Wikipedia:Reference desk/Archives/Science/2015 November 30

Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is an archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

November 30

Estimating arm diameters using heights and weights

I know the most common way to determine the arm diameter is by measuring it. There are formulas for estimating the diameter of the arm. I saw in anthropometry of the upper arm article that the only formulas for it includes the amount of fat and muscles in the arm. I use the diameter of my middle upper arm as proxy to determine middle upper arm diameters of other people by comparing heights and weights with mine. I came up with the formula:

$d_{e}={\frac {\sqrt {\frac {w}{w_{m}}}}{\frac {h}{h_{m}}}}\times d_{m}$
or
$d_{r}={\frac {w}{h^{2}}}\times 110.8$ then $d_{e}={\frac {d_{r}}{\sqrt {\frac {d_{r}}{d_{m}}}}}$

where $w$ is weight of another person, $w_{m}$ is my weight, $h$ is height of another person, $h_{m}$ is my height, $d_{m}$ is diameter of my arm, $d_{r}$ is the recipe arm diameter, and $d_{e}$ is the estimated arm diameter of another person

Arm diameter has inverse relationship with height and direct relationship with weight, meaning a taller person who weigh the same as the other have skinnier arms than that other, while person who has the same height but weigh more than the other have fatter arms than that other.

If you want to estimate arm diameters of people you know using these formulas, you can use my height of 164.8 cm (64.88 in), my weight of 48 kg (105.82 lb), and diameter of my middle upper arm of 7.076 cm (2.786 in) as proxies. If you want to know the circumference of their arms, just multiply the resulting arm diameters by pi. I obtained my arm diameter by repeatedly measuring it until I measured the circumference of my middle upper arm then dividing by pi.

--PlanetStar 04:34, 30 November 2015 (UTC)

Was there a question? (P.S. my arms aren't even symmetrical so not sure how well a formula is going to work). --DHeyward (talk) 07:18, 30 November 2015 (UTC)

The question is about the reliability of the formulas. I judge the formulas are reliable enough for me to use, a reason why I made this formula in the first place. At least these formulas roughly estimate the diameters of the arms; they may or may not match the actual arm diameters. Arm diameter in this sense means diameter on the front side of the middle upper arm while person's arm is pointing down in the relaxed state (not twisting) or putting arm on the platform (like on the table) with the front facing up. I know that arms are not symmetrical, I noticed it by looking at my arm. You can estimate the diameter of your arm by using one of the formulas above by using your weight and height and then measuring it to see how accurate your results are. To increase the accuracy, you'll may need weight and height in fractions of a pound and inch (or kilogram and centimeter) respectively as seen in the last paragraph of my previous post. PlanetStar 01:08, 2 December 2015 (UTC)

I believe in more accurate formula for measuring the circumference of the mid-upper arm in the scholarly page without taking into account amount of fat and muscle mass but must take the person's age into account. See here. The resulting arm circumference can be divided by pi to reveal the arm diameter. PlanetStar 04:45, 4 December 2015 (UTC)

110 AC & 220 AC

As I have seen most electric appliances designed for110 AC do safely run on 220 AC also. Will this grinder [1] be also safe when run on 220 AC ? I am asking this because many of you will be having far more PRACTICAL know-how, especially about particular devices as such.27.255.155.93 (talk) 04:54, 30 November 2015 (UTC)

How do you know 110 will work safely on 220? ←Baseball Bugs ^{What's up, Doc?} carrots→ 05:12, 30 November 2015 (UTC)

Some appliances are labeled to work with either, a feature that can easily be accommodated with certain designs of a Switched-mode power supply#Input rectifier stage. Others might have that internally but not be certified for more than one country's or receptacle-style's standard. Without knowing the actual product information (manufacturer and part#) of the item in question, there's no way to know. And we're definitely not allowed to provide professional advice about what might or might not be "safe" in various contexts. DMacks (talk) 05:28, 30 November 2015 (UTC)

You need to look at the appliance's labeling or manual. It should tell you what it's designed for. Running anything on electrical input it's not designed for can be very dangerous, with a risk of fires, electrocution, and all that. --71.119.131.184 (talk) 06:47, 30 November 2015 (UTC)

