The rule of thumb that a thermometer measures the 'number of times molecules bump into it (per unit of time)' is borne out in practice (or the number of times and how intense those bumps are, but in this instance the intensity is constant and the number clearly falls).
So unsurprisingly, when he turns on the vacuum pump, the thermometer shows a lower temperature, and when he opens the valve to let air back in, the thermometer shows room temperature again (plus a bit for friction).
Sucking out some molecules doesn't actually reduce the average kinetic energy of the remaining ones, it reduces their number, so as a generalisation, we would expect there to be less 'heat' inside the chamber, however defined.
Local Council Efficiency
1 hour ago
12 comments:
The kinetic energy of the air molecules in the gas being decompressed by the piston of the pump is lower after decompression as the gas molecules colliding with the retreating piston will bounce off with reduced velocities.
Don't forget photons.
Din, a few molecules which get sucked into the pump will bounce back, that is true. But most that hit the pump never return, leaving only ones that are unaffected.
When he releases the valve, the reverse happens, plus a bit for friction.
If I were you, I'd just accept the fact that lower layers of air have higher pressure and higher temperature, and these two go down in step as you go up in height.
Those are just facts. The explanation is precisely as I have it, gravity etc.
JJ, explain?
Is this a Joke or is this serious .
Genuine question is this Post meant as a Joke or is it serious.
These are basic gas laws. Not anyone's opinion.
Temperature is not "collisions per unit time". If that were true then if he had continued the demonstration by adding more air and letting it cool to steady state then the resulting chamber of air would contain more molecules and more collisions per unit time and it would be hotter. And it is not.
Din, whatever the explanation, do you dispute the basic facts that the lowest levels have highest temperature and highest pressure, and that pressure and temperature fall with increasing height?
Physics is about observing the world and trying to explain it. If you don't accept facts about the world, then what is the point?
Yes it is observed that pressure and temperature fall with increasing height.
"If that were true then if he had continued the demonstration by adding more air and letting it cool to steady state then the resulting chamber of air would contain more molecules and more collisions per unit time and it would be hotter. And it is not"
No it wouldn't be hotter, precisely because it has been allowed to "cool to steady state". It wouldn't contain "more collisions per unit time", because the molecules would contain less energy and so they would be moving slower and so they would not be colliding with each other so much" The correct wording is: "If that were true then if he had continued the demonstration by adding more air and letting it cool to steady state then the resulting chamber of air would contain more molecules but fewer collisions per unit time and would be at the same temperature, which is what we observe"
B, thanks.
Seems like I was wasting my time.
B, always worth a try.
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