问题 问答题

如图所示,M,NM',N'是两组平行的光滑金属导轨,都水平放置,M,NM',N'的上方,二者如图所示连接着,间距分别为dd/3.空间有方向竖直向上的匀强磁场,磁感应强度大小是B.A.b是两根质量都是m,长度、电阻都相同的金属棒,分别放在两导轨上,并都与导轨垂直,其中a棒用两条长为l的细线悬挂在支架上,线伸直且棒与导轨接触良好.现将a棒向右拉,使两线伸直且处于水平位置,从静止释放,a摆下与导轨接触后继续向左摆动,至最大高度时用手接住,此时线与竖直方向的夹角为60°.求

(1)当a离开导轨后,b棒的运动速度大小及方向.

(2)若导轨电阻不计,由于电磁辐射而损耗的能量可以完全忽略,求b棒上生成的焦耳热.

 

答案

(1),方向向左 (2)

(1)a导线刚接触导轨时的速度,a导线刚离开导轨时的速度

a导线与导轨接触的短暂过程中,ab导线组成的闭合电路中有感应电流,两棒都将受到磁场力,由于两棒中的电流大小相等,但a导线在电路中的长度是b导线的3倍,因此a导线受到的磁场力也总是b导线的3倍,从而a导线的动量变化量的大小等于b导线的动量变化的3倍,即m(v2-v1)=3mv’,可解出b导线产生的速度大小,方向由左手定则判定是向左.

(2)根据能量守恒定律,电路中通过电阻生热损失的能量等于系统减少的机械能,即Q=mgl-mgl/2-mv’2/2=,

由于ab导线的电流大小总相等,但a导线在电路中的部分是b导线的3倍,因此b导线中损失的热量是总量的1/4,即

阅读理解

American cities are similar to other cities around the world; In every country. Cities reflect the values of the culture. American cities are changing, just as American society is changing.

After Wolld Wa Ⅱ,the population of most large American cities decreased; bowever. The populaton In omthe Sun Belt cities increased. Los Angeles and Houston are eitit where populatfon shife(转移)to and from the oity reflect the changing values of Amorican society. In the late 1940s and early 1950s,city residents(居民)became wealthier. They had more children so they needed more space, They moved out of their apartments in the city to buy their own homes. They bought houses in the suburbs(郊区).

Now things are changing. The children of the people who left the cities in the 1950s are now adults. Many, unlike their parents, want to live in the cities. They continue to move to Sun Belt cities and older ones of the Northeast and Midwest. Many young professionals are moving back into city. They prefer the city to the burbs because their jobs are there; or they jus enjoy the excitement and possibii that the city offers.

his  people     shift is bringing problems as well as benefits. Countless poot people  must leave  their apartments in the city because the owners want to sell the buildings or make apartments for sale instead of for rent. In the 1950s, many poor people did not have enough money to move to the suburbs; now many of these same people do not have enough money to stay in the cities.

Only a few years ago, people though that the older  American cities were dying. Some city residents now see a bright,  new fiture,  Others see only problems and comfiels the thing is sure many dying cities are alive again.

1. What does the author  think of cities all over the world?

A. They are alive.                        B. They are hopeless.

C. They are similar.                            D. They are different.

2.Why did American city residents want to live in the suburbs after World War Ⅱ?

A. Because older American cities were dying.

B. Because they were richer and needed more space.

C. Because cities contained the wont pare of society.

D. Because they could hardly afford a live in the city.

3. According to the 4th paragraph, a great many poor people in American cities_________.

A. are faced with housing problems

B. are forced to move to the suburbs

C. want to sell their buildings

D. need more money for daily expenses

4. We ear comude  from the text that ________.

A. American cities are changing far the worse

B. people have different views on American cities

C. many people are now moving from American cities

D. the population is decreasing in older American cities

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