CaptainPanic

I'm not sure that my post will actually answer the question. I've given it some thought, and I'm not 100% sure myself. One problem is that you use poor wording, as you already knew: what do you mean by "stable"? If the crystal XZ is unstable, then it would form, and react to another shape crystal. If it's less favorable (which I'll describe below in more proper words), then... well... just read the following. There are a number of factors that could influence the crystal growth, and I think these concepts overlap. The parameter that describes whether a reaction will occur is the Gibbs Energy. There is plenty of info online (e.g. wikipedia). Basically, if the change of the Gibbs energy of reaction is negative, the reaction will proceed. [math] \Delta G < 0 [/math], favored reaction (Spontaneous) [math] \Delta G = 0 [/math], Neither the forward nor the reverse reaction prevails (Equilibrium) [math] \Delta G > 0 [/math], disfavored reaction (Nonspontaneous) Often, crystals will grow on a surface. This surface (you call it a "base crystal") will actually change the [math] \Delta G[/math]. So, in other words, if a certain type of crystal is already there, the type that will grow on it might be the same (since this is very often favored). the answer is therefore: "it depends...". I am not sure if kinetics are also linked to the [math] \Delta G[/math]. I can imagine that with two equally favorable crystals don't necessarily have to form at the same speeds.

This is easiest done experimentally. The calculations of the resistance ("pressure drop" is a better term) on a straw with a clamp, and most likely some corners aren't very easy. In addition, the pressure drop is dependent on how hard you blow. What's important is the Reynolds number. Let's see: Flow = 2 liters / s = 0.002 m3/s - that's just an estimated value, dependent on the patient. Cross-sectional area of a 1 cm tube = 3.14*0.005^2 = 7.85E-5 m2 Therefore the velocity in the straw is 25 m/s. Reynolds (Re) = density*velocity*diameter / viscosity = 1.2 * 25 * 0.01 / 0.001 = 300 That should be a laminar flow. The thinner straw has a flow 4 times higher since it's about twice as thin. That makes the flow turbulent (flows generally become turbulent at about Re > 1000). That will greatly increase the "resistance" (pressure drop). I am sorry that I am not in the mood to calculate the pressure drop in a theoretical case. Perhaps later (but no guarantees).

I am educated a bit better than your "ordinary citizen", but I'm not 100% sure what you're talking about. The "ordinary citizen" clearly has a vocabulary which is significantly smaller than yours... and it uses maths to solve every-day problems, not to study its own brain or something.

That is the most worrying thing. We aim for "economic growth", but the above statement shows that either that aim is wrong, or the definition of "economic growth" is wrong. The majority of our world population don't want a very small group of people to become very rich while taking huge risks and thinking only about shares on the stockmarket and the next quarterly results. The majority of the population see "economic growth" as a subjective measure of their fun and comfort in life, which is not easily quantified... and further regulation could very well cause an "economic growth" to most of us.

The density hardly changes, so it doesn't matter how deep or how cold the water is. A certain velocity will translate into a certain height above the water level. Look up the equations and new words that you learned in this thread... You cannot learn physics without studying, I'm afraid. You really have to put a bit of effort

Basically, you must ask yourself with every slide: "why does my audience want to know this?" And don't ask yourself: "why do I want to tell this?" Think about your audience, not about yourself. Most important is: Know what you want to say: know your main message, know your arguments. Then if you make your presentation, make sure to put in the important bit, and leave out the unimportant bit. Explain difficult words to your non-scientific audience using analogies (a polymer = like a chain, a membrane = like a sieve etc...). Don't give the boring official definition, unless it's the only way to avoid a mistake. It's alright to refresh the audience's memory at some point, and hint again at the explanation of the difficult words... especially if you have many definitions (which perhaps you should avoid in the first place - you want to speak in the same language as your audience!!) Don't use abbreviations. TLA's are very confusing. (TLA = three letter abbreviations). The use of abbreviations makes things sound very scientific, but it's often very confusing.

