AndrewT said:
You need to decrease temp to make the steam condense. You can compress till the cows come home, but it will not change the steam back to water. Temperature MUST fall for this to happen.
Yes, change the air pressure and the boiling point changes. That has nothing to do with compression.
This puts me in mind of the time Rosie O'Donnell, one of the greatest progressive intellects ever, told us that 9/11 was probably an underhanded plot by the Bush Administration. After all, she said, about 9/11: 'miraculously, the first time in history, steel was melted by fire. It is physically impossible. '
SY said:
Sorry Sy, I'm not seeing how all of my analogies are inapt.
Lets try this:
Picture a driver firing into a tank of water. Apply a signal to produce sound waves. The cone of the driver moves forward and backward with the signal. Increased pressure, decreased pressure. The result of the forward, increased pressure is not compression. The result is molecules get pushed, not only forward, but sideways, every direction, but primarily forward. The molecules are being displaced, as the cone moves forward. This displacement (same as a pebble falling into a pond) is the origin of the wave.
If the driving force of the cone were powerful enough, and the displaced water didn't have anywhere to go, then compression would take place. The driving force would need to be huge though, and the vessel to contain the water would need to be impressive indeed.
You guys are hung up on the term "compression wave" and this is understandable. I see the term as descriptive of what a wave is "trying" to do, and most likely does do with air.
Liquids and solids are different though. The force required to compress these is beyond the ability of a normal sound wave.
MJL21193 said:
Nope ... certainly didn't miss that, but I did find an answer on this page:
http://www.spiraxsarco.com/resource...es-and-heat-transfer/what-is-steam.asp#head87
"Compression at constant temperature below the critical point however, will result in liquefaction of the vapour as it passes from the superheated region into the wet steam region.
The critical point occurs at 374.15°C and 221.2 bar a for steam. Above this pressure the steam is termed supercritical and no well-defined boiling point applies."
What this says to me is that you can compress steam (vapor) back into water (liquid) without reducing the temperature. Oh well, at least you had some fun.
All that means, John, is that the driver couples more efficiently to the medium- for a given displacement, the pressure change is far higher than in a gas. But... the fact remains that when you send a wave through water, no matter how you generate it, there are regions of higher and lower pressure, and the higher pressure regions contain more molecules per unit volume than the lower pressure region. The average pressure need not (and generally will not) change.
MartinQ said:
Nice find and a good read. Everyone should have a look.
I stand corrected once again. At a constant temp (say above 100*C) steam can be compressed to liquid. One glitch: What happens when you release the pressure? It will return to steam of course.
When I said the temperature needs to drop for water to be in a liquid state, I did mean at normal pressure.
MartinQ said:
Oh, I'm not loosing steam and you need not gloat over your minor "victory". At least I have the spine to admit when I'm mistaken about something.
Anyhow, the topic is water, not steam. Compressing water with sound, in fact.
Have any evidence to conclusively contradict me there?
wakibaki said:
Nice observation.
This is kind a' like waterproof wood finishes
Sound is a Compression Wave
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Sound is a Compression Wave
The School for Champions is an educational website that shows you how to achieve your dreams.
SY said:
These regions of higher and lower pressure are regions of high molecular excitement and low. There is no meaningful increase in density as there is no meaningful compression.
You really need to look at this in a different way: How much pressure can a sound wave exert? Is it enough to overcome all of the forces involved in order to increase density? Is it not a more reasonable assumption that it will be easier to move something rather than compress it, especially a medium that requires huge force to compress?
MJL21193 said:
Since sound does not travel infinitely fast through water, it is self-evident (obvious?) that water is being compressed when sound is being transmitted, otherwise, it would not travel as a WAVE. There is no physical substance with an infinitely high (or zero) Young's Modulus.
If water was absolutely incompressible, the transmission of force (sound) through a body of water would arrive at the distant end at the same time it is applied (infinitely fast).
If you hold a steel rod, or, a wooden down, and you thrust it in one direction, it will APPEAR to us that the far end moves at the same time as the end you are holding, though it does not. There is some delay, it is simply too small for us to observe without instruments. Your suggested "experiments" of trying to compress water, and, your hydraulic example are no different. They will appear to be as you say, to the unaided observer. But, with scientific instruments, the delay of the force transmitted through the hydraulic column, and the degree of the compressibility proportional to force of water can be detected.
Suppose you have a sample of water so small it consists of only two molecules. Now, evidently, the density is proportional to the distance between the molecules. So, if you pass a sound wave through the water sample, and in doing so cause the two molecules to move closer together and farther apart, then the sound has changed the water's density.
QED.
Discussions like this, after a point IMHO, become more interesting for what they teach us about psychology than physics.
-Henry
QED.
Discussions like this, after a point IMHO, become more interesting for what they teach us about psychology than physics.
-Henry
Re: This is kind a' like waterproof wood finishes
A recent graduate? C+ for effort.
Everyone sees "compression wave" and that's it, close the book. Look deeper, try to understand that compression is related to pressure, and some things do not like to be compressed. Pressure will still bear, but unless there's enough, it will not compress.
No one has commented on my query about steel, and how sound travels through that. Compression too?
As mentioned by sreten above, stiffness or elasticity has a big part to play in sound transmission. The ability of a medium to conduct sound effectively is determined by it's density and it's ability to resist permanent change from applied forces.
When you drop a steel ball from a distance onto a hard surface, it will rebound as it works to return it it's original shape. A lead ball will not bounce, but will deform.
In neither of these instances does the ball compress appreciably.
Ed LaFontaine said:
A recent graduate? C+ for effort.
Everyone sees "compression wave" and that's it, close the book. Look deeper, try to understand that compression is related to pressure, and some things do not like to be compressed. Pressure will still bear, but unless there's enough, it will not compress.
No one has commented on my query about steel, and how sound travels through that. Compression too?
As mentioned by sreten above, stiffness or elasticity has a big part to play in sound transmission. The ability of a medium to conduct sound effectively is determined by it's density and it's ability to resist permanent change from applied forces.
When you drop a steel ball from a distance onto a hard surface, it will rebound as it works to return it it's original shape. A lead ball will not bounce, but will deform.
In neither of these instances does the ball compress appreciably.
WTF is "molecular excitement"? As for sound waves in steel...
http://en.wikipedia.org/wiki/Phonons
Pretty pictures, too.
http://en.wikipedia.org/wiki/Phonons
Pretty pictures, too.
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