Wikipedia:Reference desk/Archives/Science/2014 February 23

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February 23

revolver shot underwater:

looking at this gif,

http://i.imgur.com/bBbYFdY.gif

How does it even shoot underwater? 1) How does it even ignite, without any oxygen? 2) Isn't it too wet to ignite? 3) How can the hammer have enough force, given that it's dampened by water which slows it down incredibly. I don't understand how this can work. Then again, I don't know anything about the subject. Could you explain? 212.96.61.236 (talk) 00:39, 23 February 2014 (UTC)

All of the chemistry required to create the explosive force to expel the bullet is contained in the cartridge's primer and propellant (gunpowder, which includes the oxygen atoms released from oxidizer Potassium nitrate: KNO₃ ) The cartridge is (hopefully) watertight.   ~:71.20.250.51 (talk) 01:03, 23 February 2014 (UTC)

On the hammer, the spring there is going to be pretty strong, and the small frontal area of the hammer moving through water does not provide enough drag to slow it way down. People often assume guns need oxygen from the atmosphere, but if that were true, guns would already not work, even in air. The cartridge (reinforced and held in place by the chamber) needs to withstand tens of thousands of PSI pressure. If things were not sealed properly the gun could blow up when you shot it. Friday (talk) 01:18, 23 February 2014 (UTC)

[e/c] Although the water does slow down the velocity of the hammer, it does not do so "incredibly" – or not enough to matter. (See also: Newton's second law: f = ma)   ~:71.20.250.51 (talk) 01:21, 23 February 2014 (UTC)

Many guns reliably fire underwater. The bigger issues are 1) pressure damage to the gun after firing, 2) problems with the cycling on semi-automatic weapons, particularly if they're gas operated, and 3) ineffectiveness of the projectile in water. Shadowjams (talk) 01:43, 23 February 2014 (UTC)

We have a related article: Underwater firearm — The most important distinction is that it fires darts instead of slugs due to hydrodynamic properties. ~:71.20.250.51 (talk) 02:05, 23 February 2014 (UTC)

That's so they don't travel sluggishly. Clarityfiend (talk) 02:19, 23 February 2014 (UTC)

Note that those "darts" aren't like normal darts, in that they lack fins, and have a massive shell behind them. I wonder what kind of recoil is caused by firing such massive shells and if the concussion wave is a problem, causing ear damage and such. I also wonder if something like an underwater RPG might not work better, since it provides continuous thrust as opposed to just using the initial momentum of the shell. StuRat (talk) 16:49, 23 February 2014 (UTC)

Or a miniature version of a torpedo. Somewhere it was mentioned that rockets are quite inefficient at low speed; that's one of the reason why the Gyrojet gun was discontinued, the other ones being its low reliability and the ammo cost.

Keep in mind that most pistol rounds and even most assault rifle rounds are subsonic, and the underwater projectiles look heavier and thus even slower. IMO you don't save much if you accelerate the projectile gradually rather than suddenly, if the muzzle velocity is decidedly subsonic in both cases. - ¡Ouch! (^{hurt me} / _{more pain}) 08:01, 24 February 2014 (UTC)

The difference is that the maximum speed is much lower, so the drag is so much less. For a bullet, you have to give it a high initial speed, in order to make it to the target, and this means more drag. That limits the effective range by quite a bit.

Note that similar issues come up with bullets in the air, only at greater ranges. That is, if you want to shoot a projectile 10 miles, it either needs to be huge, to have enough inertia to overcome that much air resistance, or it needs to have a rocket attached. StuRat (talk) 15:14, 27 February 2014 (UTC)

The mythbusters episode Bulletproof water tests firing guns into water. They discover that abovwe a certain caliber of bullet, it shatters due to the decelleration when coming into contact with the water - the lower caliber bullets are generally slower so survive. Would this also happen with large caliber weapons fired underwater? Or I guess they would also be accelerated less? 80.254.147.164 (talk) 16:21, 25 February 2014 (UTC)

IIRC, it depended on the velocity more than anything else. An ounce slug, which is subsonic even in air, damaged the container, but hi-vel bullets like .50 Browning disintegrated in water rather than hitting the container.