Waw, I am on the edge of overweight according to this BMI However, I am tall, and something just doesn't seem right... because I am not fat. I have a healthy amount of body fat and I get enough exercise. Let's do a test... let's take some top-athletes and see if they're FAT. Kobe Bryant: 198 cm, 93 kg. BMI = 23.7 (nearly overweight) LeBron James: 203 cm, 113 kg. BMI = 27.4 (overweight) Shaquille O'Neal: 216 cm, 147 kg. BMI = 31.5 (seriously overweight) I think I made my point. As mentioned before, I think that this BMI is flawed, especially for tall people. Measure a large amount of healthy people, and give that data to somebody skilled in statistics, and I am sure that the correlation will look different than this BMI. But then the other coefficient, the IQ, would prevent people from successfully calculating it

Backing down from one of the biggest issues of the political calendar? They're a bunch of cowards. And anyway, it's not as if the republicans are gonna take it easy when they win one political victory. IMHO, the republican party has transformed from a political party into a gossip machine that throws mud at the democrats 24/7, regardless of the political issues of the day. The tsunami of mud thrown at the democrats will keep coming anyway. So, if you want to get some independent people to vote for you, at least show some backbone. (I'm not saying that the democrats play it fair, but they do seem to address issues a little more objectively every now and then).

So, no rivers, no large lakes then? Just bring a bucket of water wherever you go, and you're fine then! I hope the Americans will enter the conversation soon... but while they remain quiet, I just advise to check out Minnesota. It's a land climate, on the North side of the USA. It has lakes and either has or borders on the prairies/steppes. Should be relatively flat in some areas. Humidity is probably an issue (it's land climate - but I'm not sure).

If space were really cold, you'd expect it to be very efficient to cool the earth. But while it is quite cold (when the sun isn't shining), the main issue is that there is just nothing there... And therefore it cannot take up heat. Radiation is the only way to transfer heat through vacuum, as was mentioned before. Logically therefore that radiation is dominant. Radiation is also dominant at daytime. For the earth's energy balance, radiation is the only type of energy coming in and going out (unless you want to include solar flares, spaceships and the odd meteor). You can calculate the radiation from any object. You start off with the Stefan-Boltzmann Law, and you should include the emissivity factor (all explained on wikipedia). You then also realize that you should calculate this radiation for the object that is cooling, but also for the object that is (net) receiving the radiation. Then you should compare that to (convective) heat transport (which I admit is a pain in the ass to calculate). For a perfect black body, the radiation at 298K is: [math]j^{\star} = \sigma T^{4}. [/math] with: [math]\sigma=\frac{2\pi^5 k^4}{15c^2h^3}= 5.670 400 \times 10^{-8} \textrm{J\,s}^{-1}\textrm{m}^{-2}\textrm{K}^{-4} [/math] [math]j^{\star}=5.6704\cdot{10^-8\cdot{298^4}}=447 W/m2 [/math] which is most likely smaller than heat transport. Liquid-liquid heat transport generally proceeds at 1000 W/m2K (1000 W/m2 per Kelvin). So, with 10 K difference, it's already 10,000 W/m2 Gas-liquid or gas-solid heat transfer will be an order of magnitude smaller, but will still be larger than radiation. The exact value where radiation will be dominant is hard to calculate... but you can be sure that it won't suddenly become dominant at lower temperatures. Only at higher temperatures.

I don't believe that Cyclonebuster actually attempts to point out a flaw in the laws of physics. He's merely trying to find out what the physics are behind his "trick". And I believe that this has been answered by now.

IR radiation isn't my expertise, but isn't it true that different materials emit at different wavelengths (which are also a function of the temperature)? In addition, the human skin, or your clothing reflect different wavelengths with different efficiency. So, certain wavelengths will hardly heat you at all, while others are absorbed almost entirely and will heat you efficiently. I can't think of how to write it down, but I would evaluate this with a proper heat balance (on one side the wavelength and flux of the radiation, and the other side the selectivity of the type of wavelength are the main factors).

Regarding carbohydrates: Some keywords are: sugars, sugar polymers, starch, cellulose. I am afraid that you'll still end up with wikipedia being one of the best sources of information... The main problem with questions such as the one asked here is that the answer can be just a webpage (wikipedia) or a BSc + MSc + PhD combined. We cannot choose which field you wish to study. And in addition, the people working in this field(s) will hardly ever use a website as reference. I know that in my personal field of science, it's either general Google info, or textbooks (made of dead trees). I wouldn't know how to direct anyone to a quality webpage for chemical engineering. (But I do know a good forum and wikipedia). The next logical question is if you want to study the physical properties, the possible reactions, the way they function in life (that's biology and related fields), etc. The better you know your keywords, the better your Google search.

