[Excellence]

So my friend is trying to explain to me that if you have ice in a cup of ice water, that all the water everywhere in the cup will be the same temperature of the ice, about 32 degrees farenheit. I tell him, that great, but when you get to a larger system, it's not going to be very useful. And he says that it's still true. I bust out with, everything takes time, so it will take time for any state change where the ice is to absorb heat that is over on the other side of the system, and in that time there will be difference, not to mention things like the jetstream in the ocean, and no uniformity of heat being applied to the glass via the external environment, so on and so forth..


The whole thing got kicked off when I said that you could cook ramen noodles the same time in the boiling water twice and get very different results (I posited it may be the difference between throwing them in the water when it just starts boiling compared to when it's all out boiling.) This is where he first stated that it would be the same temperature everywhere once it started boiling. To which my reply was, see the water boil in a location in the pot is probably a better indicator of temperature than what the thermometer was telling you anyway, if you weren't able to measure a difference.


So I finally find this: "Non-equilibrium thermodynamics applies to situations where the system under study is not in thermodynamic equilibrium but can be broken into subsystems which are sufficiently small to be in equilibrium, while still being large enough that thermodynamics is applicable to them" Which I think is the deal breaker showing that time and distance and external forces really do matter, and are better accounted for using this model of thermodynamics as upposed to the only one he probably learned as was droning on to me about.


What do you guys think?

 

[AnthonyP]

Quote:

So my friend is trying to explain to me that if you have ice in a cup of ice water, that all the water everywhere in the cup will be the same temperature of the ice, about 32 degrees farenheit. I tell him, that great, but when you get to a larger system, it's not going to be very useful. And he says that it's still true. I bust out with, everything takes time, so it will take time for any state change where the ice is to absorb heat that is over on the other side of the system, and in that time there will be difference, not to mention things like the jetstream in the ocean, and no uniformity of heat being applied to the glass via the external environment, so on and so forth..

you are right.


even with a glass it should be different. Just much faster. but you could probably do a simple test, get a big glass, add hot water, get a few straws put them in different areas of the glass. then add the ice, wait a bit (just enough for the ice to start to melt) and sip from the straws


an other test, put water in a glass and let it get to room temp, Get a small bowl, put an ice cube in the glass and an other in the plate

 

[oneobgyn]

Calling Morbius!!!

 

[Rikard]

To complicate things a bit, if I remeber correctly you also has to account for the energy required for the change from liquid phase to gas phase in the boiling example. It´s possible that all the water has the same temperature even if only a little of it is boiling compaired to when all of it is boiling.


/Rikard

 

[aphexist]

If he thinks that a system reaches thermodynamic equilibirum instantly, then please have him explain why the universe hasn't reached that state yet. Heat death is a not-so-quick way to destroy the universe, fortunately.


The ice example: You put ice in the glass and the energy of the warmer water is transferred to the _surface_ of all the ice that is exposed to the water. As the ice melts, the water around the ice loses energy. The water surrounding THAT water transfers energy, more water is losing energy and decreasing in temperature. Eventually the glass will reach equilibrium, but if there is a smaller ratio of ice to water, the ice will melt completely before this happens (assuming room temp).


Boiling Water: The thermodynamic system within the pot has different measuring points for temperature as well. If the pot is on the range, then all the heat is concentrated at the bottom of the pot. This equilibrium will probably take much less time because of currents forming within the pot (heat and bubbles of vapor rising, displacing the hot water with cold).

 

[markrubin]

the water will at different temperatures throughout the glass: you will never reach total equilibrium

 

[FrantzM]




I can only love you people .. Interesting OT, but really, has the High End Audio or Video reached such a state of (equilibrium ) that we have not much to discuss about?

 

[Morbius]

Quote:

Originally Posted by Excellence The whole thing got kicked off when I said that you could cook ramen noodles the same time in the boiling water twice and get very different results (I posited it may be the difference between throwing them in the water when it just starts boiling compared to when it's all out boiling.) This is where he first stated that it would be the same temperature everywhere once it started boiling. To which my reply was, see the water boil in a location in the pot is probably a better indicator of temperature than what the thermometer was telling you anyway, if you weren't able to measure a difference.

Excellence,


The problem as described is not fully posed. One needs the boundary conditions. The pot of

water is losing heat out the sides and top, and there is a heat input at the bottom from the stove.

In general, one would need to know the ambient temperature.


That would allow one to calculate the temperature distribution in a stagnant pot of heated water.


However, once boiling started - and bubbles were rising - then the water is being mixed - and

is pretty much the same temperature - your friend is correct in that regard.


Don't confuse boiling and temperature. You can have an entire pot of water at 212F / 100C

which is the boiling point - and have no boiling. Then you put heat in at the bottom - and the

water that boils turns to steam at 212F / 100C. Water, or any other material for that matter;

doesn't change temperature when it changes phases - going from a liquid to a gas. The heat

goes into converting more liquid into gas; rather than raising temperature. But the temperature

of the non-boiling liquid is still at the boiling point. It just doesn't have the additional heat -

the "latent heat" or "heat of vaporization" - needed to change it to a gas. So you can't say that

one area is "hotter" [ higher temperature ] , just because it's boiling.


Additionally, you can "superheat" water - if the water doesn't have "nucleation sites" - that

is something to trigger the formation of a bubble. In a metal pot, the microscopic imperfections

and scratches can serve as nucleation sites. One place one can encounter "superheated"

water is with water in a glass container in a microwave. The glass or ceramic bowl may be

too smooth to allow nucleation - and the microwave heats the water without disturbing it.