It could depend on distance, too: after they cracked their container, they tested the other guns in a large pool. 217.255.151.74 (talk) 06:39, 26 February 2014 (UTC)

Real world energy measurements

Looking at Orders of magnitude (energy) we are missing the range from microgram to decigram microjoule to decijoule. But in this range would be many energies in every day experience, eg energy of moving animals or falling raindrop, perhaps peel off a post it note, or the sound energy in a smashing wineglass. But where can I find any of this in reliable sources so as to add to this page? Graeme Bartlett (talk) 08:32, 23 February 2014 (UTC)

Use joules (energy) in your search, not grams (mass). — My quick search didn't find anything useful. ~:71.20.250.51 (talk) 10:48, 23 February 2014 (UTC)

Searching on erg may also help. -- ToE 13:54, 27 February 2014 (UTC)

Raindrop#Speed gives, "A drop with a diameter of 3 mm has a speed of approximately 8 m/s." with a reference. If we assume it is a spherical drop of pure water, it's kinetic energy would be 0.5 * 4/3 * pi * (1.5 mm)^3 * 1 g/ml * (8 m/s)^2 = 4.5 x 10^-4 J. -- ToE 17:51, 23 February 2014 (UTC)

You or I can calculate the energy, but is that not original research from the Wikipedia point of view? I am looking for previously published values. Perhaps some textbooks have examples. Graeme Bartlett (talk) 20:16, 23 February 2014 (UTC)

It's more a matter of WP:SYNTH than WP:OR. You're effectively taking one document that says that "kinetic energy is calculated from mass and velocity using this equation" and another one that says "the mass and velocity of a typical raindrop is..." - and from that, you're forming a synthesis that the kinetic energy of a raindrop is that. It's not original research - every part of the information was derived from reliable sources. The "synthesis" part is the problem here. WP:SYNTH does give us a little leeway here though. It says "Routine calculations do not count as original research, provided there is consensus among editors that the result of the calculation is obvious, correct, and a meaningful reflection of the sources." - and I'd strongly argue that in this case, converting mass+velocity into kinetic energy via the standard formula is an extremely routine calculation. HOWEVER, I don't like that you assume that the drop is spherical (it's clearly not) or that the water is pure...both of those are original research - unless you have some reference that says otherwise. Perhaps it would be OK to say "4.5 x 10^-4 J - The energy of a such-and-such gram raindrop hitting the ground at so-and-so meters per second."...with a reference showing that this is a reasonable estimate for the mass and velocity of a raindrop. SteveBaker (talk) 14:28, 24 February 2014 (UTC)

The shape of rain drops depend upon their size

I should have worded that better. I did not mean "assuming a spherical raindrop" as much as I meant, "assuming that a 3mm raindrop refers to a drop with volume equal to that of a 3mm diameter sphere", and indeed that is the usage in the cited paper (eg. "diameter of a sphere with the same mass"). This makes sense because their mathematical model is predicting the velocity of a given falling drop as a function of time since release, taking into account the changes in drag induced by the changing shape of the drop. When they label one curve as a "3.0 Drop Diameter" on their "Speed vs. Distance" graph, they aren't changing the name of the drop as its shape changes.

Their model produces terminal velocities which match well with experimental results, and perhaps an older paper they cite is more useful to us. Ross Gunn and Gilbert D. Kinzer, 1949: The Terminal Velocity of Fall for Water Droplets in Stagnant Air. J. Meteor., 6, 243–248; (pdf), includes a table for terminal velocities for drops of "Equivalent drop diameter" ("calculated from the mass") ranging from 0.1 mm (0.27 m/s) to 5.8 mm (9.17 m/s). This corresponds to kinetic energies ( (π/12) ρ_w d³ v² for ρ_w = 1.0 g/ml ) ranging from 1.9×10^-11 J to 4.3×10^-3 J, over eight orders of magnitude (as would be expected from 3*log(5.8/0.1)+2*log(9.17/.27)=8.35). That allows us to choose one or two well placed choices within the range of typical raindrop and drizzle droplet sizes. -- ToE 13:52, 27 February 2014 (UTC)

Temperature scale

What is the most useful temperature scale in the world? Celcius is used in most of the world, Fahrenheit is US only, Kelvin is for science geeks, and no one uses Reamur although it is on my national exam paper 140.0.229.39 (talk) 10:32, 23 February 2014 (UTC)

The "most useful" depends on who is using it and for what purpose (as you already pointed out).unsigned post contributed by 71.20.250.51 (talk)

When you say "Farenheit is US only" that is only something that has come about fairly recently. To quote from the Farenheit article

The Fahrenheit scale was the primary temperature standard for climatic, industrial and medical purposes in English-speaking countries until the 1960s. In the late 1960s and 1970s, the Celsius scale replaced Fahrenheit in almost all of those countries — with the notable exception of the United States — typically during their metrication process.

