问题 选择题

如图所示,是重庆一中某实验小组制作的一个可调节亮度台灯原理图,其特别之处在于可以实时查看电流大小。一个标有“6V 3.6W”字样的小灯泡L和最大阻值为50Ω的滑动变阻器R串联后接在电源电压U恒为6V的电路中,设灯泡电阻不变,则下列判断正确的是 (?  )

A.无论如何移动滑片P,电路中总功率不会超过2W

B.移动滑片P,电路中总功率最小值为0.6W

C.当滑片P由a滑向b的过程中,滑动变阻器两端的电压变大

D.当滑片P由b滑向a的过程中,灯泡L越来越亮

答案

答案:B

A 小灯泡的电阻为RL=

当滑动变阻器滑片在b端时,电路的总电阻最小,总功率最大

P最大=

B  当滑动变阻器滑片在a端时,电路的总电阻最大,总功率最小

P最小=

C 串联电路中导体两端电压与其阻值成正比,滑片P由a滑向b的过程中,滑动变阻器接入电路的阻值减小,所以两端电压减小.

D   当滑片P由b滑向a的过程中,滑动变阻器接入电路的阻值增大,电路电流减小,灯泡电阻不变,由公式P=I2R知,灯泡功率减小,所以亮度变暗.

故选B.

填空题
阅读理解

C

At present, in many American cities expecially, many teachers in the public schools say they are underpaid. They point to jobs such as secretary or truck driver, which often pay more to start than that of a teacher. In many other fields, such as law, medicine, computer science, a beginning worker may make more than a teacher who has taught for several years.

Teaching has never been a profession that attracted people interested in high salaries. It is by history a profession that has provided rewards in addition to money—the satisfaction of sharing knowledge, of influencing others, of guiding young people. But in the past several years, there are more difficulties in teaching, for many, than there are rewards.

Unruly students, especially in big cities, large classes and a lack of support from the public in terms of money and understanding have led many public school teachers to leave the profession.

As a result, many of the best students, who would have chosen teaching as their life career in the past, are going into other fields.

Another reason for this change in teacher candidates is the changing status of women in the United States. Until the late 1960s and 1970s, one of the most popular choices for women was teaching. But as other professions, such as law and medicine opened up to women, women stopped pouring into teacher training programs. Thus, a major pool of excellent candidates for the teaching profession dwindled.

Bit by bit government officials and others realized that the status of the teacher had suffered. They talked about change. But the change in a vast society like the United States is not easy. People’s attitudes have formed over many years, and sometimes change takes many years.

63.The underlined word “that” in Paragraph 1 refers to “      ”.

A.money             B.job

C.secretary          D.truck driver

64.What is the present situation of the teaching?

A.Teachers work harder and get underpaid.

B.Teahcers have no opportunities to work in other fields.

C.Teaching can attract best students to work as a teacher.

D.Teaching can provide rewards as well as high salaries.

65.Many public school teachers turn to other professions because        .

A.the government doesn’t finacially support them

B.they have to work longer hours than a lawyer

C.their students refuse to listen to them

D.they are not fairly treated

66.The author believes that change in teachers’ status in the United States       .

A.is not great     

B.is impossible

C.influences people’s attitude

D.needs time