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September 23[edit]

volatile bases[edit]

I can sort of get (but not fully) why NaOH isn' simply a base analogue of HCl, looking at Fajans' rules. Why isn't for example, aluminum hydroxide volatile like HF is? Also is there any way to get an ammonium salt to distill as a complex ... I see that ammonium acetate decomposes rather than say form a cute NH4-OOCCH3 complex. (After all acetic acid distills as a dimer...) John Riemann Soong (talk) 03:08, 23 September 2010 (UTC)[reply]

I assume ammonium complexes don't distill because at high temperature you boil off NH3 (the complex is formed by a reversible equilibrium of stable components and LeChatelier says they are shifted by loss of a volatile component). In their salt forms, the boiling point is expected to be high due to intermolecular ionic attraction. That is, you can easily visualize a low-energy "decomposition" pathway. Salt complexes that don't dissociate upon boiling would require very unstable and/or nonvolatile components in order to keep "together" (e.g., BF₃•OEt₂) or no other "easy" reaction they could undergo with thermal activation energy. Ammonium chloride sublimates, but you have to reduce the pressure to allow that to happen. I assume that at the cooler temperatures where "→gas" transition occurs at lower pressure, the acid/base equilibrium is not as shifted based on volatility so you can evaporate the complex rather than dissociating it (T∆S term less important at low T). HF is actually less volatile than one might expect based on its low mol-wt and covalent structure with no unsatisfied octets. But "less volatile than extremely volatile" is still "pretty volatile" (it H-bonds intermolecularly just like your acetic acid dimer case). DMacks (talk) 13:49, 23 September 2010 (UTC)[reply]

standing waves[edit]

Does it completely not matter how two opposite waves are timed? I don't see anything in the equations for the difference in phase between two waves. If those waves (travelling in opposite directions) happened to be superimposed such that they always negated the other, wouldn't we get a string without vibration? I understand there's a trig identity that says otherwise, but conceptually I want to see why we get positions with invariant amplitude. John Riemann Soong (talk) 04:20, 23 September 2010 (UTC)[reply]

(Not sure if I totally understand the question.) The two waves only cancel out when they are lined up, but at the instant this happens the string still has non-zero velocity, which continues the motion. Once they pass through each other and become out of phase again the shape of the wave reappears. The relative timing of the two opposite waves is determined by the fact that each one is reflecting at the end of the string and becoming the other. That's why there will always be a node at each end, and why only certain resonant frequencies can exist on the string in a standing wave. Rckrone (talk) 05:02, 23 September 2010 (UTC)[reply]

I guess I want to ask another way. When waves are reflected at a frequency that is not a harmonic or a fundamental, what happens to all the energy that would normally be retained by a standing wave? John Riemann Soong (talk) 10:25, 23 September 2010 (UTC)[reply]

Other shaped waves can still live on the string in perpetuity (in the absence of friction), they just won't form a standing wave. Sort of as a side note, any shaped wave can be thought of as a sum of standing waves by taking the Fourier series. Rckrone (talk) 13:10, 23 September 2010 (UTC)[reply]

Yes, but the motion is damped somewhat. There is a resonance effect when you say oscillate a piece of string: the string doesn't budge very much unless there are standing waves.John Riemann Soong (talk) 14:10, 23 September 2010 (UTC)[reply]

In an experiment with a high-energy beam,[edit]

In an experiment with a high-energy beam, hydrogen atoms each weighing $1.67\times 10^{-24}g$ strike a target with a velocity of $2\times 10^{9}cms^{-1}$ . If $10^{15}$ atoms arrive each second, and the target is a lump of brass of 500 g thermally insulated, find how long it will take for the temperature of the brass to rise by 100°C. (Specific heat of brass = $0.38JK^{-1}g^{-1}$ 115.178.29.142 (talk) 05:12, 23 September 2010 (UTC)[reply]

Sounds like a homework question. Think about conservation laws. Think about how you can estimate the incoming energy. John Riemann Soong (talk) 05:18, 23 September 2010 (UTC)[reply]

I figure the amount of energy required is

0.38*100*500=19000J

but where does this energy come from? The kinetic energy of the hydrogens?115.178.29.142 (talk) 05:22, 23 September 2010 (UTC)[reply]

The question is not implying some sort of chemical reaction causing the brass to rise in temperature. So the only other source of energy is in the hydrogen stream which arrives with a high kinetic energy, and then is absorbed in the brass where it has almost no kinetic energy. Dolphin (t) 05:36, 23 September 2010 (UTC)[reply]

Do you remember how to find out the kinetic energy of a particle given its mass and its velocity? John Riemann Soong (talk) 09:15, 23 September 2010 (UTC)[reply]

Gelling time of methylcellulose[edit]

This is coming from a food perspective, not a laboratory perspective. Let us assume I have a solution of 5% methylcellulose in a liquid medium at 40 degrees C. I then add an equal amount of hot liquid at 80 degrees celsius to raise the temperature of the solution to 60 degrees, the gelling temperature of some types of methylcellulose.