Most older people in the UK still think of climatic temperature in terms of farenheit rather than celsius although, perversely, we often use farenheit to describe days that are "too hot" and celsius when it is "really cold" (minus 10°C sounds much colder than 14°F). Richerman (talk) 11:30, 23 February 2014 (UTC)

BBC weather forecasts give temperatures in Celsius, but they still always give the Fahrenheit equivalent of a couple of them, presumably for those "older people", so Fahrenheit is not quite dead here yet. AndrewWTaylor (talk) 13:52, 23 February 2014 (UTC)

Fahrenheit is more useful because it's a finer gradient. ←Baseball Bugs ^{What's up, Doc?} carrots→ 14:27, 23 February 2014 (UTC)

Of course. My room is frigid at 20°C, and sweltering at 21°C; I am oh so uncomfortable in this wretched metric regime. If only there were some way I could express an intermediate temperature using intervals that are only 5⁄9 as large—that would be the only true, perfect, red-blooded way. TenOfAllTrades(talk) 16:51, 23 February 2014 (UTC)

• Kelvin, because it allows physical laws to be stated in their simplest form. It would be possible to invent an even simpler scale, because thermodynamics shows that temperature can be expressed in units of 1/energy. Looie496 (talk) 15:11, 23 February 2014 (UTC)
  • The average citizen would likely scoff at using Kelvin, as its range of temps that humans can tolerate is pretty narrow. I can see Al Roker saying, "The overnight low will be pretty cold, about 253 degrees." No. ←Baseball Bugs ^{What's up, Doc?} carrots→ 15:26, 23 February 2014 (UTC)

Bugs and Looie are both correct, Fahrenheit is best for fine distinctions, and Kelvin, whose zero is absolute zero, is perfect for scientific calculations. Celsius is just as arbitrary as Fahrenheit. It's magical thinking to think that the freezing and boiling points of water need to be 0 or 32 rather than 212 or 100. But Fahrenheit divides that range into 180 intervals, while centigrade is almost only half as precise. And yes, I can not only tell a one degree F difference on the the thermostat, I can almost always correctly guess where it's set within the normal range. μηδείς (talk) 21:23, 23 February 2014 (UTC)

Frederick Reif in his by now classic textbook "Fundamentals of Statistical and Thermal Physics" defines the absolute temperature as a dimensionless quantity. Then Boltzmann's constant has the dimensions of energy. Alternatively, you can make Boltzmann's constant to be dimensionless and give temperature the dimensions of energy. In any case, as Looie points out, there is no need to have a special unit for temperature. Count Iblis (talk) 17:02, 23 February 2014 (UTC)

Okay, Count Iblis, I'll take the bait. How do you define absolute temperature as a dimensionless quantity, without resorting to Planck units (or a closely related system of natural units)? —Quondum 17:36, 23 February 2014 (UTC)

It's the standard statistical thermodynamics treatment, except that you don't introduce units for temperature. This is what Reif does in Chapter 3 of his book. The usual arguments lead one to conclude that in thermal equilibrium between two systems the quantities:

$\beta \left(E\right)\equiv {\frac {\partial \log \left(\Omega \right)}{\partial E}}$

are equal for both systems

Reif then points out that $\beta$ has the dimensions of energy, and that therefore you can introduce a dimensionless parameter T by writing:

$kT\equiv {\frac {1}{\beta }}$

where k is some constant with the dimensions of energy whose magnitude can be chosen in some convenient arbitrary way.