An easy first-order check is if the power cord is removable/replaceable. If it's permanently connected, it likely is not interchangeable. The cord in the picture looks permanently attached so you would need a converter to make it run on 220V AC. It's not safe to cut the supplied plug off and put a different voltage or amperage plug in its place. The manufacturer will usually provide an appropriate replacement cord for products that can run in multiple countries. (computers, DVD players, various electronics often have a cord that can interchanged for the appropriate electrical system - they look like this for the consumer US market [2] - the female part connector is universal and works for many countries). --DHeyward (talk) 07:16, 30 November 2015 (UTC)

One counter example to the removable power cord check is a laser printer -- with at least the few Brother models I've seen all having removable cords. I would not be surprised if the power supply for their electronics accepts a wide range of input voltage, but the drum heater itself if AC powered and is not so tolerant. Fortunately, our 110 vac printer was protected by varistor which ensured that, when it was plugged unto a 220 outlet sans transformer, the built in fuse blew before any other damage was done. Unfortunately, I removed the the quite literally blown (into bits) varistors and replaced the fuse so that I could continue to use it while waiting for replacement varistors. It worked fine until it was again plugged directly into 220 vac, at which point the drum heater gave up its magic smoke. -- ToE 13:25, 30 November 2015 (UTC)

Where did you get a compatible replacement cord? If it's not tolerant, they really shouldn't be using the universal removable cord. The whole point of interchangeable cords is that they are literally interchangeable on different systems. It's supposed to be safer, not just convenience/cost savings for the manufacturer. Now, if it was a proprietary removable cord, you shouldn't have been able to find it in the wrong voltage. For example, even though some electronics can run either 220V/50Hz or 110/60Hz, I don;t think you will find the 220V U.S. version of the cord as it isn't as safe. On lamp cord plugs, the wider blade is the neutral and it corresponds to the wire inside the ridged insulation of the cord. The line side is the smaller blade, smooth insulation and is supposed to energize the least amount of area. --DHeyward (talk) 21:51, 30 November 2015 (UTC)

Are you sure about that? I don't see anything in IEC 60320 suggesting that any devices using them have to be compatible with 100-120V and 220-240V. I've had kettles, rice cookers and a wall paper steamer C15/C16 connectors and I strongly suspect none of them would work properly at 100-120V although I suspect they weren't dangerous. Particularly in the past, many desktop computer PSUs which almost universally use C13/C14 PSUs didn't always support 100-120V. I'm not sure that many of the CRT monitors which likewise used C13/C14 all supported 100-120V either. Likewise, I'm not convinced that all casette radios, CD players, hifi systems or VCRs I used with C7/C8 cables supported 100-120V (although some may have). The proprietary satellite decoded provided by Sky NZ also uses a C7/C8 and only mentions 230V at the bottom, although it's not that likely to be used outside NZ, except perhaps Australia and some of the Pacific Islands. And since it belongs to the company and you aren't supposed to do that, possibly they wouldn't mention even if it did support 100-120V.

In any case, even for those devices that do support 100-120V, I'm fairly sure a number of them required selection of the voltage, including again some of the cheaper PSUs (definitely), probably those CRT monitors which did support 100-120V and those casette radios, VCRs, etc. I'm not sure I see a reason why a selectable 100-120V/220-240V option is fine, but only supporting 220-240V with a clearly labelled input voltage isn't. It would help in a situation like ToE but not in many cases where a person fails to check their device is actually able to tolerate 100-120/220-240V.

Even with a limited input supply requirements, removable power cords still have a number of advantages. The most obvious one remains the easy of supplying them to different countries. Presuming the device is really tolerant of 220-240V and any frequency, this would mean most Europlug and UK plug (which remember is also used in Malaysia, Singapore, HK and some other places) as well as Australia/NZ (which admitedly is a small often ignored market) could use the device without a end of power plug adapter. Even if the frequency was limited to 50 Hz, most countries could still use it. The others would be easier transport (the cord doesn't have to be sticking out all the time), safe replacement of damaged cords etc.

Nil Einne (talk) 03:31, 1 December 2015 (UTC)

From the lede of IEC 60320:

Different types of connector (distinguished by shape and size) are specified for different combinations of current, temperature and earthing requirements. Unlike IEC 60309 connectors, they are not coded for voltage; users must ensure that the voltage rating of the equipment is compatible with the mains supply.

