问题 问答题

如图1所示,在一倾角为37°的粗糙绝缘斜面上,静止地放置着一个匝数n=10匝的圆形线圈,其总电阻R=3.14Ω、总质量m=0.4kg、半径r=0.4m.如果向下轻推一下此线圈,则它刚好可沿斜面匀速下滑现在将线圈静止放在斜面上后.在线圈的水平直径以下的区域中,加上垂直斜面方向的,磁感应强度大小按如图2所示规律变化的磁场(提示:通电半圆导线受的安培力与长为直径的直导线通同样大小的电流时受的安培力相等)问:

(1)刚加上磁场时线圈中的感应电流大小I=?

(2)从加上磁场开始到线圈刚要运动,线圈中产生的热量Q=?

(最大静摩擦力等于滑动摩擦力,sin37°=0.6,cos37°=0.8,g取10m/s2.)

答案

(1)由闭合电路的欧姆定律I=

E
R

由法拉第电磁感应定律E=n

△B•S
△t

由图,

△B
△t
=0.5T/S

S=

1
2
πr2

联立解得I=0.4A

(2)设线圈开始能匀速滑动时受的滑动摩擦力为Fμ,则mgsin37°=Fμ

加变化磁场后线圈刚要运动时nBIL=mgsin37°+Fμ,其中L=2r

由图象知B=B0+kt=1+0.5t

由焦耳定律Q=I2Rt

联立解得Q=0.5J

答:(1)刚加上磁场时线圈中的感应电流大小为0.4A;

(2)从加上磁场开始到线圈刚要运动,线圈中产生的热量为0.5J.

阅读理解

阅读理解。

     When we think of Hollywood (好莱坞), we think of films and famous film stars. They are part of

Hollywood's history. Today people make films on other places too. Not all famous film stars live in Hollywood.

But Hollywood is still a very special city in Los Angeles, California (洛杉矶, 加利福尼亚).

    You can easily see where Hollywood is in Los Angeles. There is a big sign on the hills, it says

"HOLLYWOOD". The whole letters are fifty feet tall. You can see the sign from far away. The Hollywood

sign is a famous landmark (标识) in Los Angeles. Many postcards show this famous Hollywood landmark.

      In the hills of Hollywood, there is also an open - air theatre called the Hollywood Bowl. It has seventeen

thousand seats and a very special stage (舞台). The design (设计) of the stage was by the great American

architect (建筑师) Frank Lioyd Wright. You can listen to all kinds of music at the Hollywood Bowl.

1. The best title for the reading should be ___________.

A. Hollywood

B. Famous Films and Film Star

C. The History of Hollywood

D. Famous Hollywood Film Star

2. We learn from reading that ___________.

A. "HOLLYWOOD" can just be seen (被看见) from far away

B. Hollywood is now a place only for travellers

C. all the films are made in Hollywood

D. Hollywood is one part of Los Angeles

3.  The word "special" in the reading means ____________.

A. new

B. rich

C. famous

D. unusual

4. At the Hollywood Bowl ___________.

A. all postcards are made

B. only 7,000 people can get together

C. music can be enjoyed

D. you can hardly listen to different kinds of concerts

5.  It seems that ___________.

A. some famous film stars don't choose to live in Hollywood

B. film stars don't think Hollywood a good place to make films

C. the Hollywood Bowl is bigger than any other open - air theatre in the world

D. the Hollywood Bowl is a hill in Hollywood

问答题 案例分析题