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In Vol I,Chap 1-2 of the Feynman lectures on Physics,Feynman talks about how a change in pressure of a fixed amount of gas enclosed in a piston can cause its temperature to increase/decrease. He concluded by saying that:

"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."

What is the significance of the word “slowly” in the above context? What is the difference when the compression/expansion isn’t done slowly but quickly instead?

On second thoughts,shouldn't the compression/expansion be done quickly so as to capture the temperature change before the heat gained is lost to the surroundings and vice versa for heat loss?

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    $\begingroup$ Slow means slow in comparison to the average velocity of the gas molecules and the speed of sound in the gas. Imagine what happens if you try to move the piston supersonically! $\endgroup$ – CuriousOne Jan 2 '15 at 10:35
  • $\begingroup$ Since the atoms/molecules in the gas pick up speed from the collision with the moving piston,wouldn't it just mean that they pick up even more speed? Even from a piston moving supersonically? $\endgroup$ – Charles Jan 2 '15 at 12:14
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    $\begingroup$ If you compress the gas supersonically it will become very hot because a lot of extra kinetic energy is added by the piston. If you remove the piston supersonically, the atoms can't follow the piston, so they can't transfer their kinetic energy to it as it moves. In both cases the gas will be hotter than with a piston that is moving slowly (i.e. adiabatically). $\endgroup$ – CuriousOne Jan 2 '15 at 12:18

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