So, what you see in this treatment is that it's the same as the usual treatment from other textbooks, except that Reif doesn't bother to introduce a dimension for temperature. It's completely analogous to cgs units in electromagnetism where you don't bother with introducing a dimension for electric charge. Count Iblis (talk) 18:02, 23 February 2014 (UTC)

Isn't "some constant with the dimensions of ... whose magnitude can be chosen in some convenient arbitrary way" almost the definition of a unit for measurement? You had me thinking that there might be a natural way to find dimensionless units for temperature (and equivalently energy) without using the Planck constant, but the arbitrary choice eliminates this route. —Quondum 20:38, 23 February 2014 (UTC)

I think you read Iblis too fast. He didn't say you make temperature dimensionless. He said you make Boltzmann's constant dimensionless. --Trovatore (talk) 20:40, 23 February 2014 (UTC)

Yeah, I did miss half of it, and hence misinterpreted. You can choose which of two things are considered dimensionless, but not in a way that gives anything new. —Quondum 20:57, 23 February 2014 (UTC)

Celsius would be the scale that most humans could relate to. Simplistically, 0 degrees for water turning to ice, 100 degrees for water turning to steam. Icy roads and cups of tea. Easy. The Rambling Man (talk) 17:08, 23 February 2014 (UTC)

Digressions aside, the OP's question does not have a single answer. In physics, a scale with zero at absolute zero is the most useful, and for that the Kelvin is the most used. For everyday use, a linear scale familiar to most people is the most useful, and hence will be location-dependent. The Scale of temperature article lists eight linear temperature scales, but at present typically only Celsius and Fahrenheit are widely used in everyday contexts, and their usefulness here will depend on what the people in the area use. There were other scales that were in use for very high temperatures, before these could be related to lower temperatures – e.g. for baking pottery. —Quondum 17:25, 23 February 2014 (UTC)

Is Celsius really the most relatable scale, TRM? "Ice is zero, steam is 100" are just arbitrary choices of numbers as well as situations for them. They're convenient situations to measure for calibrating measuring devices, but that's "sciency" not lay public. For example, the most common temperature uses seem to be weather and cooking. The former goes nowhere near as high as 100 °C, so it might be saner to call the bp of water 200 (still a nice round value), then weather is fairly close to 0–100 (again/still assuming the arbitrary choice of "liking" a certain range of values, but also gaining the finer-grained detail of F vs C). The latter goes well over 100 °C. DMacks (talk) 20:36, 23 February 2014 (UTC)

Yes, I think it is. The original question was what was the "most useful temperature scale in the world", not the one which a scant few physicists may use, not one which a scant few "Imperialists" would use, probably the most useful one would be the one that most people could relate to. Of course we can go below 0 and above 100 but most humans are aware of water, ice and steam. Many modern scales, like, ooh, percentages, go from 0 to 100. Not saying that's a direct read-across, but most answers here really are thinking too hard about the answer. No-one outside scientists really care about Kelvin, those arguing for the granularity of Fahrenheit are bonkers (decimal point anyone? Check 98.6 for instance...). Anyway, no doubt this thread should be archived as there's clearly no objective answer, just a lot of opinion. The Rambling Man (talk) 21:12, 23 February 2014 (UTC)

Do we have to do "American imperialism" on the science desk? Isn't that sorta 1970s anyway? --Trovatore (talk) 21:20, 23 February 2014 (UTC)

Ach, you know what I mean. People who use Imperial measurements by default....! The Rambling Man (talk) 21:22, 23 February 2014 (UTC)

We don't call them that, never have. "Imperial" is used to distinguish when y'all's version of a unit is different from ours (e.g. the Imperial gallon vs the US gallon). --Trovatore (talk) 21:26, 23 February 2014 (UTC)

Sorry, don't really care. I've answered the point. The Rambling Man (talk) 21:27, 23 February 2014 (UTC)

I like Fahrenheit for weather measurements because 100 is hot and 0 is cold. Bubba73 ^{You talkin' to me?} 02:16, 24 February 2014 (UTC)

The thing that gives Americans a bad image on topics like this is that people who have never extensively used metric measurements are so certain that they are so evil and bad. As someone who worked for Australia's Bureau of Meteorology when Australia metricated, I just have to say, it wasn't really all that hard. We all survived. And, maybe because of global warming, where we used to think 100 degrees F meant a hot day, we now seem to do that for days over 40 C. (And that's 104 F!) Nobody in Australia uses Fahrenheit for anything now, not even old people. (Like me.) HiLo48 (talk) 02:40, 24 February 2014 (UTC)

Oh, please, HiLo. Everyone knows that it is you personally as an editor and a person who are both evil and bad, and, for that matter, evil. Trying to foist that criticism off on Americans is silly. Only Kelvin has any scientific justification, or maybe Robin Thicke. Centigrade is just easier for small-browed water-bigot cretins who find subtracting 32 from 212 too difficult to deal with. μηδείς (talk) 03:40, 24 February 2014 (UTC)

