问题 填空题

如图所示电路.电源电压恒定不变,滑动变阻器的最大阻值R1为20Ω,定值电阻R2、R3的阻值分别是10Ω、20Ω,两个电流表的电阻不计.闭合开关S,当滑动变阻器的滑片P置于R1中间位置时,电流表A1和A2的示数之比为______;当滑动变阻器的滑片P置于R1最右端时,电流表A1和A2的示数之比为______.

答案

当滑片P置于R1中间位置时,R1=10Ω;

并联部分的电阻为R=

R1R2
R1+R2
=
10Ω×10Ω
10Ω+10Ω
=5Ω,

电路中的总电阻为R=R+R3=5Ω+20Ω=25Ω,

电路的总电流为I=

U
R
=
U
25

并联部分的电压为U=IR=

U
25
×5Ω=
U
5

R1支路的电流为I1=

U
R1
=
U
5
10Ω
=
U
50

所以两电流表的示数之比为I1:I=

U
50
U
25
=1:2;

当滑片P置于R1最右端时,R1,R2都短路,电路为R3的简单电路,两电流表测电路中的电流;

根据串联电路各处的电流相等可知,两电流表的示数相等,即1:1.

故答案为:1:2,1:1.

选择题
阅读理解

阅读理解

      Most people want to be happy, but few know how to find happiness. Money and success alone do

not bring lasting happiness. Aristotle, a Greek thinker, said, " Happiness depends upon ourselves." In

other words, we make our own happiness. Here are a few suggestions to help you be happier.

     The first secret of happiness is to enjoy the simple things in life. Too often, we spend so much time

thinking about future - for example, getting into college or getting a good job - that we fail to enjoy the

present. You should enjoy life's simple pleasures, such as reading a good book, listening to your favorite

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healthier lives.

      Another secret to living a happy life is to be active, and have hobbies where you forget your problems and time. Many people experience this dancing, or playing a sport, such as swimming. You can forget

about your problems, and only think about the activity. Finally, many people find happiness in helping

others. Studies show that people feel good when they spend their time helping others. If you want to feel

happier, do good things for someone. You can help a friend with his or her studies, go shopping to buy

food for an elderly person, or simply help out around the house by washing the dishes.

1. The best title of the passage is ___________.

A. Money Make You Happy                

B. The Secrets of Happiness

C. Ideas for Helping Other People to Be Happy  

D. Good Friends Make You Happy

2. The writer thinks that _____________

A. everyone knows how to live a happier life

B. it's wrong to spend time on work

C. hobbies take up too much time

D. doing good things for someone can make you happier.

3. Which of the following is not mentioned in the passage?

A. Reading a good book                

B. Traveling to a foreign country

C. Playing a sport                      

D. Spending time with close friends