问题 多选题

如图,在光滑水平面上用恒力F拉质量为m、边长为a、总电阻为R的单匝均匀正方形铜线框,线框从位置1开始以速度v0进入磁感应强度为B的匀强磁场,到达位置2时线框刚好全部进入磁场,在位置3时线框开始离开匀强磁场,到达位置4时线框刚好全部离开磁场.则(  )

A.在位置1时,线框右侧边MN的两端电压为Bav0

B.线框从位置1到位置2的过程与从位置3到位置4的过程产生的感应电流方向相反

C.线框从位置1到位置2的过程与从位置3到位置4的过程所受的安培力方向相同

D.从位置1到位置2的过程中流过线框某一截面的电荷量等于

Ba2
R

答案

A、在位置1时,线框MN边切割磁感线产生感应电动势为 E=Bav0,MN的两端电压为 U=

3
4
E=
3
4
Bav0.故A错误.

B、根据楞次定律判断可知:线框从位置1到位置2的过程产生逆时针方向的感应电流,从位置3到位置4的过程产生顺时针方向的感应电流,故B正确.

C、根据左手定则可知,线框从位置1到位置2的过程与从位置3到位置4的过程所受的安培力方向均向左,方向相同,故C正确.

D、根据感应电荷量经验公式q=

△Φ
R
可知,从位置1到位置2的过程中流过线框某一截面的电荷量 q=
△Φ
R
=
Ba2
R
.故D正确.

故选:BCD.

阅读理解

Daniel was always making excuses for forgetting to do things. His usual excuse was: “ I was too busy.” or “I didn’t have time.”

Daniel’s parents were worried about him. “ He shouldn’t go through life making excuses all the time.” his father said. “ No one will trust him to do anything. He won’t be able to keep a job. No sensible(明智的)girl will marry him.”

“I don’t know what we can do.” his mother said.

“I have an idea.” his father said. “If this doesn’t work, nothing will.”

The next day was Daniel’s birthday. He was really looking forward to getting exciting presents. He woke up, expecting his parents to wish him a happy birthday. But all day what they said was “Good morning, Daniel. Hurry, or you will be late for school.” There were no presents, not even a card. He was really disappointed.

“Never mind.” he thought. (3) “There’ll be a surprise for me after school.” But there wasn’t. Sadly he said to his parents at dinner, “Today’s my birthday.”

“Oh,” his father said. “So it is. I forgot.”

“So did I.” his mother said.

“How could you forget?” Daniel asked. “There must be a reason.”

“Well, yes.” his father said. “We have a really good excuse for forgetting your birthday, Daniel. We didn’t remember your birthday because we were too busy.”

Poor Daniel! He never had a silly excuse for not doing something again.

小题1:判断Daniel got his birthday present at last(   )。

小题2:He thought there would be ________for him afte school.

小题3:What did Daniel feel when he didn’t receive any presents?

_______________________________________________________.

小题4:What do you learn from the story?_____________________________________.

小题5:翻译划横线句子?

_________________________________________________________________.

判断题