美国宝石学院（GIA）将钻石的荧光强度级别划分为（）等级A.三个 B.四个 C.五个

如图所示，在竖直向下，场强为E的匀强电场中，长为l的绝缘轻杆可绕固定轴O在竖直面内无摩擦转动，两个小球A、B固定于杆的两端，A、B的质量分别为m₁和m₂（m₁＜m₂），A带负电，电量为q₁，B带正电，电量为q₂．杆从静止开始由水平位置转到竖直位置，求：

（1）在此过程中电场力所做的功为多少？

（2）在竖直位置处两球的总动能为多少？

若将轻杆弯折成如图所示的“Γ”形，两边互相垂直、长度均为

1

2

，两端各固定一个金属小球A、B，在竖直向下，场强为E的匀强电场中，可绕过O点的水平轴在竖直平面内无摩擦转动．已知A球质量m₁=m，电量为+q，B球质量m₂=7m/2，B球也带负电，电量为-q．现将“Γ”形杆从OB位于水平位置由静止释放，求：

（3）OB杆能转过的最大角度为127°，则该电场强度的大小为多少？

（4）当两球的速度达到最大时，OB杆转过的角度为多大？

In the course of working my way through school, I took many jobs I would rather forget. But none of these jobs was as dreadful as my job in an apple plant. The work was hard; the pay was poor; and, most of all, the working conditions were terrible.

First of all, the job made huge demands on my strength. For then hours a night, I took boxes that rolled down a metal track and piled them onto a truck. Each box contained twelve heavy bottles of apple juice. I once figured out that I was lifting an average of twelve tons of apple juice every night.

I would not have minded the difficulty of the work so much if the pay had not been so poor. I was paid the lowest wage of that time—two dollars an hour. Because of the low pay, I felt eager to get as much as possible. I usually worked twelve hours a night but did not take home much more than $ 100 a week.

But even more than the low pay, what made me unhappy was the working conditions. During work I was limited to two ten-minute breaks and an unpaid half hour for lunch. Most of my time was spent outside loading trucks with those heavy boxes in near-zero-degree temperatures. The steel floors of the trucks were like ice, which made my feet feel like stone. And after the production line shut down at night and most people left, I had to spend two hours alone cleaning the floor.

I stayed on the job for five months, all the while hating the difficulty of the work, the poor money, and the conditions under which I worked. By the time I left, I was determined never to go back there again.

49. Why did the writer have to take many jobs at that time?

A. To pay for his schooling.

B. To save for his future.

C. To support his family

D. To gain some experience

50. The following facts describe the terrible working conditions of the plant EXCEPT ______.

A. loading boxes in the freezing cold                     B. having limited time for breaks

C. working and studying at the same time        D. getting no pay for lunch time

51. What is the subject discussed in the text?

A. The writer’s unhappy school life.               B. The writer’s eagerness to earn money.

C. The writer’s experience to earn money.      D. The writer’s hard work in an apple plant.

52. How is the text organized?

A. Topic—Argument—Explanation

B. Opinion—Discussion—Description

C. Main idea—Comparison—Supporting examples

D. Introduction—Supporting examples—Conclusion

在黑火药、烟火药制造加工、储存场所，对电气设备选用的规定是( )

A．应选用本质安全型
B．应选用尘密结构型
C．应选用隔爆型
D．不应安装电气设备

“你看给我方的折扣定为3%是否妥当”这句话属于( )发问。

A．诱导式

B．协商式

C．借助式

D．探索式

在数据库加锁技术中，一个事务如果对某一数据项加了排他(exclusive)锁，它将保持此锁直到该事务结束。这是保证事务哪一种特性的重要措施？

A．原子性(atomicity)

B．一致性或可串行性(consistency or serialize ability)

C．永久性(durability)

D．隔离性(isolaty)