问题 问答题

如图所示,两个水平放置的平行光滑金属导轨之间得距离为L,电阻不计,左端串联一个定值电阻R,金属杆ab的电阻为r质量为m.匀强磁场的磁感应强度为B.杆在恒力F作用下由静止开始运动.

(1)杆的运动速度为v时,感应电动势及感应电流的大小.

(2)求出金属杆的最大速度(3)已知金属杆达到最大速度时运动位移为s,求此过程中电路产生的焦耳热Q.

答案

(1)设速度为v时,ab产生的感应电动势为 E=BLv       

此时回路中的感应电流为I=

E
R+r
=
BLv
R+r
                   

(2)金属杆所受安培力为 F=BIL=

B2L2v
R+r
                    

金属杆的加速度为a=

F-F
m
            

可见,a随着速度的增加而减小,金属杆做加速度逐渐减小的加速运动,最后做匀速运动.

由④可知,当a=0时速度最大,最大速度为 vm=

F(r+R)
B2L2
           

(3)由功能关系得  FS=

1
2
mv2m
+Q                             

其中Q是系统产生的焦耳热

Q=FS-

mF2(R+r)2
2B4L4

答:(1)杆的运动速度为v时,感应电动势及感应电流的大小是

BLv
R+r

(2)金属杆的最大速度是

F(r+R)
B2L2

(3)此过程中电路产生的焦耳热是FS-

mF2(R+r)2
2B4L4

选择题
阅读理解

English is the most widely used language in the history of our planet. One in every seven human beings can speak it. More than half of the world’s books and three quarters of international mail are in English. Of all languages, English has the largest vocabulary — perhaps as many as two million words.

However, let’s face it: English is a crazy language. There is no egg in an eggplant, neither pine nor apple in a pineapple and no ham in a hamburger. Sweet-meats are candy, while sweetbreads, which aren’t sweet, are meat.

We take English for granted. But when we explore its paradoxes (探讨它的矛盾), we find that quicksand can work slowly, boxing rings are square,  public bathrooms have no baths in them.

And why is it that a writer writes, but fingers don’t fing, grocers don’t groce, and hammers don’t ham? If the plural of tooth is teeth, shouldn’t the plural of booth be beeth? One goose, two geese — so one moose, two meese?

How can a slim chance and a fat chance be the same, while a wise man and a wise guy are opposites? How can overlook and oversee be opposites, while quite a lot and quite a few are alike? How can the weather be hot as hell one day and cold as hell the next?

English was invented by people, not computers, and it reflects (反映) the creativity of human beings. That’s why, when stars are out, they are visible (能看见的); but when the lights are out, they are invisible. And why, when I wind up my watch, I start it; but when I wind up this essay, I end it.

1. According to the passage ______.

A. sweet-meats and sweetbreads are different things

B. there should be egg in an eggplant

C. pineapples are the apples on the pine tree

D. boxing rings should be round

2. Which of the following is the correct plural?

A. Beeth.        B. Geese.         C. Meese.        D. Tooth.

3. Which of the following includes two items which have the similar meaning?

A. A wise man and a wise guy.    B. Overlook and oversee.  

C. Quite a lot and quite a few.     D. Hot as hell and cold as hell.

4. The underlined words “wind up” in the last paragraph probably mean “______”.

A. blow        B. roll up         C. get hurt        D. finish

5. Through the many paradoxes in the English language, the writer wants to show that human beings are ______.

A. clever      B. crazy        C. lazy         D. dull