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在导热的气缸内用质量不计的轻活塞，封闭一定质量的理想气体，活塞可沿气缸无摩擦地滑动，开始气缸开口向上竖直放置，活塞相对于底部的高度为h．现缓慢向活塞上加沙粒，活塞距缸底的距离减少h/4，外界大气的压强为P0，温度始终保持不变，活塞面积为s．求：

（1）最后缸内气体压强多大，所加沙粒的质量多大；

（2）从微观角度解释气体压强变化的原因；

（3）在此过程中缸内气体是吸热还是放热，为什么．

答案

（1）该过程是等温变化，开始气体的压强为p0，气体体积为v1=sh

加上沙子的质量为m，气体压强为p，气体体积为v2=

3hs
4
由玻意耳定律得：

p0h=p（h-

1
4
h）… ①

加沙子后由平衡方程：P=P0+

mg
s
…  ②

由①式得P=

4
3
P0…③

联立②③式解得：m=

p0s
3g

（2）温度不变分子热运动的平均动能不变，体积减小，单位体积内的分子数增加，分子密集程度增大，气体压强变大．

（3）在此过程中外界对气体做正功，且内能不变，据热力学第一定律△U=W+Q知，气体放出热量．

答：（1）最后缸内气体压强多大，所加沙粒的质量是

p0s
3g
；

（2）温度不变分子热运动的平均动能不变，体积减小，单位体积内的分子数增加，分子密集程度增大，气体压强变大．；

（3）在此过程中缸内气体是放热．

完形填空
完形填空。
     I fell in love with Yosemite National Park the first time I saw it, when I was 13. My parents took us
there for camping. On the way out, I asked them to wait while I ran up to E1 Capitan, a   1   rock
of 3,300 feet straight up. I touched that giant rock and knew   2   I wanted to climb it. That has
been my life's passion (钟爱) ever since---   3   the rocks and mountains of Yosemite. I've long
made Yosemite my   4  .
     About 15 years ago I started seeing a lot of   5  , like toilet paper, beer cans, and empty boxes,
around the area. It's   6   me why visitors started respecting the place   7  and treated such a
beautiful home-like place this way.
     I tried  8   trash(垃圾)myself, but the job was too big. I would   9   an hour or two on the
job, only to find the area trashed all over again weeks later. Finally, I got so   10   it that I decided
something had to change.
     As a rock-climbing guide, I knew   11  about organizing any big event. But in 2004, together with
some climbers, I set a date for a  12  . On that day, more than 300 people   13  . Over three
days we collected about 6,000 pounds of trash. It was amazing how much we were able to  14  . I
couldn't believe the   15   we made ---the park looked clean!
     Each year volunteers come for the cleanup from everywhere. In 2007 alone, 2,945 people picked up
42,330 pounds of trash and   16 132 miles of roadway.
     I often hear people   17   about their surroundings. If you are one of them, I would say the only
way to change things is by   18   rather than complaining. We need to teach by  19   . You
can't blame others   20   you start with yourself.
( ) 1. A. distant      
( ) 2. A. immediately  
( ) 3. A. imagining    
( ) 4. A. garden        
( ) 5. A. material      
( ) 6. A. beyond        
( )  7. A. more          
( )  8. A. throwing away
( )  9. A. kill          
( )10. A. satisfied with
( )11. A. something    
( )12. A. cleanup      
( )13. A. dropped out  
( )14. A. demand      
( )15. A. plan        
( )16. A. crossed      
( )17. A. talk        
( )18. A. doing        
( )19. A. method      
( )20. A. although    		B. huge        
B. finally    
B. painting    
B. home        
B. resources  
B. against    
B. most        
B. picking up  
B. save        
B. delighted in
B. anything    
B. party      
B. showed up    
B. receive     
B. visit      
B. measured      
B. complain      
B. thinking      
B. explanation
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C. gradually    
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C. everything  
C. picnic      
C. looked around
C. accomplish  
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C. covered      
C. argue        
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C. example      
C. when         		D. loose      
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D. overcome    
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D. designed    
D. quarrel    
D. watching    
D. research    
D. useless    

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