Ask MetaFilter questions tagged with thermodynamics

Listen to me kid, this is important...

bonobothegreat — Fri, 18 Sep 2009 16:53:17 -0800

"There are only two types of motion in the universe: oscillation or rotation and you can't have exclusively one without a trace of the other. That's the reason machines wear out." Does this statement statement have any basis in fact and can you tell me where it might have originated? I heard that from an in-law's father many years ago. He was an old, old guy who'd been a car mechanic and blacksmith. He said it with the sort of intensity that people usually reserve for things they've cooked up themselves but I always wondered if it was original.

Why do double-paned windows have a 2cm gap?

unmake — Fri, 06 Mar 2009 20:46:04 -0800

Why is a 2cm gap between window panes optimal? I came across this 2cm/0.75in figure while researching solar oven design. It appears to also apply to regular housing windows, and walled insulation in general. Nowhere have I found a justification.

Is it based on some engineering principle, or law of thermodynamics?

Thought experiment on entropy

zachawry — Mon, 23 Feb 2009 20:28:54 -0800

ThermodynamicsFilter: Assume I have a perfectly insulated room, a battery with a fixed amount of power, and an air-conditioning/heating unit..... If I use the power in the battery to power the unit to COOL the room (and assume that all the power contained in the battery is used), then obviously the extent to which the room is cooled depends on the efficiency of the AC unit. This is because you are working against entropy, and you will always lose some energy to heat. How much depends on the efficiency of the AC unit.

Now, here's the tricky question: Suppose I use all the power in the battery to HEAT the room. In this case, will the room always be heated the exact same amount no matter the efficiency of the heating unit? After all, since inefficiency will ultimately be thrown off as heat anyway, then it should make no difference, right?

(Note that the imaginary room is "perfectly insulated," so the speed with which the heat is released should not be taken into account.)

Aluminum foil turned brown; should I be worried?

taramosalata — Sun, 14 Dec 2008 14:15:11 -0800

I made (delicious!) mashed sweet potatoes in a conventional stove-top pot, covered the pot with Reynolds aluminum foil, then put it in a still warm but off-position oven (simply to save stove-top and counter space for the moment while wanting it to stay at room-temperature). Retrieving the pot a few hours later, I found that the inner surface of the foil had turned a light brown. It was also moist from the condensation, but had remained clean, unoiled, unburnt. I've cooked quite a bit and never seen this. What could be causing such discoloration, and is it a worrying symptom (of, say, toxins in the foil)? Plus: if -- as FAQs on the subject seem to agree -- kitchen aluminum foil is 'the same' on both sides, even though one side is shiny and the other matte, does this discoloration phenomenon indicate otherwise?

Ideal wok height above the flame?

Jsn7821 — Sun, 14 Dec 2008 13:45:52 -0800

How high above the flame should I have my Wok/frying pan/etc? Is full blast always the hottest setting? When I have my gas stove on full blast the flames push out around to the sides of the wok, and if I turn it down just a little they are more centered (but much smaller). Which is more efficient for cooking with a Wok?

Would it be better to rig up something to hold my Wok slightly higher up so the highest-setting flames don't get "smushed" around the sides of the Wok?

I have read a few conflicting things...

Temperature and bathroom scales

knave — Sun, 08 Jun 2008 08:15:04 -0800

Does temperature (ambient) affect the reading of an analog bathroom scale? The reading I'm getting on my scale just dropped by 3 or 4lb, coinciding with a sharp change in air temperature (from 60F to 90F). I know these things are not perfect to begin with, so I could just chalk it up to the scale being quirky. (One day this winter, my weight randomly shot up by 3 or 4lb, and stayed there.)

My level of hydration hasn't changed, so I'm not blaming water (even though it seems a reasonable explanation if hot weather = sweat). I try to maintain a steady water intake.

advanced thermodynamics

complience — Sat, 31 May 2008 16:52:32 -0800

explain further The Second Law of Thermodynamics I asked a few months ago for the good members of Metafilter to explain to me the 2nd Law and I got a great response.

But the more I learn about the second law the more confused I seem to became.

I understand the 2nd law to be this: expression of the universal law of increasing entropy,

or to put it more simply

"energy is moving about"

basicly meaning:

"nothing is free" or more correctly "profit can never be made"

is there anthing wrong with what I understand?

Can't stop thinking about fridges and winter...

advicepig — Thu, 21 Feb 2008 08:07:11 -0800

I can't stop thinking about fridges and winter. Would it save much energy to freeze gallon containers of water outdoors and put those in the fridge now and then? It's Minnesota winter over here and I want to make the most of it ;)

Crystal formation is an increase in entropy?

XMLicious — Wed, 06 Feb 2008 16:51:52 -0800

ScienceFilter: Creationists, crystals, and thermodynamics. A common red herring argument that I've encountered as advanced by Creationists is that by Newton's Second Law of Thermodynamics evolution should not be possible. (According to Wikipedia that argument was originated by a biochemist named Duane Gish, incidentally.) My understanding is that among other things, one reason why this is an erroneous argument is that while it might apply to a closed system the environment on Earth is constantly being pumped full of heat via sunlight and other solar radiation.

But never mind that, that's just the context. I was thinking about it and it occurred to me that in terms of orderliness increasing, the formation of ice crystals in freezing-temperature water or quartz crystals in liquid hot magma that has cooled to the correct temperature both seem to represent an increase in orderliness of the matter in question.

Something I've heard is that formation of crystals isn't strictly due to loss of heat. Supposedly you can have a quantity of water at a static temperature around freezing and if it's turbulent it will remain liquid but when it becomes still the ice crystals will begin to form. (Although hmm, maybe loss of turbulence would be a loss of heat.)

