问题 选择题

如图是荷质比相同的a、b两粒子从O点垂直匀强磁场进入正方形区域的运动轨迹,则(  )

A.a的质量比b的质量大

B.a带正电荷、b带负电荷

C.a在磁场中的运动速率比b的大

D.a在磁场中的运动时间比b的长

答案

A、荷质比相同的a、b两粒子,因电量无法确定,则质量大小无法比较.故A错误.

B、初始时刻两粒子所受的洛伦兹力方向都是竖直向下,根据左手定则知,两粒子都带负电.故B错误.

C、根据轨迹图可知,a粒子的半径大于b粒子,根据qvB=m

v2
r
得,r=
mv
qB
,则
q
m
=
v
Br
,因它们荷质比相同,即半径越大时,速率越大.故C正确.

D、粒子在磁场中的运动周期T=

2πm
qB
,由图可知,a粒子的圆心角大于b粒子,则知a在磁场中的运动时间比b的短.故D错误.

故选:C.

完形填空
完形填空。
                                                           What do you hear?
     A Native American and his friend were in downtown New York City, walking near Times Square
in Manhattan. It was during the lunch hour and the __1__ were filled with people. Cars were honking,
taxis were __2__ around corners, sirens (汽笛)were howling, and the sounds of the city were almost
__3__. Suddenly, the Native  American said,"I hear a cricket."
     His friend said,"What? You must be __4__. You couldn't possibly hear a cricket in all of this __5__!"
     "No, I'm sure of it," the Native American said,"I heard  a __6__."
     "That's crazy," said the friend. The Native American listened __7__ for a moment, and then walked
across the street to a big cement plant where some bushes were __8__. He looked into the bushes,
beneath the branches, and sure enough, he __9__ a small cricket. His friend was utterly __10__.
     "That's incredible(不可思议)," said his friend."You must have superhuman ears."
     "No," said the Native American."My  __11__ are not different from yours. It all depends on what
you're listening for."
      "But that can't be," said the friend."I could never __12__ a cricket in this noise."
     "You, it's __13__,"came the reply."It depends on what is really __14__ to you.Here, let me show
you."He __15__ into his pocket, pulled out a few coins, and discreetly __16__ them on the sidewalk.
     And then, even with the noise of the __17__ street still blaring in their ears, they noticed every head
within twenty feet __18__ and look to see if the money that tinkled on the __19__ was theirs.
     "See what I __20__?" asked the Native American."It all depends on what's important to you."
( )1.  A.supermarkets  
( )2.  A.waiting  
( )3.  A.deafening  
( )4.  A.enthusiastic  
( )5.  A.environment  
( )6.  A.siren  
( )7.  A.carefully  
( )8.  A.waving    
( )9.  A.watched    
( )10.A.interested  
( )11.A.ears    
( )12.A.raise  
( )13.A.wrong    
( )14.A.impressive  
( )15.A.reached  
( )16.A.distributed  
( )17.A.deserted  
( )18.A.nod    
( )19.A.pavement  
( )20.A.perform  
B.streets      
B.parking      
B.frightening  
B.cautious    
B.noise        
B.horn        
B.patiently    
B.planting    
B.located      
B.glad        
B.hands        
B.feed        
B.true        
B.extraordinary
B.got          
B.hid          
B.crowded      
B.bend        
B.counter      
B.possess      
C.restaurants
C.screaming  
C.sickening  
C.uncertain  
C.scene  
C.cricket  
C.silently  
C.fading  
C.followed  
C.amazed  
C.eyes  
C.catch  
C.foolish  
C.important  
C.broke  
C.placed  
C.narrow  
C.turn  
C.floor  
C.indicate 
D.lanes    
D.lining  
D.weak    
D.crazy    
D.situation
D.call    
D.nervously
D.growing  
D.played  
D.lucky    
D.legs    
D.hear    
D.terrible
D.necessary
D.cut      
D.dropped  
D.empty    
D.lift    
D.square  
D.mean   
单项选择题