Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Jeffrey Collins Oman on 06/11/2003 14:05:35
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What is a vacuum? Why does water stay in the straw when you put your finger on the top of the straw?
If you suked the air out of a glass bottle woud it collapse like a plastic bottle does?
Thankssss
Jeff
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Howdy JCO - Atmospheric pressure keeps the liquid in the straw. A glass bottle would shatter rather than collapse.
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A vacuum is a space that's devoid of matter and from which the air has been removed.
It's also a machine used to clean floors that is too big, too noisy and too awkward.
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There's a great book named: "The Book of Nothing" by John Barrow that discusses vacuums, voids and other good stuff. It's very well written and packed full of info that a layman such as myself can understand. Briefly though, I think he refers to a vacuum as a point in space at zero energy, or near zero energy as I don't think a zero energy state is possible.........but I could be, and probaly am, incorrect. Anybody??
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A state of zero energy in matter is unattainable, for both matter and void. A void is very near zero energy when separated from matter by great distances, but there is ambient energy in the electromagnetic spectrum permeating just about every space of the known universe. Energy can propagate through a void quite readily.
However, by zero energy, it's entirely possible he is not referring to energy of all sorts but energy of a specific type. I'm pretty sure a perfect vacuum does not exist, however. Even in deep space, there are particles of matter ejected by stars and such floating around. A concentration of vapor will always flow from a space where it has high potential to a space of lower potential in order to achieve material equilibrium.
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If I remember right, the estimated density of matter in intergalactic space (out between galaxies)is something like one or two atoms per cubic meter. The temperature, if that has much meaning is about 3 Kelvin. I'm not sure where that comes from, and I'm too lazy to research it just now, so don't throw anything heavy at me if I'm wrong.
Oh, I almost forgot: if the glass bottle is thick enough it won't break or collapse. Bell jars in old labs were made of glass and could be pumped down to a very good vacuum (nothing like interstellar space, but I doubt there is enough pressure to make any difference).
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John
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Good info, John. To reinforce the point that temperature doesn't necessarily depend on matter but rather the environment: The bell jar John talks about, when it is evacuated the temperature does not decrease. There is still thermal equilibrium with the environment. Heat can propagate through a vacuum in the form of electromagnetic rediation.
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Although, presumably, as the jar is evacuated there is a cooling effect, affecting the residual volume, which subseqently equilibrates with the surroundings ?
"I never forget a face, but in your case I'll make an exception"
- Groucho Marx
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Right Chris, as the bell jar is evacuated, it does cool inside according to P1*V1/T1 = P2*V2/T2. In this case the volume is fixed (assuming the jar does not collapse, P2 is lower, and thus T2 must be lower for the equation to hold.
This is how refridgerators and air conditioners work. You compress a gas (e.g. Freon) which makes it heat up, then run it through a radiator to remove the heat (this is the hot blast you feel outside the A/C), then run it into a coil where it can expand. It will cool, and you run it through another radiator and blow air through it and into your house. Freon has a special twist in that it actually condenses to a liquid when it cools, thus taking advantage of the latent heat of vaporization in the process.
Welcome to the forum Jeff. I bet you never thought an innocent question like yours could generate so much irrelevant disucssion, huh?
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John
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I thought that zero motion and absolute zero were attainable, but it would be impossible to measure them because that would stir the molecules at least a tiny bit.
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UScaV:
The laws of thermodynamics make it pretty clear that absolute zero is unattainable. Without getting into long equation derivations (I will if you want me to) let's just say that the second and third laws are broken because, with absolute zero temperatures, you would be able to create a 100% efficient heat engine (violates 2nd law) and you would have a condition of zero entropy. (violates third law, sort of) There's a way to cool paramagnetic materials using magnetic fields in an adiabatic container that proves the latter part. ..again, long and complex but I'll explain it if you're really excited about it.
I think you're confusing the attainability of absolute zero with Heisenburg's Uncertainty Principle. My knowledge of quantum mechanics hasn't progressed beyond knowing that means that you can't measure directly a quantum system because the very act changes the state of the system.
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I believe that if you are at or near absolute zero, the classical laws of thermodynamics would break down and you would have to resort to quantum mechanics to describe the situation. Zero motion would indeed imply zero momentum and thus infinite position uncertainty. I'm not sure how that works in quantum calculations. Infinity is a big problem [:)].
Is there someone with a better knowledge of quantum mechanics that can address the zero motion question?
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John