The Brother HL-2140 sitting next to me (110V - 120V ~ 50/60Hz 8.6A) has an IEC 60320 C14 inlet with this specific cord (p/n 8121-0740) connecting to the mains via an IEC 60320 C13 plugging into the printer and a NEMA 5-15P plugging into the wall outlet, but if I were in the UK I could buy this C-13 to BS 1363 cord (described as a "Standard UK Computer Power Cord") and plug the printer into 220 vac mains much to its detriment. -- ToE 03:55, 1 December 2015 (UTC)

Well, color me shocked. The whole point of all the various U.S. NEMA plugs is to prevent things like a 20A appliance being plugged into a 15A receptacle (120V/20A plugs accept both 15A and 20A plugs). An electric dryer receptacle is 240V/30A and has a unique configuration. Not sure why/how these devices pass UL and CE standards if it's that easy to buy a replacement cord that is a fire hazard. Kind of defeats the purpose of a universal component that is compatible with multiple grids. --DHeyward (talk) 02:06, 2 December 2015 (UTC)

Where have you seen that "most electric appliances designed for 110v AC do safely run on 220v AC also"? That statement is FALSE and DANGEROUS to make. Apologies for shouting, but you should not connect any electric motor to a voltage higher than that for which it was designed. Dbfirs 09:51, 30 November 2015 (UTC)

Electrical appliances designed for 110V AC DO NOT RUN SAFELY on 220V AC. Perhaps you have been fooled by laptops computers that can run on both 110V and 220V AC. I have news for you. Those laptops run on 18V DC. Their charger can take both 110V AC and 220V AC. 110.22.20.252 (talk) 10:53, 30 November 2015 (UTC)

The questioner may also have been misled by hearing that 110 vac / 60 Hz appliances often run safely when used with a transformer plugged into 220 vac / 50 Hz, so that the appliance is receiving 110 vac / 50 Hz. (See mains frequency.) The question of running a 60 Hz motor at 50 Hz or vice versa is more subtle and a web search on "running 60hz motor at 50hz" yields much advice, with this discussion by Keith Cress summing things up nicely. While much depends on the nature of the load, it is generally safer to overspeed a 50 Hz motor to 60 Hz than vice versa. I've done both, and the few problem's I've had have been while underspeeding a 60 Hz motor at 50 Hz. -- ToE 13:46, 30 November 2015 (UTC)

One way to tell is if they have a toggle switch, usually by where the power cord plugs in, to switch between them. Do not use a 110-120V volt device on 220-240V unless you are absolutely certain it's designed for that, or there could be a risk of fire. (Using a 220-240V device at 110-120V isn't as likely to be dangerous, but could damage some devices. Others will just run slowly or be dimmer, in the case of incandescent lights.) StuRat (talk) 13:38, 30 November 2015 (UTC)

Be careful with intrests in given information. A motor might be designed by changeing manually its parallel or inline connected parts. Refer datasheets, not relay information of a reseller, only. Todays active PFCs are a buck converters behind a rectifier, used in switching mode power supplies. Some Rice cookers split the heaters into two 115 volts parts, see drawings on commons.[3] A former solution was the voltage selector switch. Convetional Transformers were configured with it by two independent 115 volts inputs, SMPSs use the delon circuit, used in PSUs for computers. --Hans Haase (有问题吗) 11:46, 1 December 2015 (UTC)

Quicksand: Shear-Thinning or Shear-Thickening?

Is quicksand a shear-thinning or shear-thickening non-Newtonian fluid? I have seen various sources that support either case, but do not know which ones can be expertly verified and trusted.

Here's a brief, freely accessible Nature communication [4] that says:

“ The higher the stress, the more liquid the quicksand becomes. ”

--But it's complicated. The viscosity also increases with time after the liquefaction, unlike suspensions of clay or sand alone. See the figures for more details, and this [5] recent full Nature article that discusses some modern research on mechanisms that control jamming an flow in similar systems (it is sadly not freely accessible, ask at WP:REX if you'd like a copy). SemanticMantis (talk) 15:19, 30 November 2015 (UTC)

Full text version available at archives-ouvertes here. Mikenorton (talk) 12:41, 2 December 2015 (UTC)

Measuring Solar Radiation Pressure

What equipment do I need if I want to measure solar radiation pressure close to the sun? (Hypothetical question, I'm not really going to launch a satelite and measure it) 77.127.164.102 (talk) 12:45, 30 November 2015 (UTC)