Now, there speaks a master rationalist. Any population resists change; the difference is in the impetus by the legislators. It does not reflect on the American people, even though HiLo's observation applies. And even though it does not reflect on the people, it is difficult to resist feeling superior, forgetting that we were dragged through the process kicking and screaming, decrying the confusion it creates, and the (then) youth being bemused at how the previous generation fails to adapt (I too come from a country in which the metrication process did not stall). One never really fully adapts to a unit system that was not in use at the time one went to school. —Quondum 06:46, 24 February 2014 (UTC)

I understand that the US is a democracy, and the American people elect those legislators. So surely it does reflect on the American people? HiLo48 (talk) 06:56, 24 February 2014 (UTC)

Surely you don't buy into the myth that a democracy is an efficient expression of the nature or even the will of the people? That's like saying you can efficiently guide a blind racing driver around a track by sending him Braille postcards. —Quondum 07:24, 24 February 2014 (UTC)

@Quondum: that's a beautiful phrase, truly. Nonetheless, there now are blind racing drivers [1] and in the era of the text message a Braille postcard could be relevant to a race. Your analogy, however perfect, is breakable, and so can be the original case. Democracy may have been a vacuous dream, like "all men are created" equal was during a century of slavery, and yet, sometimes the dream turns out to be possible after all. Wnt (talk) 14:39, 25 February 2014 (UTC)

@Wnt: You are stretching the content of the link severely... but anyway, I had something like in Scent of a Woman in mind. And I was not trying to imply that democracy is inherently broken or that it cannot still achieve much of what it is projected to be, only that to attribute too much of some outcome to a particular detail associated with one of many inputs to a complex system is not a valid deductive process. This thread is so far off topic already that I'm surprised it hasn't been hatted yet... —Quondum 17:21, 25 February 2014 (UTC)

Those legislators have to do things that appeal to at least some of the voters. Those voters are American people. Obviously a chunk of them don't want change. HiLo48 (talk) 07:36, 24 February 2014 (UTC)

Show me someone who says that "13 meV per degree of freedom" is a comfortable temperature, and that is the Geek to be admired. Though I suppose he'd be an electron chauvinist for not making that "2.07 zeptojoules"... Wnt (talk) 05:27, 24 February 2014 (UTC)

Is that 13.0 meV? The implicit ±0.5 meV uncertainty is enough to more than span the "comfortable" range. :) —Quondum 07:03, 24 February 2014 (UTC)

Summing up,

• Celsius gives us double digit temperatures we can encounter in real life, at the price of quite arbitrary zero and step size. OTOH, it is easy to reproduce in a classroom, and it was what could have passed as a natural constant in Celsius's time. (The boiling point does vary a lot with pressure, though.)
• Kelvin has the most "natural" zero temperature and is great for physics, but bad for everyday life - its zero is so far down that all the double digit temperatures are really cold (although Finns and Alaskans might challenge that...) and 243K is not my idea of "10% colder than 270K" either. The step size, the same as Celsius, is a bit on the arbitrary side, though.
• The Fahrenheit points are really arbitrary, the zero being the lowest temperature he could make; the step size seems awkward from a 21st-century point of view – the 180 steps between freezing and boiling water are a coincidence; OTOH, multiples of 6, 12, and 60 were quite common during that time, which could have been the reason why the F scale wasn't abandoned earlier. By another coincidence, -39°F is -39°C, the freezing point of mercury thermometers. Actually, according to the article, it's -38°F. Damn. There goes another rule of thumb.
• Reaumur is "a less decimal Celsius" and thus slightly less useful all things considered.

I'd say Celsius is the most useful scale for everyday use; negatives are cold and mean slippery roads, and 30 or more is hot... or among Hawaiians, anything below 30 is cool. Of course, Fahrenheit can be justified just as well (by those who wouldn't touch decimal units with a 3.048-meter pole, but the reasoning that "it uses the double digit range better than Celsius" sort of defeats itself). Kelvin and its F-cousin Rankine make simpler scientific formulas.

I wouldn't blame ±0.6 degrees Celsius for an uncomfortable room temperature; IMO it's a humidity thing more than temperature. - ¡Ouch! (^{hurt me} / _{more pain}) 07:49, 24 February 2014 (UTC)

The most useful is whatever you are used to and are comfortable with using. −40 °C (−40 °F) is the temperature that User:One.Ouch.Zero was looking for. Also Celsius had 100 as the freezing point and 0 as the boiling point. CambridgeBayWeather (talk) 09:54, 24 February 2014 (UTC)

The problem with this question (and the reason for all of the controversy about the answers) is that we aren't being asked what the scale has to be useful for.