So is an ice crystal actually a higher entropy state than the equivalent amorphous mass of water?

Water expands when it freezes. I don't know why, van der Waals bonds or something, right? But other substances become more dense when they change state to a solid, so a given mass would lose volume. Isn't this kind of like all the air molecules in a room leaping into one corner of it - just the example that's presented as absurd in explanations of statistical mechanics?

I know that orderliness isn't the same thing as heat and isn't really the opposite of entropy. I was hoping that anyone who feels they've got a thorough understanding of thermodynamics could expound on what a thermodynamic analysis of the formation of crystals would be. And anything you can say about the relationship of order to entropy and thermodynamics would be interesting too.

In an eerie coincidence there was this recent post about the second law. The crystals must be telling me things.

Can someone please explain the The Second Law of Thermodynamics to me, with examples?

complience — Tue, 05 Feb 2008 08:25:08 -0800

Can someone please explain the The Second Law of Thermodynamics to me, with examples? I'm reading The User Illusion: Cutting Consciousness Down to Size. Trying to understand Information Theory and entropy. For this I need to understand the 2nd law of Thermodynamics.. which i understand to be:

"Energy spontaneously tends to flow only from being concentrated in one place to becoming diffused or dispersed and spread out."

And an example of this is a rock will always fall if you let go of it a great height Or high pressure will always leak into a low pressure.

But this where I get confused.. that is surely Newtons theory of Cause and Effect in action not the second law.

So my question is really.. what is the difference between the second law of thermodynamics and newtons law of cause and effect?

Great Sites btw:-
www.secondlaw.com
www.2ndlaw.com

Why do sugars like to cyclize in solution, thermodynamically speaking?

perissodactyl — Wed, 12 Dec 2007 06:31:38 -0800

What is the correct thermodynamic explanation for the fact that all monosaccharides with 5 or more backbone carbon atoms occur predominately in their cyclic form in solution?

Forgetted term from physics/thermodynamics

mr_roboto — Sat, 18 Aug 2007 22:29:16 -0800

There's a name for the principle stating that the bulk properties of a substance are a consequence of the ensemble average of the behaviors of the individual molecules comprising that substance. What is it? I cannot for the life of me remember this word, and I might be off a little bit on the concept, too. I cannot figure out a way to search for this. Trust me, I've tried.

How long in the freezer to chill a Coke from 89F to 35F?

zeoslap — Thu, 30 Nov 2006 10:46:43 -0800

So I just bought a can of Coke from a machine, and it came out hot so how long will it take to chill it in the freezer? The machine is usually is 35F inside, the display was showing 89F. So if the Coke was at 89F how long will it take to reach 35F assuming the freezer is at 32F?

The "steam" cloud

zek — Tue, 24 Oct 2006 09:18:47 -0800

Science question. Why is it that when you kill the heat to a pot of boiling water, it suddenly emits lots of condensation clouds? Shouldn't it be making those "steam" clouds while it boils?

Why do you cook with the shiny side of the tinfoil in?

nebulawindphone — Sat, 17 Jun 2006 16:28:35 -0800

When cooking with tinfoil, why do you put it with the shiny side towards the food? If we cooked food by shining bright lights on it, it would make sense to put the light-absorbent side out — but we don't (usually) cook food that way. If we cooked food until it became glowing hot, it would make sense to put the reflective side in — but we don't do that either.

In fact, I was under the impression that most cooking techniques worked by conduction or convection, not radiation. So why do the reflective properties of the foil matter at all?

Why does blowing on food make it cooler?

macinchik — Thu, 30 Mar 2006 10:23:58 -0800

Why does blowing on food make it cooler? I was eating some beef stew last night and it occured to me I had no real understanding of why blowing on something makes it cool down. I suppose I should start with an understanding of what it is for something to be hot.

I understand heat as being a function of the motion of molecules. Something that is hot has more energy. This is about all I know.

unexplained pressure differential

Galvatron — Sat, 10 Dec 2005 22:48:46 -0800

My old apartment had extremely leaky single-pane windows, so in the winter my roommate and I would always cover them with tightly sealed heat-shrink plastic film. Why would the plastic nearly always be bulging inward? My intuition tells me that the plastic should sometimes bulge inward and sometimes outward, as atmospheric pressure fluctuates. I didn't ever notice the plastic flexing significantly towards the outside. Also, I might have expected the plastic to bulge outward when the winds picked up, due to Bernoulli's principle--didn't happen. What's the deal? Why was my apartment constantly behaving like a low pressure zone?

If it makes any difference, the apartment was on the lower level of a two-story building, with the windows at ground level.

Future energy sources

riffola — Tue, 09 Aug 2005 13:10:57 -0800

It's obvious that none of the current energy producing methods we have has enough power to make say space travel affordable and fast for everyone. So no Star Wars / Jetsons type flying spaceships and cars for now. But are there any up and coming energy sources that will be able to do this? Also when the future energy source is cheap, how will we avoid overheating the planet?

How does a deep fat fryer carbonize things if they aren't being burned? Or, how can they be burned if not exposed to oxygen?

twine42 — Thu, 22 Jan 2004 09:34:01 -0800

When something burns it uses up it's fuel/calorific content and turns into a small blob of black which is known to chemists as 'carbon' and to the rest of us as 'bugger'. This I can cope with.

If you stick the same item (or an unburnt relative of said product) into a deep fat frier and heat it for too long, it goes black and creates this same black substance. There has been no oxygen for the the energy to burn with. Where did the energy go?