How long will it take the new solution to gel? Is it instantaneous? One the order of seconds? Minutes? Thanks. → ROUX ₪ 19:55, 23 September 2010 (UTC)[reply]

Anyone? My google-fu is failing me, and there's a dish I'd like to use this concept in but do not have the resources for significant experimentation. → ROUX ₪ 21:24, 24 September 2010 (UTC)[reply]

Allow 10 to 15 minutes; during which time you continuously stir. The recipe presumable gives these temperatures as being right for the grade called for in the recipe to give the optimum hydration time, which gives plenty of time to thoroughly mix a large cauldron of the stuff without it going lumpy (see table on page two and there is some mixing guidence towards the end). When in doubt it helps to read the manufactures literature. If it's a DOW product you can double check by sending them an e-mail thats at the end of this information sheet:Product Selection Guide for METHOCEL Food Gums--Aspro (talk) 10:20, 25 September 2010 (UTC)[reply]

Close.. but not quite what I was asking. Let us assume as I said in my question that the methylcellulose is already hydrated and disperesed, it's just not hot enough to gel. → ROUX ₪ 16:58, 25 September 2010 (UTC)[reply]

Most common prescription drugs[edit]

Just out of curiosity, I'm trying (and failing so far) to find a list of the most commonly used prescription drugs in the USA. All I've found so far are vague groupings, like "anti-depressants", rather than the names of specific drugs. Since I'm not using this for professional research, if someone happens to know the numbers for Canada, UK, or some other wealthy nation, I'd be quite happy to see that, too. (Also, I'm aware that there's a unit-of-measurement problem, so I'll take whatever units you've got.) Thanks! --M @r ē ino 20:00, 23 September 2010 (UTC)[reply]

This would be a start for England. - Jarry1250 ^{[Who? Discuss.]} 20:43, 23 September 2010 (UTC)[reply]

Take a look here: http://drugtopics.modernmedicine.com/drugtopics/article/articleList.jsp?categoryId=7604 —Preceding unsigned comment added by 76.169.33.234 (talk) 05:49, 24 September 2010 (UTC)[reply]

Does Viagra require a prescription... Googlemeister (talk) 13:12, 24 September 2010 (UTC)[reply]

Thanks! --M @r ē ino 13:32, 24 September 2010 (UTC)[reply]

Resolved

I know you've marked this as resolved, but I wanted to point out that the lists on the modernmedicine site can be a tad misleading. It isn't taking into account that many popular medications are prescribed in combination with other medications. For example, there are many pills that contain a medication along with hydrochlorothiazide (hctz). The list shows how many times hctz has been prescribed by itself as one ranking. It shows each combination as a separate ranking. So, you don't see the total number of times hctz has been prescribed either by itself or in combination with other medications. -- kainaw ™ 13:52, 24 September 2010 (UTC)[reply]

Anything left to discover after electricity?[edit]

The discovery and putting to work of electricity has revolutionised society, yet centuries ago was only known in relation to static electricity and lightning. Is electricity the end of the line, or are there other as yet little used phenomena that could potentially have as much as an impact as electricity? 92.15.8.96 (talk) 20:37, 23 September 2010 (UTC)[reply]

How would we know what we don't know? → ROUX ₪ 20:39, 23 September 2010 (UTC)[reply]

That was not the question. 92.15.8.96 (talk) 20:44, 23 September 2010 (UTC)[reply]

Well it was, actually. → ROUX ₪ 20:48, 23 September 2010 (UTC)[reply]

"...are there other as yet little used phenomena that could potentially have as much as an impact as electricity?" was the question. 92.15.8.96 (talk) 20:54, 23 September 2010 (UTC)[reply]

Lasers are making the transition from laser pen toys to useful technology at the moment, I'd say Chaosandwalls (talk) 21:11, 23 September 2010 (UTC)[reply]

Making the transition? Are CDs still fancy executive toys, then? Algebraist 21:14, 23 September 2010 (UTC)[reply]

Yes. WikiDao ☯ (talk) 21:51, 23 September 2010 (UTC)[reply]

I thought someone would mention quantum hocus-pocus, which I do not understand. 92.24.188.86 (talk) 22:45, 23 September 2010 (UTC)[reply]

Certainly Quantum computing looks promising, and is "still in its infancy". WikiDao ☯ (talk) 23:41, 23 September 2010 (UTC)[reply]

And there's plenty of mileage in materials science, from graphine to nanotechnology. Arguably we're right now taking what used to be known only in dead tree form and making it do new wonders on the internet. 3D printers look quite promising. None of these are quite the same as electricity, but all are in nascent states with the potential to be game changing. --Tagishsimon (talk) 23:50, 23 September 2010 (UTC)[reply]

