问题 问答题

如图甲所示,光滑且足够长的金属导轨MN、PQ平行地固定的同一水平面上,两导轨间距L=0.20cm,两导轨的左端之间连接的电阻R=0.40Ω,导轨上停放一质量m=0.10kg的金属杆ab,位于两导轨之间的金属杆的电阻r=0.10Ω,导轨的电阻可忽略不计.整个装置处于磁感应强度B=0.50T的匀强磁场中,磁场方向竖直向下.现用一水平外力F水平向右拉金属杆,使之由静止开始运动,在整个运动过程中金属杆始终与导轨垂直并接触良好,若理想电压表的示数U随时间t变化的关系如图乙所示.求金属杆开始运动经t=5.0s时,

(1)通过金属杆的感应电流的大小和方向;

(2)金属杆的速度大小;

(3)外力F的瞬时功率.

答案

(1)由图象可知,t=5.0s时,U=0.40V.

此时电路中的电流(即通过金属杆的电流)I=

U
R
=1.0A

用右手定则判断出,此时电流的方向由b指向a.

(2)金属杆产生的感应电动势E=I(R+r)=0.50V

因E=BLv,所以0.50s时金属杆的速度大小v=

E
BL
=5.0m/s

(3)金属杆速度为v时,电压表的示数应为 U=

R
R+r
BLv.

由图象可知,U与t成正比,由于R、r、B及L均与不变量,所以v与t成正比,即金属杆应沿水平方向向右做初速度为零的匀加速直线运动.

金属杆运动的加速度a=

v
t
=1.0m/s2

根据牛顿第二定律,在5.0s末时对金属杆有:F-BIL=ma,

解得F=0.20N.

此时F的瞬时功率P=Fv=1.0W.

答:(1)通过金属杆的感应电流的大小为1.0A,方向由b指向a.

(2)金属杆的速度大小为5.0m/s.

(3)外力F的瞬时功率为1.0W.

阅读理解

阅读理解。

     People from other countries will find many different surprising things in Britain.

     Greek: A man from Greece sees a special way of driving in Britain. People drive on the left in the

 British street, but the British people never feel surprised. The Greek man said, "I've been in Britain for

 nearly a month now. But at first, I couldn't always remember to drive on the left. Every time, my friend  

 reminded me. "

     Japanese: The Japanese people can't understand why British people keep their shoes on at home. In

 Japan, people walk in the room without shoes. If a visitor keeps his shoes on at home in Japan , the

 Japanese people think he is very impolite.

     Spanish: The Spanish people mind the way that the British people greet each other. They themselves

 seem to be afraid of touching. When they meet, they just smile or nod but never kiss or hug in Spain. But

 the British people think kissing is a very usual greeting when they meet!

     There is an old saying, " When in Rome, do as Romans do". When we go to a foreign country, we

should follow the etiquette of country.

1. The Greek visitor _____.

    A. always drives on the left

    B. used to drive on the left

    C. drove correctly in Britain with his friend's help

2. _____ think it's impolite to wear shoes at home.

    A. The Japanese

    B. The Greek

    C. The English

3. When greeting, the Spanish _____

    A. kiss each other

    B. hug each other

    C. smile at each other

4. _____ people seem not to be afraid of touching each other.

    A. The Spanish

    B. The British

    C. The Japanese

5. Etiquette in the world        .

    A. is the same .

    B. is important

    C. is sometimes different and interesting

选择题