I solved my own problem. I found it in a series of books called "Landolt-Börnstein", Volume 1, Densities of Liquid systems and their heat capacities (Springer-Verlag, Berlin, 1977). I'm not sure how the copyright works with handbooks like this. Am I allowed to scan it, and put it online? (I mean, the handbooks have their info from other authors as well, right?). Because I don't know, I won't do it (yet). Anyone with further info about SFN rules can PM me (thanks in advance). Anyway, I am sure that I can copy the references. The data seems to be from (very old) references: Bose, Z. physik. Chem., 58, (1907) 585 Blacet, Leighton u. Barlett: J. physic. Chem., 35, (1931) 1935 For further information, the book refers to: Rivkin, S.L., Singarev, M.R.: Teplofix. vysokich Temperatur 2 Nr. 1 (1964) 39/47 ... although the language with that last reference might be tricky. There, now the references will be found by google as well

About question 2: please read this. It explains all you need to know about the word "doctor". I don't understand question 1. Sorry.

Your body radiates heat... and a cold object will not radiate as much. So, indeed, the balance will be that you radiate more to the cooler than the cooler to you. The net effect will be that you cool. However, you mention that people will be there. So, I assume there will be air too. At low temperatures, you can radiate as much as you want, but convective effects will be much greater. Radiation is a 4th order function of the temperature. So, at low temperatures, this effect becomes negligible, and at high temperatures it becomes dominant. Other forms of heat transfer are often a function of the temperature difference (1st order) and will almost be as effective at low temperatures as at high ones (perhaps even more effective). You need a near perfect vacuum to reach low temperatures and have radiation as the dominant effect. To answer your question: it seems that 3He is used for cryogenics, sometimes in combination with 4He. But why do you want a cooling machine that can go to such low temperatures? There must be easier ways to cool your beer.

When you're still in highschool, a common way of thinking about a new idea is: "Nobody seems to use this idea, so it probably will not work". Then, after you finish university, you realize that the correct version is: "Nobody seems to use this idea, so probably nobody thought of it yet"... Which is something that you should always test twice, because often somebody DID think of it, and it probably will not work after all.

Coldest temperature in winter? Or coldest on average? With a choice between a land climate and a sea climate, those are 2 very different things. My initial guess is somewhere near the Great Lakes. I find points 2 and 4 a bit conflicting. Why not live near the sea if you like lots of water and humidity? Your dream area in the USA might also be called "Canada". But I never said that because if I did, the mounties would hunt me down and drown me in a barrel of maple syrup.

I'd go back to last weekend, and do it again. No, it's not that I am not a very adventurous person... last weekend was just that good.

Hmpf. I could have voted... but I missed it. Probably because I was on holiday when this thing opened and closed again. Anyway, I prefer polls where I can simply click on the bottom option (which by default should always be: "I don't know what this poll is about, but I just felt like clicking").

D'oh. That was a pretty poor way to say what I wanted to say. Let's try that again: According to the link posted above (4 posts back), the lower explosion limit of methane is 4.4% by volume. Any concentration higher than that (but lower than the higher explosion limit) and it can explode.

Perhaps strangely, but I disagree. What we should be doing in this world isn't more cooperation. We should have more communication. If from communication we find that cooperation has benefits, then it will happen nearly automatically... as a result.

Sadly, in 4 years, nobody even mentioned the important keywords here: Lower Explosion Limit and Higher/Upper Explosion Limit. According to the link posted above, the lower explosion limit of methane is 4.4% by volume. Any concentration higher than that and it can explode. Here's a list of explosion limits for a bunch of gases. Natural gas can be assumed to be mostly methane. http://www.engineeringtoolbox.com/explosive-concentration-limits-d_423.html

It couldn't be practically achieved also before today

Mushrooms like humidity. Lightning usually occurs when it rains. Humidity, rain... wet.