问题 计算题

如图所示电路,灯泡标有“6 V 0.3 A”,R1为定值电阻、R为滑动变阻器。当开关都闭合,滑动变阻器的滑片位于b端时,灯恰能正常发光,电流表的示数为1.5 A。当只闭合开关S和S1,滑动变阻器的滑片位于c点时,电压表的示数为3V,灯泡的电功率为P1;当只闭合开关S和S2,滑动变阻器的滑片位于d点时,电压表的示数为4V,滑动变阻器的功率为P2。灯丝的电阻与电源电压保持不变,求:

(1)电源电压和定值电阻R1的阻值;

(2)P1和P2的比值。

答案

解:

(1)当开关都闭合、滑片位于b端时,灯泡L和R并联在电源两端,等效电路图如图甲所示,灯正常发光,电源电压:U= U额定=6 V ,IL=0. 3A,由并联电路特点,I=IL+I1,I1=I-IL=1.5 A-0.3 A=1.2A,

根据欧姆定律I= U/R,定值电阻阻值: R1= U/I1=6 V/1.2 A=5 Ω ;

(2)只闭合开关S和S2,滑动变阻器滑片P位于c点时灯与R串联,等效电路如图乙所示。Uc=3 V,根据串联电路特点U=UL+Uc,UL=U-Uc=6 V-3V=3V,

根据灯的铭牌可知,RL= U额定/I额定 =6 V/0. 3 A=20 Ω,

由P=U2/R得:P1=UL2/RL=(3V)2/20 Ω=0.45W,

只闭合开关S和S2,滑动变阻器滑片P位于d点时,R1与R串联,等效电路如图丙所示,Ud=4 V。

根据串联电路特点 U= UL'+Ud,UL'=U - Ud =6 V - 4 V =2 V ,

根据欧姆定律,I= UL'/R=2 V/5 Ω=0.4 A, P2= Ud·I3=4 V ×0.4 A =1.6W ,

P1/P2= 9/32 。

     

阅读理解

阅读理解。

     When I met him, I had a lot of anger inside of me. I've lived my whole life in Spanish Harlem, but in

my neighborhood, there are shoot-ups all the time. I know kids who have been shot or beaten up. I have

friends who ended up in prison. I could have ended up that way, too, but Mr. Clark wouldn't let that happen.

     Mr. Clark worked long hours, making sure I did my work. My grades rose. In fact, the scores of our

whole class rose. One day, he took our class to see The Phantom of the Opera, and it was the first time

some kids had ever been out of Harlem. Before the show, he treated us to dinner at a restaurant and taught

us not to talk with our mouths full. We did not want to let him down.

      Mr. Clark was selected as Disney's 2000 Teacher of the Year. He said he would draw three names out

of a hat; those students would go with him to Los Angeles to get the award. But when the time came to draw

names, Mr. Clark said,"You're all going."

     On graduation day, there were a lot of tears. We didn't want his class to end. In 2001, he moved to Atlanta,

but he always kept in touch. He started giving lectures about education, and wrote a bestselling book based

on his classroom rules, The Essential 55.

      In 2003, Mr. Clark took some of us on a trip to South Africa to deliver school supplies and visit

orphanages(孤儿院). It was the most amazing experience of my life. It's now my dream to one day start a

group of women's clubs, helping people from all backgrounds.

1. Without Mr. Clark, the writer _____.

A. might have been put into prison

B. might not have won the prize

C. might have joined a women's club

D. might not have moved to Atlanta

2. The Essential 55 is _____.

A. a show

B. a speech

C. a classroom rule

D. a book

3. How many students'names were finally drawn out of a hat by Mr. Clark?

A. None

B. Three

C. Fifty-five.

D. All.

4. In the passage, the writer intends to tell us that _____.

A. Mr. Clark went to South Africa because he liked travelling

B. Mr. Clark helped to set up a group of women's clubs

C. a good teacher can help raise his or her students' scores

D. a good teacher has a good influence on his or her students

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