You could measure the total of solar wind and radiation pressure, by using a pair of solar sails, each on a mast, and attach a sensitive strain gauge to each mast. (You need a pair of sails, on opposite sides, to prevent rotation.) You might find the pressure is too low to measure directly, so do better to measure the change in position of the ship, over time, and infer the radiation pressure from that. Of course, this assumes that you aren't so close that all this sensitive equipment will be destroyed. StuRat (talk) 13:29, 30 November 2015 (UTC)

Thanks! But how do we differ the radiation pressure's effect from the solar wind's effect? (I don't know a lot about the topic, sorry if my question sounds silly) 77.127.164.102 (talk) 14:20, 30 November 2015 (UTC)

For clarity: solar radiation pressure is the pressure produced by absorption or reflection of light -- photons -- emitted by the Sun. Solar wind pressure, on th eother hand, is the pressure produced by stopping or reflection of charged particles -- mostly electrons and protons -- escaping from the Sun. The numbers for solar radiation pressure at various distances from the Sun are given here: Radiation_pressure#Solar_radiation_pressure. At 0.2 a.u. from the Sun, the radiation pressure is 227 mkN / m² = 2.27×10^-4 Pa. The formulas for solar wind pressure are given, for example, here: [6] or here: [7]: turns out, a typical star like our Sun emits an order-of-magnitude of 10⁹ kg of solar wind per second, and typical speed of solar wind particle is in the ballpark of 600 km/s. Using these (very approximate) numbers, we get the following. Let us first find how much solar wind crosses a 1 m² area per second. Since 0.2 a.u. = 3×10⁷ km = 3×10¹⁰ m, the 0.2 a.u. sphere around the Sun would have an area of 4 × pi × (3×10¹⁰ m)² = 1.1×10²² m². Assuming the particles are emitted by the Sun uniformly in all directions (which is not quite accurate, but this is only a rough estimate), we get that the mass of solar wind crossing one m² of this imaginary sphere is roughly 10⁹ / 10²² = 10^-13 kg/m²/s. Now, the pressure is simply the change in momentum flux. Thus, for 600 000 m/s particle velocity we find - assuming the solar wind particles are stopped and do not bounce back - the pressure of 6×10⁵m/s × 10^-13 kg/m²/s = 6×10^-8 kg/m/s² = 6×10^-8 Pa - way smaller than the radiation pressure. Hope this helps. Dr Dima (talk) 02:20, 1 December 2015 (UTC)

the temperature in the "outskirts" of the sun

Do we know what the temperature is in certian points or areas close to the sun? (I'm thinking about apprx. 0.2 au from the sun) 77.127.164.102 (talk) 13:05, 30 November 2015 (UTC)

At that distance you have basically empty space, which doesn't have a well-defined temperature. You could ask for the temperature of the very low concentration of gas that is present, and the answer would be very high, but it is meaningless for practical purposes because the amount is too low to conduct heat to a macroscopic object. Heating at that location is determined by solar radiation, which is independent of temperature. Looie496 (talk) 13:19, 30 November 2015 (UTC)

At that range, heating from coronal mass ejections might be an issue. StuRat (talk) 13:59, 30 November 2015 (UTC)

The questioner may also be interested in reading Black-body radiation#Temperature relation between a planet and its star. -- ToE 14:32, 30 November 2015 (UTC)

Specifically, that where

$T_{P}=T_{S}{\sqrt {\frac {R_{S}{\sqrt {\frac {1-\alpha }{\overline {\varepsilon }}}}}{2D}}}$

gives a temperature at 1 au from the sun of 254 K (-19 °C) for a sphere with the albedo and emissivity (but not the greenhouse effect) of Earth, of 274 K (1 °C) for those of the Moon, and 279 K (6 °C) for a gray (flat spectrum) ball, these temperatures would be √5 times higher at D = 0.2 au, giving 569 K (296 °C), 613 K (341 °C), and 623 K (350 °C) for a sphere with those respective albedo and emissivity values. -- ToE 15:24, 30 November 2015 (UTC)

See Corona, specifically the "Coronal heating problem" section. A recent study [8] of a comet's tail suggests that magnetically driven turbulence heats the corona/solar wind after it leaves the sun. I believe the temperature can be calculated from [9] but they don't seem to give a temperature graph directly that I can see - (paper from 1967, men were men, giants walked the earth, and people didn't reprint the same semilog graph with different scales, I guess. Wnt (talk) 15:16, 30 November 2015 (UTC)

