问题 计算题

(10分)如图所示,一矩形线圈在匀强磁场中绕OO′轴匀速转动,磁场方向与转轴垂直。线圈匝数为n,电阻为r,长为1,宽为2,角速度为ω。磁场的磁感应强度为B。线圈两端外接电阻为R的用电器,和一个理想交流电流表。求:

(1)线圈中产生的最大感应电动势Em

(2)电流表的读数I。

答案

(1)nBωl1l2(2) 

1)Em = nBωS=nBωl1l2       (4分)

(2)由           (2分)

             (2分)

求出    (2分)

本题考查交变电流的形成和有效值的计算,由公式Em = nBωS可求出电压最大值,由欧姆定律可计算电流最大值

单项选择题
阅读理解

阅读理解。

     We walked in so quietly that the nurse at the desk didn't even lift her eyes from the book. Mum pointed at

a big chair by the door and I knew she wanted me to sit down. While I watched with mouth open in surprise,

Mum took off her hat and coat and gave them to me to hold. She walked quietly to the small room and took

out a wet mop (拖把). She pushed the mop past the desk and as the nurse looked up, Mum nodded and said,

"Very dirty floors."

     "Yes, I'm glad they've finally decided to clean them," the nurse answered. She looked at Mum strangely and

said, "But aren't you working late?"

     Mum just pushed harder, each swipe (拖一下) of the mop taking her farther and farther down the hall. I

watched until she was out of sight and the nurse had turned back to write in the big book.

     After a long time Mum came back. Her eyes were shining. She quickly put the mop back and took my hand.

As we turned to go out of the door, Mum bowed politely to the nurse and said, "Thank you."

     Outside, Mum told me:"Linda is fine. No fever."

     "You saw her, Mum?"

     "Of course, I told her about the hospital rules, and she will not expect us until tomorrow. It's a fine hospital.

But such floors! A mop is no good. A brush (刷子) is needed."

1. When she took a mop from the small room,what Mum really wanted to do was . [ ]

A. to clean the floor

B. to please the nurse

C. to see a patient (病人)

D. to surprise the story-teller

2. When the nurse talked to Mum she thought Mum was a _____.[ ]

A. nurse

B. visitor

C. patient

D. cleaner

3. After reading the story what can we know about the hospital? [ ]

A. It is a children's hospital.

B. It has strict rules about visiting hours.

C. The conditions there aren't very good.

D. The nurses and doctors there don't work hard.

4. From the text we know that Linda is most likely _____. [ ]

A. the story-teller's sister

B. Mum's friend

C. the story-teller's classmate

D. Dad's boss