do with your

mom and dad, and what we're going to do with ourselves now that we're in the situation we are in."

     "But we can't do anything about it, Grandpa," I said, surprised that here was someone with whom I could

share my feelings and someone who was in the same "boat" as I was .

     And that's how the most unbelievable friendship between my grandfather and me started. Sitting there in

the dark, we talked about our feeling and fears of life---from how fast things change, to how they sometimes

don't change fast enough. That night, because the power went out, I found a new friend, with whom I could

safely talked about all my fears and pains, whatever they may be.

      Suddenly, the lights all came back on. "Well," he said, " I guess that means you'll want to go now. I really

like our talk. I hope the power will go out every few nights!"

1. I wished to get out of the house because _______.

A. I was angry about my parents' quarrel

B. I found nobody to share my feelings with

C. I wanted to escape from the dark house

D. I planned to tell my friend about my trouble

2. Grandpa was happy to see me because _______.

A. he could discuss the problem with me

B. he had not seem me for a long time

C. he was afraid of darkness

D. he felt quite lonely

3. What can be inferred from the passage?

A. The grandchild was eager to leave.

B. They would have more chats.

C. The lights would go our again.

D. It would no longer be dark.

单项选择题