问题 问答题

如图所示,一水平放置的平行导体框架宽度L=0.5m,接有电阻R=0.20Ω,磁感应强度B=0.40T的匀强磁场垂直导轨平面方向向下,仅有一导体棒ab跨放在框架上,并能无摩擦地沿框架滑动,框架及导体ab电阻不计,当ab以v=4.0m/s的速度向右匀速滑动时.试求:

(1)导体ab上的感应电动势的大小.

(2)要维持ab向右匀速运行,作用在ab上的水平力为多大?

(3)电阻R上产生的焦耳热功率为多大?

答案

(1)导体ab垂直切割磁感线,产生的电动势大小:

E=BLv=0.40×0.50×4.0 V=0.80 V  ①

(2)导体ab相当于电源,由闭合电路欧姆定律得回路电流:

I=

E
R+r
=
0.80
0.20
 A=4.0 A  ②

导体ab所受的安培力:

F=BIL=0.40×4.0×0.50 N=0.80 N  ③

由于ab匀速运动,所以水平拉力:

F′=F=0.80 N  ④

(3)R上的焦耳热功率:

P=I2R=4.02×0.20 W=3.2 W  ⑤

答:(1)导体ab上的感应电动势的大小为0.80V.

(2)要维持ab向右匀速运行,作用在ab上的水平力为0.80N.

(3)电阻R上产生的焦耳热功率为3.2W.

选择题
阅读理解

T-shirts out; uniforms in

School uniforms(制服) are becoming more and more popular across the U.S.A. That’s no surprise, because they offer many benefits. They immediately end the powerful social sorting and labeling (标记) that come from clothing. If all students are dressed in the same way, they will not pay too much attention to their clothing, and some of them will not be laughed at for wearing the “wrong” clothes.

Some people are against the strict rule of school uniforms, but they do not realize that students already accept a kind of rule — wanting to look just like their friends. The difference is that the clothing students choose for themselves creates social barriers(障碍); school uniforms tear those barriers down.

As in other places, uniforms remind the wearers of their purposes and duties. For example, when a man or woman puts on a police uniform, he or she becomes, for a time, the symbol (象征) of law and order. The uniform means to the wearer his or her special duties and sends the same message to everyone the wearer meets. People with different jobs wear uniforms of one kind or another. For students, the school uniform reminds them that their task for the six or seven hours they are in school is to get an education.

Some parents are unhappy about uniforms, saying that school uniforms will affect their children’s “creativity.” First, as noted above, the clothes students choose to wear do not necessarily express their individuality (个性). They just copy their classmates. Second, students have the rest of the day to be as creative as they like. While they’re in school, their job is to master reading, writing, and maths; this should take up all the creativity they have. Mastery of those skills will be good for the students to build up their creativity in every way.

小题1:In Paragraph 1, the word “benefits” probably means       .

A.tasks

B.messages

C.differences

D.advantages小题2: From the passage we learn that uniforms in general       .

A.prevent the wearers from being laughed at

B.help the wearers keep their duties in mind

C.are seen as a symbol of power

D.help to create social barriers小题3: Some people are against school uniforms because       .

A.they fail to realize that students have accepted the uniforms

B.they believe that uniforms will make students less creative

C.they don’t agree that uniforms can remove social barriers

D.they think that school uniforms are too popular小题4:The author would probably agree that       .

A.it makes no difference whether to wear school uniforms or not

B.students’ individuality may not come from school education

C.students’ creativity is related to the clothes they choose

D.school uniforms help to create equality among students小题5: School uniforms are becoming more and more popular and important because _____

A.they can stop the powerful social sorting and labeling.

B.if all students are dressed in the same way, they will not pay too much attention to their clothing.

C.uniforms can remind the wearers of their purposes and duties

D.school uniforms will affect their children’s creativity.