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问题 问答题

如图所示，电路中R0为定值电阻，R为滑动变阻器，总阻值为R，当在电路两端加上恒定电压U，移动R的滑片，可以改变电流表的读数范围为多少？

答案

设滑动变阻器滑动触头左边部分的电阻为Rx．电路连接为R0与Rx并联，再与滑动变阻器右边部分的电阻R-Rx串联，

干路中的电流：I=

U
R并+R-RX
=
U
R0RX
R0+RX
+R-RX
，

电流表示数：I′=

R0
R0+RX
I=
R0
R0+RX
•
U
R0RX
R0+RX
+R-RX
=
UR0
RR0+RRX-
R2X
=
UR0
RR0+(
R
2
)2-(RX-
R
2
)2
，

由上式可知：当Rx=

R
2
时，I最小为：Imin=
4UR0
4RR0+R2
；当Rx=R或Rx=0时，I有最大值，Imax=
U
R
；　

即电流表示数变化范围为：

4UR0
4RR0+R2
～
U
R
；

答：电流表示数变化范围是：：

4UR0
4RR0+R2
～
U
R
．

单项选择题

单相满管流的流动方式为（）

A.凝水呈无压流动

B.凝水靠水泵动力或势能差充满整个管道截面的流动

C.凝水不充满整个管道截面，靠管路坡度流动

D.管内为凝水、蒸汽的混合物，靠疏水器后的背压流动
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填空题


In the following passage, there are 20 blanks representing the words that are missing from the context. You are to put back in each of the blanks the missing word． The time for this section is 20 minutes.
Where do you really come from And how did you get (1) where you live today DNA studies suggest that all humans today (2) from a group of African ancestors who—about 60,000 years ago— (3) a remarkable journey.
The Genographic Project is seeking to chart new (4) about the migratory history of the human species by (5) sophisticated laboratory and computer analysis of DNA contributed by hundreds of (6) of people from around the world． In this unprecedented and real-time (7) effort, the Genographic Project is closing the gaps of what science (8) today about mankind’s ancient migration stories.
The Genographic Project is a five-year (9) partnership led by National Geographic Explorer-in-Residence Dr. Spencer Wells. Dr. (10) and a team of renowned international scientists and IBM researchers, are (11) cutting-edge genetic and computational technologies to analyze historical patterns in DNA (12) participants around the world to better understand our human genetic roots. (13) three components of the project are: to gather field research data (14) collaboration with indigenous and traditional peoples around the world; to invite (15) general public to join the project by purchasing a Genographic Project Public Participation Kit; (16) to use proceeds from Genographic Public Participation Kit sales to further (17) research and the Genographic Legacy Fund which in (18) supports indigenous conservation and revitalization projects. The Project is anonymous, non-medical, (19) , non-profit and non-commercial and all results will be placed in the (20) domain following scientific peer publication.

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