• A weather forecast on TV would be hopelessly hard to follow in Kelvins because all of the temperatures would be about the same.
• Doing low temperature physics in Fahrenheit...or even Celsius would be a nightmare.
• If you're doing science, then Kelvin is really the only absolute temperature scale you can use because all of the data in all of the textbooks and all of the equations relating to absolute temperatures an SI units are in Kelvin. It's the only absolute scale that we have.
• However, if you're doing relative temperature calculations (eg how much temperature rise would you get if you applied X joules of heat to Y grams of water) - then either celsius or kelvin is OK because for relative temperatures, they are the same thing.
• If you live in a country where meters and kilograms are like a foreign language, then making people switch to Celsius without changing all of those other units seems kinda pointless.
• If you live in a country where feet and pounds sound like something out of the 18th century - then using Fahrenheit is madness.
• Anything other than Celsius, Kelvin and Fahrenheit would be confusing to absolutely everyone - so those are clearly non-starters.

The word "Celsius" should be taken to mean "Degrees Kelvin above the freezing point of water"...and if you understand that it's just shorthand for that - then we're really only using two systems - Kelvin and Fahrenheit. Even people who use Fahrenheit often say things like "twelve degrees below freezing" to describe 20 deg F - so it's not like this is an unfamiliar shorthand for all of us.

For people who are not scientists, temperatures relative to absolute zero are not really of interest - and having big, very similar numbers marked on your home heating system thermostat would be ridiculous. So we're really only down to Fahrenheit or Celsius - with celsius being a universal shorthand for "degrees kelvin above freezing". I grew up with Celsius - but I live in the USA where anything non-science related is likely to be in Fahrenheit...so I'm entirely familiar with both systems. I know that a 100 degF days in Texas is freaking hot because it's higher than body temperature...which matters a lot...so it's handy that it's a nice round number. But for day-to-day use, I have to say that Fahrenheit sucks. I really *REALLY* care whether my water pipes are going to freeze - and having some arbitrary number for that (32 degF) sucks. Choosing zero as the freezing point of water under normal day-to-day conditions just makes a ton of sense from a practical perspective. There is no physical thing that happens at 0 degF to warrant that being the zero point...but since our world contains so much water - and that phase change is so critical to us - a zero degree freezing point is the perfect lower end. The higher end is less important.

For day-to-day weather-related temperatures - a scale that went from 0 (freezing point of water) to 100 (typical human body temperature) then I'd be fine with that scale for day-to-day use. But neither of the scales do that - so Celsius wins for me...as it does for by far the majority of the people in the world.

This whole thing boils down to "Why doesn't America adopt the metric system?" - which is a frequently argued topic, both here and elsewhere - and doesn't bear repeating or continuing.

SteveBaker (talk) 15:13, 24 February 2014 (UTC)

And the answer is, "Because there's no compelling reason to." Someone farther up says something about voters resisting change. That's a bit misleading. In fact, it's not even a topic of conversation. Except maybe outside the US. ←Baseball Bugs ^{What's up, Doc?} carrots→ 15:34, 25 February 2014 (UTC)

To what degree is this a matter of civil liberties? In the U.S., the First Amendment is extremely important, and is the immediate objection to any harsh scheme of metrification. As explained at the Fahrenheit article, it is still used in some "unregulated sectors" in non-U.S. countries. As it also is relevant to Wikipedia, is useful to note that anarchy's most defining drawback (and at other times, its strength) is (to some, surprisingly) an extreme resistance to change. There's simply no one in a position to say that "everybody has to use this now", and if the argument to do so is not really very compelling, it can take a very long time indeed to happen. Wnt (talk) 17:21, 25 February 2014 (UTC)

Temperature scales that form linear functions with respect to energy (0=0) are the most useful. Kelvin is. Celsius and Fahrenheit are not (Kelvin is the linearization of Celsius). E=f(T)=kT -> f(T1+T2) = f(T1) + f(T2) (plus using Kelvin shows how fragile people are :) ) --DHeyward (talk) 19:59, 26 February 2014 (UTC)