问题 问答题

(4分)如图所示,黑匣子上有A、B、C三个接线柱,内部只有一个电压恒为6V的电源和两个定值电阻R1、R2(R1>R2).当电流表分别连接到接线柱AB、AC、BC时,电流表示数如下表所示:

(1)请将上表填写完整:          

(2)第3次实验时,电流为零,说明BC之间     (有/没有)电源.

(3)若用导线同时把AB、AC连接起来,电路可能消耗的总功率为     

答案

(1)如下表:

实验序号123
电流表位置接AB之间接AC之间接BC之间
电流表示数/A0.40.60
电路的总电阻/Ω1510无法计算
(2)没有  (3)①电路消耗的总功率为6W ②电路消耗的总功率为3.6W

题目分析:(1)根据欧姆定律得:RAB=U/IAB=6V/0.4A=15Ω     RAC=U/IAC=6V/0.6A=10Ω

(2)因为电流为零,说明电路两端中没有电压或电路处于开路,这里是一个内部结构正常的电路,所以当电流表分别连接到接线柱BC时应是电路两端中没有提供电压的电源引起的

(3)根据RAB=15Ω,RAC=10Ω;而内部是两个定值电阻R1,R2(R1>R2),因此可能的情况有:

①R1=RAB=15Ω,R2=RAC=10Ω,电阻R1、R2串联在BC之间,再根据BC之间没有电源可知电源应接 在AB、AC之间。如图:

所以若把AB,AC连接起来时,只有R1、R2并联连入,根据功率公式P=U2/R1+U2/R2=(6V)2/15Ω+(6V)2/10Ω=6W

②可判断AC之间是一个定值电阻R1=RAC=10Ω;AB之间是这两个定值电阻R1、R2串联,则R2=RAB-RAC=15Ω-10Ω=5Ω再根据BC之间没有电源可知电源应接在AB、AC之间.所以结构如图:

所以把AB,AC连接起来时,导线AB对R2短路,只有R1连入,根据功率公式P= U2/R1=(6V)2/10Ω=3.6W

阅读理解

It is hardly surprising that clothing manufacturers (生产商)follow certain uniform standards for various features(特征)of clothes.What seems strange,however,is that the standard adopted for women is the opposite of the one for men.Take a look at the way your clothes button.Men’s clothes tend to button from the right, and women’s from the left.Considering most of the world’s population—men and women — are right-handed,the men’s standard would appear to make more sense for women.So why do women’s clothes button from the left?

History really seems to matter here.Buttons first appeared only on the clothes of the rich in the l7th century,when rich women were dressed by servants.For the mostly right-handed servants,having women’s shirts button from the left would be easier.On the other hand,having men’s shirts button from the fight made sense,too.Most men dressed themselves,and a sword drawn from the left with the right hand would be less likely to get caught in the shirt.

Today women are seldom dressed by servants,but buttoning from the left is still the standard for them.Is it interesting? Actually,a standard,once set,resists change.At a time when all women’s shirts buttoned from the left,it would have been risky for any single manufacturer to offer women’s shirts that buttoned from the right.After all,women had grown so used to shirts which buttoned from the left and would have to develop new habits and skills to switch.Besides,some women might have found it socially awkward to appear in public wearing shirts that buttoned from the fight,since anyone who noticed that would believe they were wearing men’s shirts.

小题1:What is surprising about the standard of the clothing industry?

A.It has been followed by the industry for over 400 years.

B.It is different for men’s clothing and women’s.

C.It works better with men than with women.

D.It fails to consider right-handed people.小题2:What do we know about the rich men in the 17th century?

A.They tended to wear clothes without buttons.

B.They were interested in historical matters.

C.They were mostly dressed by servants.

D.They drew their swords from the left.小题3:Women’s clothes still button from the left today because _______.

A.adopting men’s style is improper for women

B.manufacturers should follow standards

C.modern women dress themselves

D.customs are hard to change小题4: The passage is mainly developed by ________.

A.analyzing causes

B.making comparisons

C.examining differences

D.following the time order

判断题