Then you open the microwave after heating and disturb the water - you put a spoon in the

water - and the superheated water now has nucleation sites - and flashes to steam - and your

bowl of water "explodes" in the microwave. Lots of people have been burned by this all too

common kitchen accident. [ You may also remember from your junior and senior high

science classes that when you boil water in a flask or beaker - you add "boiling chips" - little

pieces of broken ceramic - in order to provide nucleation sites so that you boil the water without

superheating it. ]


As far as cooking noodles or whatever - this also gets complicated - since it depends on the

heat transfer characteristics between the water and noodles - which can be affected by

things like dissolved gas. You can cook in water fresh out of the tap - and then let it cool to

room temperature and repeat the process - and you get different results - because the first

heating drove out dissolved gas.


You get this problem also posed at the other end of the phase diagram - freezing - i.e. does hot

water freeze quicker than cold? In general, NO. However, if you don't do the experiment

carefully, you can fool yourself. The heated water may have less dissolved gas - so your cold

water should be heated and allowed to cool. If you put a tray of heated water in the freezer -

it may melt the layer of frost on the metal shelf - so you get better contact, that is, better heat

transfer conducting heat out of the hot tray than you get with a tray of cold water sitting on a

layer of frost.


However, if you do the experiments carefully - one finds no discrepancies with the Laws of

Thermodynamics.

 

[Morbius]

Quote:

Originally Posted by FrantzM I can only love you people .. Interesting OT, but really, has the High End Audio or Video reached such a state of (equilibrium ) that we have not much to discuss about?

Frantz,


Some people "cryo" their cables - I wonder what "boilded cables" sound like?

 

[FrantzM]

You know what?


After reading Greg reply. I will tell you:... Bring more OTs of this nature on.. Most educational... His answer is Asimovian in its thoroughness and simplicity.. Great Greg!. I love physics and you are renewing the emotion... Thanks

 

[Morbius]

Quote:

Originally Posted by FrantzM You know what? After reading Greg reply. I will tell you:... Bring more OTs of this nature on.. Most educational... His answer is Asimovian in its thoroughness and simplicity.. Great Greg!. I love physics and you are renewing the emotion... Thanks

Frantz,


Thank you for the comparison to Isaac Asimov.


I met Isaac Asimov when he gave a lecture at M.I.T. when I was a graduate student there.


I used my little portable recorder to tape his lecture, and then went up on stage and had

Dr. Asimov autograph the cassette.

 

[ChrisWiggles]

Morbius is right on, of course!


Yeah, there might be small differences, but all the water is going to be pretty much near boiling, so the differences in a regular sized pot are not large.

Quote:

I said that you could cook ramen noodles the same time in the boiling water twice and get very different results

This almost made my soda come out my nose. Leave it to high-end guys to have debates about whether you can cook ramen noodles and have them come out differently!!! I will admit to having eaten a lot of ramen noodles, but they usually always come out equally sh*tty...

 

[Ron Party]

Maybe Chris, but 7 days in on an arduous backpacking expedition Ramen tastes like gourmet.


---------------------------------

Ron Party

 

[Mooneyass]

Just explain to the guy the meaning of gradients. The guy obviously has no understanding of physics and especially thermaldynamics, which is very complex and conceptual.


Although I am only an undergraduate level Engineer, I learned a lesson that a good portion of PH.D's dont learn: The more you know, the more you realize how little you actually know.


Wes

 

[Swampfox]

Quote:

Originally Posted by Excellence So my friend is trying to explain to me that if you have ice in a cup of ice water, that all the water everywhere in the cup will be the same temperature of the ice, about 32 degrees farenheit.

Who said ice has to be 32 degrees F?


SM

 

[ChrisWiggles]

it's melting...


And presumably we're near sea level and not in the andes mountains...


and i think god probably said so


so then we invented the metric system because it's just SOOO much easier. what now, god? WHAT NOW!? booya!

 

[Greg_R]

Quote:

What do you guys think?

I think I prefer Top Ramen to regular Ramen. After all, this is the high-end forum...

 

[Swampfox]

Quote:

Originally Posted by ChrisWiggles it's melting... And presumably we're near sea level and not in the andes mountains... and i think god probably said so so then we invented the metric system because it's just SOOO much easier. what now, god? WHAT NOW!? booya!

32 F or 0 C is the maximum temperture for solid phase H2O at standard pressure, it can be colder.

 

[ChrisWiggles]

yes, but it's melting. See the water in the ice-water cup. It's melting ice, or freezing water. In any case, with standard stuff this would be 32 degrees. Unless it's a REALLY big piece of ice and it's only the outside of it that's melting at the moment, which brings us back to the original query of the thread....

 

[Swampfox]

The point is that with any logical statement that starts out with a false supposition , the conclusion is automatically rejected.


Ice is colder than 32 degrees. How much colder depends on where it came from, ie the freezer temperature. When one dumps it in water, the water exchanges energy with the ice, so the ice changes state, and the water gets colder. The water can get no colder than 32, or it changes state to solid. Thus, a mixed phase solution exists. A well mixed liquid portion will have a temperature close to the freezing point. The solid phase developes a temperate gradient, with the outside being at 32, the core colder, and the cube slowly melting, and moving the whole system toward equilibrium. Yet, the core of the ice cube will be colder than 32 degrees, until it is close to being completely melted.