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Thread: Compressing a gas: Temperature rises...

  1. #1 Compressing a gas: Temperature rises... 
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    When you compress a gas, then the temperature rises. It's because of the work done at the gas.
    However, I don't understand this happening.
    I put work into the gas, ok. Physically, the energy increases within the gas.
    But WHY? How can I imagine this?
    This would mean that the average velocity of the gas particles rises (since 3/2*k*T = Ekin average). Only because I compress the gas? How that?
    Logically, it is incomprehensible for me.
    I mean, let's assume that at the point of compression no particle of gas touches the wall of the container. Then I compress the gas only to a half of its former volume. Well, the only thing I'm really doing here is to make the space smaller for the particles to move. Why does the energy increase by just making the room smaller for the particles to move???


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  3. #2  
    The Doctor Quantime's Avatar
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    Because there are more particles in a smaller area and thus increases their probability that they will collide. Potential energy is thus converted into heat energy. At least I think thats right.


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    This is how Feynman explains it, although it has always seemed counterintuitive to me that the speed of the piston would have any significant effect on the orders-of-magnitude-higher speed of the molecules. Still, if Feynman said it I tend to believe it.

    Let us consider another situation. Suppose that the piston moves inward, so that the atoms are slowly compressed into a smaller space. What happens when an atom hits the moving piston? Evidently it picks up speed from the collision. You can try it by bouncing a ping-pong ball from a forward-moving paddle, for example, and you will find that it comes off with more speed than that with which it is struck. (Special example: if an atom happens to be standing still and the piston hits it, it will certainly move.) So the atoms are ?hotter? when they come away from the piston than they were before they struck it. Therefore all the atoms which are in the vessel will have picked up speed. This means that when we compress a gas slowly, the temperature of the gas increases. So, under slow compression, a gas will increase in temperature, and under slow expansion it will decrease in temperature.
    http://www.energyandmatter.com/pages...nexcerpt1.html
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  5. #4  
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    Yeah that makes sense to me, I had this thought before, too, but I thought it to be too easy and it wouldn't work out like that. :P
    It's kind of true because at the moment when the atom hits the piston, the piston unrelentingly keeps going forward, but the poor, weak atom is punched back.
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  6. #5  
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    i always believed that heat was the excitedness of the particles/atoms
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  7. #6  
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    Could it just be that the gas doesn't want to be compressed and therefore attempts to convert some of its total energy to kinetic energy?

    Its like this: Imagine a ramp at a 45 degree incline with a block on top. Say the coefficient of friction is 1.5- when you release the block, the small imperfections in either surface attempt to cause the block the jump up and down, but it doesn't really want to do that due to gravity, so it converts the Y-axis motion to frictional thermal energy.
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