利用霍尔效应制作的霍尔元件以及传感器，广泛应用于测量和自动控制等领域．

如图1，将一金属或半导体薄片垂直置于磁场B中，在薄片的两个侧面a、b间通以电流I时，另外两侧c、f间产生电势差，这一现象称为霍尔效应．其原因是薄片中的移动电荷受洛伦兹力的作用向一侧偏转和积累，于是c、f间建立起电场E_H，同时产生霍尔电势差U_H．当电荷所受的电场力与洛伦兹力处处相等时，E_H和U_H达到稳定值，U_H的大小与I和B以及霍尔元件厚度d之间满足关系式U_H=R_H

（1）设半导体薄片的宽度（c、f间距）为l，请写出U_H和E_H的关系式；若半导体材料是电子导电的，请判断图1中c、f哪端的电势高；

（2）已知半导体薄片内单位体积中导电的电子数为n，电子的电荷量为e，请导出霍尔系数R_H的表达式．（通过横截面积S的电流I=nevS，其中v是导电电子定向移动的平均速率）；

（3）图2是霍尔测速仪的示意图，将非磁性圆盘固定在转轴上，圆盘的周边等距离地嵌装着m个永磁体，相邻永磁体的极性相反．霍尔元件置于被测圆盘的边缘附近．当圆盘匀速转动时，霍尔元件输出的电压脉冲信号图象如图3所示．

a．若在时间t内，霍尔元件输出的脉冲数目为P，请导出圆盘转速N的表达式．

b．利用霍尔测速仪可以测量汽车行驶的里程．除此之外，请你展开“智慧的翅膀”，提出另一个实例或设想．

第三部分：阅读理解（共20小题，每题2分，满分40分）

阅读下列短文，从每题所给的四个选项（A、B、C和D）中，选出最佳答案，并在答题卡上将该项涂黑。

Today most Chinese teenagers are happy to be favored by two parents and four grandparents

in their families. But have you ever thought about how to take care of four to twelve old people

when you grow up and get married? This could be a problem for most Chinese youths as China is

entering a new stage of an aging society.

An aging society refers to one where 10 percent or more of its population is over 60. By the end of 2007, Chinese over 60 years old have made up 11.7 percent of the nation’s total population.

“China is getting old before becoming rich,” said Cai Chuang, a professor at the Chinese Academy of Social Sciences. The population increase that has powered Chinese growth for three decades will give way over the next ten years to a rapid aging of the society. This will lead to weaker economic growth, because of a lack of labor resources.

As a result of the family planning policy, it is expected that the country’s total working population will decrease after 2015.

China is not the only country getting old. More that 60 countries have become aging societies and one in three people in the European Union is a senior citizen.

However, experts said that China doesn’t have a highly developed social security network to support the old. Social security is an insurance program protecting those in need, including the old, the disabled and others.

56．An aging society is a society that        .

A．less that 10% of its population is over 60

B．10% or more of its population is over 60

C．there are four old people in a family

D．most of its population is over 60

57．If China enters an aging society,      .

A．there will not be enough labor resources

B．the family planning policy will be ended

C．everyone needs to take care of four to twelve old people

D．China will have a highly developed social security network

58．What does Cai Chuang mean by saying “China is getting old before becoming rich.”?

A．China will be an aging society, then be a developed country.

B．China will be a rich country before being an aging society.

C．An aging society will affect Chinese economy development.

D．China will stop developing because of its aging problem.

59．The passage mainly talks about         .

A．an aging society             B．Chinese social problem

C．family panning policy     D．aging problem in China