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Old 25th July 2008, 06:08 PM   #51
Daveze is offline Daveze  Australia
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Quote:
Originally posted by MJL21193


Invoking Einstein, eh?
Yes.

Quote:
Originally posted by MJL21193
What is waters natural state? Liquid, is it not? Water that has been heated to the boiling point does not compress back to water, it condenses. To do this it's temperature needs to drop. Not outside force is needed, therefore no compression.

More grade school oversimplification, I'm sure.
Yes, more grade school oversimplification. A method of teaching things is to simplify things such that they are palatable to a mind at its state of maturity, then gradually un-simplify things as the mind's understanding matures.

Example: Gravity
Primary school: Gravity holds you to the ground.
Early high school physics: Gravity is the acceleration that the Earth imposes on other objects = ~10 or 9.81m/s^2
Later physics: Gravity is an interaction between two bodies with mass. The magnitude given by G(m1+m2)/r^2.
Cutting edge physics: Gravity has something to with a theoretical particle that I don't come close to understand.

Water does not have a 'natural state'. What it (and all other substances) does is have a state/s according to a set of conditions: Temp and Pressure.
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Old 25th July 2008, 06:10 PM   #52
gedlee is offline gedlee  United States
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Quote:
Originally posted by AndrewT
no, water expands either side of 4degC.
Ice contracts as the temperature is lowered.

It's this expansion below 4degC that allows the aquatic life to survive.
A clever trick of nature that enabled colonisation of the Earth.

And if this very unique characteristic of water did not occur, life would not exist on the earth. Ice would sink when frozen and eventually all of the water would turn into ice thus killing off the evolving species. Its the ice layer that insults the water below. This has always amazed me as truely "an act of God".
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Old 25th July 2008, 06:17 PM   #53
MartinQ is offline MartinQ  Canada
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Quote:
Originally posted by gedlee
Its the ice layer that insults the water below. This has always amazed me as truely "an act of God".
What a rude act of God that is ... insulting the lower water like that. Don't they know that we accept all water as being equal now?!!?
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Old 25th July 2008, 06:37 PM   #54
AndrewT is online now AndrewT  Scotland
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Quote:
Originally posted by MJL21193
Water that has been heated to the boiling point does not compress back to water, it condenses. To do this it's temperature needs to drop. Not outside force is needed, therefore no compression.
no,
compress the steam and it turns back to water at the same temperature and releases the latent heat of vapourisation.
Change the pressure and water "boils" at different temperatures.
and the reverse, changes back to the liquid state if compressed.
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Old 25th July 2008, 07:02 PM   #55
y8s is offline y8s  United States
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water is compressible.

and you can trade pressure for temperature just like with a gas.

that's why when you go camping in the mountains, your pasta takes longer to cook--water boils at a lower temperature due to the lower atmospheric pressure so you're boiling with less heat.

if you compress water enough, it'll solidify. or if you can't do that, just lower the temperature.

the compression is VERY SMALL. same in other solids. just because you can't see it doesn't mean it's not there.

sound also travels through steel for the same reason. minute rarefactions and compressions through the material.

the compression of a solid doesn't have to be at a constant temperature. there's no constraint that says it must. when you play music in a pool, the temperature changes as the compression waves travel through it. those changes are also very small and quickly diffuse into the large thermal sink.

Here's some more detail on my comment in the other thread. A quote from a nasa page if you like your science hard:

Quote:
Sea levels crept up about 20 centimeters during the twentieth century. Most of the rise happened because water expands as it warms, though melting mountain glaciers also contributed to the change.
http://earthobservatory.nasa.gov/Lib...g_update6.html

Notice the clear differentiation between adding water from glaciers (RIP) and expansion of existing water due to increasing global temperatures.
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Old 25th July 2008, 07:15 PM   #56
SY is offline SY  United States
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John, you proposed another inapt (DC) analogy instead of answering my point. There are two ways of defining compression and, as I explained, the peaks of any acoustic wave satisfy both of them. You need to show how the First Law doesn't apply, remembering that for every compression, there's a rarefaction.
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Old 25th July 2008, 07:24 PM   #57
sreten is offline sreten  United Kingdom
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Hi,

No material is incompressible - that should be obvious.

Quote:
Compressibility

The compressibility of water is a function of pressure and temperature.
At 0 C in the limit of zero pressure the compressibility is 5.110-5 bar−1.
In the zero pressure limit the compressibility reaches a minimum
of 4.410-5 bar−1 around 45 C before increasing again with
increasing temperature.
As the pressure is increased the compressibility decreases,
being 3.910-5 bar−1 at 0 C and 1000 bar.
The low compressibility of non-gases, and of water in particular,
leads to them often being assumed as incompressible.
The low compressibility of water means that even in the deep
oceans at 4000 m depth, where pressures are 4107 Pa,
there is only a 1.8% decrease in volume.
Density and the stiffness determine the speed of sound in water,
this propogation must include elastic compression / rarefaction.
(~1.5km/s - which implies for the density - very stiff.)

/sreten.
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Old 25th July 2008, 08:37 PM   #58
Pano is offline Pano  United States
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Interesting discussion.

I'm with SY on this one. Some people can't tell the ACs from the DCs. :P

I can certainly see MJL's point though. Water doesn't compress, for general practicle purposes. It's easy to get more air into a certain volume, like a car tire, a scuba tank, a V8 engine. Stuffing more water into the same those spaces would be a lot harder.....

But on a local scale, there can be compression - such as in a sound wave. There has to be rarification too, so the sum is no overall compression of the liquid water.
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Old 25th July 2008, 08:54 PM   #59
gedlee is offline gedlee  United States
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In hydrodynamics and fluid dynamics (DC) water is assumed to be incompressible because its changes through compression are miniscule compared to the other effects. But in acoustics (AC) one cannot make this simplification because it would mean that sound could not propagate. The two fields of study are almost always completely uncoupled from each other and results from one are usually not applicable to the other. It is precisely the incompressible assumption that accounts for this.
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Old 25th July 2008, 10:04 PM   #60
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Quote:
Originally posted by MartinQ


That depends entirely on it's environment.



Compressing a substance increases its temperature (adding energy), and thus cooling a substance can lead to contraction (removing energy). So you can compress a substance by cooling.

Nearly 4 hours in the dentists chair and I have to come back to this? She's a pretty 27 year old though, and it was well worth the pain, irritation and fiscal hardship.
Bad news boys, she agreed with me.

The environment? Here on planet Earth? Water is in a liquid state that covers 71% of the Earths surface, that would qualify as natural.
So, you can compress a substance by cooling. I said that already.
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