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问题 选择题

有两条长直导线垂直水平纸面放置，交纸面于a、b两点，通有大小相等的恒定电流，方向如图所示，a、b的连线水平．c是ab的中点，d点与c点关于b点对称．已知c点的磁感应强度B1，d点的磁感应强度为B2，则关于a处导线在d点的磁感应强度的大小及方向，下列说法正确的是（　　）

A．

B1
2
+B2，方向竖直向上

B．

B1
2
-B2，方向竖直向下

C．B1+B2，方向竖直向下

D．B1-B2，方向竖直向上

答案

设a处导线在d点的磁感应强度的大小为B，方向方向竖直向下．根据通电直导线产生的磁场磁感应强度与电流成正比，a处导线在c点的磁感应强度的大小为3B，方向竖直向下．由题，两条长直导线恒定电流大小相等，则得到b处导线在c两点的磁感应强度的大小为3B，根据安培定则得到，方向竖直向下．b处导线在d两点的磁感应强度的大小为3B，根据安培定则得到，方向竖直向上．

则根据磁场叠加得

    B1=3B+3B=6B，B2=3B-B=2B，根据数学知识得到，B=

B1
2
-B2．

所以a处导线在d点的磁感应强度的大小为B=

B1
2
-B2，方向是竖直向下．

故选B

完形填空
完形填空。
     In the past all letters were sent by train or ship. Now most letters are __1__ sent by train or ship, but
some are sent by air. You send a letter __2__ so that your friend may receive it sooner. A letter __3__
about twelve days to go from England to India by train or by ship. A letter can get to India by air __4__
five days now, but __5__ the time may be made shorter. 
     __6__ people were afraid to send letters by air. They thought that the plane might fall and
their friends might not receive the letter. So they sent two letters: one letter by air and __7__ by train or
ship. They wanted to __8__ that the letters would be received. Planes are now almost as safe __9__
trains or ships. __10__ letters are sent by air,
( )1. A. still          
( )2. A. by ship        
( )3. A. takes          
( )4. A. in            
( )5. A. by then        
( )6. A. At last        
( )7. A. other          
( )8. A. know          
( )9. A. as            
( )10. A. More or less  		B. just              
B. by air            
B. take              
B. on                
B. soon              
B. At times          
B. the others        
B. see              
B. the same as      
B. Much and much     		C. already      
C. by train      
C. took          
C. after        
C. quickly      
C. At first      
C. the other    
C. hear          
C. than          
C. More and more 		D. yet          
D. by bus        
D. is taken      
D. during        
D. fast          
D. So far        
D. another      
D. make sure    
D. like          
D. Many and many
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单项选择题

快速的深呼吸可能会引起（）

A.缺氧症

B.航空性中耳炎

C.换气过度
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