At 0.2 au from the Sun it is actually Solar Wind. Its temperature is usually defined as a dispersion of particle (mainly electrons) velocities with some coefficient. Ruslik_Zero 12:55, 1 December 2015 (UTC)

String Theory

My (admittedly rudimentary) understanding of string theory is that there is no observational evidence that supports it. If that is true, is string theory enduringly popular because physicists have "nowhere else to go," or because of its mathematical elegance? Or for some other reason? — Preceding unsigned comment added by 67.86.55.229 (talk) 15:20, 30 November 2015 (UTC)

String_theory#Phenomenology says "...there is so far no experimental evidence that would unambiguously point to any of these models being a correct fundamental description of nature." For a gentler introduction to the aims and goals of string theory, see Introduction_to_M-theory. Part of why string theory is popular is because it is thought to be a good thing to research that will help lead to a Grand Unified Theory, see Grand_Unified_Theory#Proposed_theories for other frameworks that also have plenty of current research. SemanticMantis (talk) 15:41, 30 November 2015 (UTC)

See this course on superstrings, therein the professor says that superstrings aren't physics yet, they're just applied maths. It's just a promising hypothesis, but there isn't even one jot of evidence that it might be true. Tgeorgescu (talk) 00:48, 1 December 2015 (UTC)

The gist of the hypothesis is that both quantum mechanics and relativity are accurate (which is rather problematic, because they are incompatible as they have been formulated), so professors do complicated math which has to account for both as special cases. Superstrings are of course one of many possible scenarios for accomplishing this. Tgeorgescu (talk) 00:54, 1 December 2015 (UTC)

The problems for string theory go further than the lack of observational evidence. The theory actually predicts that the energies involved in proving it are beyond the reach of experiments...that makes it unfalsifiable - which is a problem that most scientists find hard to forgive in a hypothesis. SteveBaker (talk) 04:07, 1 December 2015 (UTC)

Yes, the string theory is yet to make any testable predictions. Ruslik_Zero 12:57, 1 December 2015 (UTC)

That's not what falsifiability means, and it is not the case that strong theory is unfalsifiable, especially because of what you just said - string theorists can propose experiments that could falsify the theory, we just can't do them at present. To clarify: Popperian falsifiability is about the type of statement, not the current experimental apparatus. To quote our article " A statement is called falsifiable if it is possible to conceive an observation or an argument which proves the statement in question to be false." (emphasis mine). String theory does make claims that are in princicple falsifiable, even if current technology cannot demonstrate them false. SemanticMantis (talk) 16:03, 1 December 2015 (UTC)

Quantum gravity has a testability problem that is unrelated to string theory. There just isn't any system available for study where gravitational and quantum effects are both large enough to be measurable. Dimensional analysis gives you the energy scale of the particle collider that you'd probably need to study quantum gravity directly, and it's the Planck energy, about 1,000,000,000,000,000 times the maximum energy of the LHC.

That leaves trying to theoretically combine what we know from studying quantum and gravitational systems into a unified theory, which has turned out to be incredibly difficult. One of the problems is that the Standard Model has finitely many parameters (constants of nature) only because it's a low-energy approximation, essentially the first few terms of a Taylor series. In the Taylor expansion of a final unified theory you'd expect to have infinitely many terms with infinitely many coefficients that would have to be measured experimentally, and we can't do any of those experiments let alone infinitely many of them. String theory avoids that by being unique at high energy, and that makes it very appealing. It also solves a lot of other technical problems that I don't understand. No other approach has ever solved the uniqueness problem or most of the others. It apparently can't predict anything about low-energy physics, but neither can any other approach, and other approaches can't predict anything about high-energy physics either.

