问题 计算题

某发电厂通过两条输电线向远处的用电设备供电。当发电厂输出的功率为P0时,额定电压为U的用电设备消耗的功率为P1。若发电厂用一台升压变压器T1先把电压升高,仍通过原来的输电线供电,达到用电设备所在地,再通过一台降压变压器T2把电压降到用电设备的额定电压,供用电设备使用,如图所示,这样改动后,当发电厂输出的功率仍为P0,用电设备可获得的功率增加至P2。试求所用升压变压器T1的原线圈与副线圈的匝数比N1/N2以及降压变压器T2的原线圈与副线圈的匝数比n1/n2各为多少?

答案

解:当不用升降压变压器时,根据输电线上的损耗有P0-P1=(P1/U)2r ,得

当用升降压变压器后,设发电厂电流为I1,输电线上电流为I2,用户处的电流为I3,则有

I1=P0/U0=P1/U,I22r=P0-P2

,而I3=P2/U

故升压比,降压比

单项选择题 A3/A4型题
完形填空
完形填空。

     Jean's father was a farm plane pilot in the little farming community in Northern California where she
was raised, and Jean was totally _1_ by airplanes and flying.
     In her senior year of high school, her English teacher was a Mrs. Dorothy Slaton. Mrs. Slaton was a
_2_, demanding teacher with high standards and a low _3_ for excuses. She refused to treat her students
like   _4_, instead expecting them to behave like the _5_ adults they would have to be to succeed in the
real world after graduation. Jean was _6_ of her at first but grew to respect her firmness and fairness
gradually.
     One day Mrs. Slaton gave the class a written _7_. "What do you think you'll be doing 10 years from
now?" Jean's dream was to be a pilot like her father, but as the years went by, she had been beaten
down by the _8_ and negative words she got whenever she talked about her career, so she unwillingly
gave it up and wrote something else. Mrs. Slaton collected the papers and nothing more was said. Two
weeks later, the teacher handed back the assignments, _9_ the students to face it down on the desk, and
asked this question: "If you had enough finances and unlimited   10  , what would you do? Please write it
on the   11   of your previous assignment paper."
     When the students stopped writing, the teacher asked, "How many of you wrote the same thing on
  12   sides of the paper?" Not one hand went up.
     The next thing that Mrs. Slaton said changed the course of Jean's life. The teacher leaned forward 
  13   her desk and said, "I have a little secret for you all. You do have unlimited abilities and talents. You
do have access to the finest schools, and you can arrange unlimited finances if you want something   14   
enough. This is it! When you leave school,   15   you don't  16  your dreams, no one will do it for you.
You can have   17   you want if you want it enough."
     What her teacher said touched Jean so much! In the following years, she tried her best to realize her
dream of becoming a pilot. Although it didn't happen   18  , she finally became one of the first three
female pilot trainees ever   19   by United Airlines. Today, Jean Harper is a Boeing 737 captain for
United.
     It was the power of one well-placed   20   word that gave that uncertain young girl the strength and
faith to pursue her dream.

( )1.A. touched    
( )2.A. hard-working
( )3.A. interest    
( )4.A. children    
( )5.A. responsible
( )6.A. bored      
( )7.A. lecture    
( )8.A. approving  
( )9.A. requiring  
( )10.A. work      
( )11.A. title    
( )12.A. same      
( )13.A. in       
( )14.A. hardly    
( )15.A. if        
( )16.A. make for  
( )17.A. that     
( )18.A. overnight
( )19.A. realized  
( )20.A. objective
B. attracted  
B. kind       
B. tolerance
B. boys      
B. notable    
B. frustrated
B. assignment
B. inspiring  
B. suggesting
B. chances    
B. back      
B. different  
B. on        
B. badly      
B. unless    
B. refer to  
B. what      
B. some day  
B. received  
B. blank      
C. effected  
C. strict    
C. courage    
C. girls      
C. knowledgeable
C. tired      
C. paper      
C. indifferent
C. hoping    
C. money      
C. right      
C. neither    
C. over      
C. impossibly
C. weather    
C. go for    
C. which      
C. the other day
C. accepted  
C. pessimistic
D. moved          
D. lazy            
D. help            
D. students        
D. capable        
D. scared          
D. work            
D. discouraging    
D. allowing        
D. problems        
D. front          
D. both            
D. under          
D. likely          
D. until          
D. go through      
D. when            
D. fortnight      
D. accomplished    
D. positive