The hope was always that we'd be able to derive the Standard Model from string theory plus some relatively simple assumptions, and that would be a compelling case that string theory was correct. To the extent that that has failed (and I think it has), we're screwed. But we were always screwed because we have no way to do quantum gravity experiments. That isn't string theory's fault. -- BenRG (talk) 17:55, 1 December 2015 (UTC)

It's also not surprising that we haven't been able to prove that string theory can reduce to the standard model when taken in the correct limit. We also can't formally derive all of nuclear physics from QCD, or all of chemistry from quantum mechanics. This doesn't necessarily mean that string theory is wrong or missing something, any more than it means quantum mechanics or QCD is wrong, but in those cases we have a lot of data to help us bridge the gaps with empirical guidance. As you say, the required data is still unavailable in the case of string theory. Nature may be kind and give us experimental access to quantum gravity (but it's not certain that it has to be kind). --Amble (talk) 20:08, 1 December 2015 (UTC)

S. James Gates, Jr., Ph.D. strongly opposes the idea that string theory would not be falsifiable: "So, the next time someone tells you that string theory is not testable, remind them of the AdS/CFT connection...". The idea is that AdS/CFT permits the calculation of the coupling of the "constants" of the four forces (gravitation, electromagnetism, weak and strong nuclear forces). Namely, if the electroweak unification happens before the electroweakstrong unification, the supersymmetry model will be falsified. But, if all three "constants" meet each other at the same energy in a "point" wherein all three forces unite in a electroweakstrong unification (without passing first through a electroweak unification), supersymmetry will have resisted falsification. Professor Gates suggests that the LHC will be used for testing AdS/CFT, namely to check if the electroweakstrong unification does happen as predicted, and if it does happen at the energy computed according to AdS/CFT. Tgeorgescu (talk) 02:53, 2 December 2015 (UTC)

I can't find those lectures online so I'll have to guess what he meant. The idea that grand unification (which is the usual name for what you called electroweakstrong unification) is aided by supersymmetry is an old one. It's a fairly weak argument because it only works if there is no new physics all the way up to the grand unification energy (the "vast desert"). Grand unification can't be tested directly because the unification energy is only slightly below the Planck scale, still around 1,000,000,000,000 times too large for the LHC. Whatever he said would be testable at the LHC, I doubt it was that. He may have been talking about AdS/CFT-based predictions of quark-gluon plasma behavior (which didn't do well at the LHC). These predictions are not tests of string theory as a theory of quantum gravity. The GUT unification is a prediction of the MSSM. I think finding superpartners of Standard Model particles, thus proving supersymmetry is real, would give people more confidence in superstrings, but it has nothing to do with quantum gravity directly. The quark-gluon plasma thing is an application of mathematical techniques developed in connection with AdS/CFT, but even if it worked perfectly it wouldn't mean that string theory is operating in the real world. For one thing, it's backwards: they put the quark-gluon plasma on the CFT boundary (and model it with a gravitational system in the bulk), but if something like AdS/CFT is really correct, our 3+1 dimensional world is the bulk, not the boundary. -- BenRG (talk) 06:24, 2 December 2015 (UTC)

It's not online because The Great Courses are sold for profit. There are illegal ways to get it free, though. Tgeorgescu (talk) 11:48, 2 December 2015 (UTC)

References

In case anyone is still reading, I'll remind everyone again that falsifiability is not about current experimental power. That's just silly. Then we'd have to change our categories every time a technology was improved. So it doesn't matter if the energy needed to falsify string theory is higher than LHC can do, the fact that we can conceive of an experiment that would falsify some claims of string theory make those claims falsifiable. SemanticMantis (talk) 16:59, 2 December 2015 (UTC)

What is it "Gaseouse atom"?

I tried to get information on wikipedia (and on Google) and I didn't find. 92.249.70.153 (talk) 18:31, 30 November 2015 (UTC)

That's a poser. Using the search, the term "gaseous atom" shows up in various articles, but it doesn't have an article of its own, nor does it appear in Gas, which would seem an obvious place to define it. ←Baseball Bugs ^{What's up, Doc?} carrots→ 18:37, 30 November 2015 (UTC)

Might you be looking for Monatomic gas? -- ToE 18:45, 30 November 2015 (UTC)

It's used as a term to distinguish between liquid or solid or chemically bond atoms. It is used to describe energy required to free electrons from the atom without having to worry about other effects that change it. Freeing a valence electron from a gaseous carbon atom is different than freeing an electron from a graphite sheet or freeing it from a hydrocarbon. It's a way to describe the atom as a unique and unbound entity even if that is not the way it is found in nature. Gaseous hydrogen atoms are different H₂ molecules and have different energies but "gaseous hydrogen atom" data is more useful in describing how hydrogen behaves with other atoms and molcecules. Deriving energies for H₂O is easier to start with the gaseous atom properties of valences of hydrogen and oxygen rather than energies of, say Methane and O₂. --DHeyward (talk) 21:37, 30 November 2015 (UTC)