Fusion would be pretty cool, too. WikiDao ☯ (talk) 01:50, 24 September 2010 (UTC)[reply]

Thomas Edison and Nikola Tesla, a fine pair of experimenters, thought that the Curie temperature of ferromagnetic materials could be used to make motors, but nothing was ever developed practically to exploit the phenomenon. If iron gets red hot, it is not attracted by a magnet. A small temperature variation up or down modulates the magnetic attraction. In a hot environment, this could be exploited to change temperature variation into force and work. Shield or expose the iron to heat, or turn electric current through it on and off, and you get work. Very steam-punk. In a shirt-sleeve environment, like on your desk top, the energy needed to get the iron heated close to its Curie temperature and to keep it that hot would swamp any work you could get out of the device. Around a blast furnace, or on Mercury or Venus, you might start out close to the Curie temperature. For neodymium-boron-iron the Curie temperature is around 300 C [1]. I recall that one of the aforementioned inventors took out patents in the 19th century for Curie motors, but did not reduce the idea to practice. Edison (talk) 04:45, 24 September 2010 (UTC)[reply]

I suppose anti-gravity would be quite useful for lifting heavy physics textbooks and such like. Sean.hoyland - talk 05:49, 24 September 2010 (UTC)[reply]

Some really speculative and revolutionising Physical phenomena would be time travel and faster than light communication but it is probably impossible. Using small black holes to convert mater to energy by Hawking radiation would be a big step. A more realistic step is High-temperature superconductivity at normal temperatures.

Some other more or less revolutionizing technological innovations that does not use new physics are artificial intelligence, a constructive prof that P=NP, nanotechnology, self-replicating machines, genetic engineering (Synthia ), cheep (space based?) solar power, space colonization (Dyson swarm), Tether propulsion, Brain–computer interface, Mind uploading, Cheap and efficient Closed loop life-support and life span extension.

Nuclear technology could also revolutionize the world much more than it has in both bad and good ways, a full-scale nuclear war would have a very big impact on the society. Many innovations we take for granted are still revolutionizing the third-world. In the long term there can of-curse be unknown Physical phenomena that will have great technological use. --Gr8xoz (talk) 06:29, 24 September 2010 (UTC)[reply]

The current increase in energy use is about 3% per year or 20 times in 100 years. If this continue we will need to colonize space before year 2400 since our energy use will exceed the solar energy hitting the earth. It will also exceed the radiative cooling capacity of earth.--Gr8xoz (talk) 06:47, 24 September 2010 (UTC)[reply]

Even when staying away from quantum mechanics, there are several phenomena and things that have been proposed as a future big thing. The main reason for this is probably that researchers must demonstrate a usefulness of their research to get grants. So in no particular order:

Artificial diamonds are today only used in cutting tools and as coating on glass lenses. If the production of these take off, they could transform everything from computers to space elevators and kitchen utensils.
Cymatics which is mostly a party trick today, could become important for assembling things in zero-gravity. [2]
Lots of things animals do and make could become as widespread as plastics and microchips one day. for instance the goo of a snail: [3].
Crosstalk is annoying in today's electronics, but is central to some amazing circuits that have been "evolved".
The memristor exploits a non-standard effect in materials and could become a big thing in computing
The random scattering of light and electric signals through a tangled mess of cables and materials was just that, until the random matrix results of Terence Tao, along with reservoir systems in machine learning showed very practical implications.
The BZ reaction is another one of those science fair tricks, which hint at a whole field of science with all kinds of applications. This includes all kinds of self-assembling processes that creep and crawl around us every day but which we have so far not been able to mimick.

the list could go on and on. EverGreg (talk) 08:51, 24 September 2010 (UTC)[reply]

Away from physics, synthetic biology and protein engineering are very much in their infancy and it is difficult to know what they could achieve. David Wendell's photosynthetic foam hints at what may be possible in the future as biology and engineering combine. Smartse (talk) 11:12, 24 September 2010 (UTC)[reply]

What about Neuroscience, Artificial neural network, and BCM theory. Also DNA computing and Biocomputers.Smallman12q (talk) 16:42, 27 September 2010 (UTC)[reply]

Percentage of deaths from infectious diseases in New York[edit]

I was surprised to hear an American lady remarking on BBC radio that currently 9% of deaths in New York are due to infectious diseases. It seems like a big proportion, although far less than the 60% of a century or more ago. I never realised that modern city dwellers have a significant risk of catching a fatal disease off other people.

a) What diseases are these? b) How does this figure compare with other cities around the world, including for example London? Thanks 92.15.8.96 (talk) 20:43, 23 September 2010 (UTC)[reply]

Here's a link to the program in question, I was also a bit surprised by that stat. Between them infectious diseases must each have a small effect adding up to 9%. List of causes of death by rate has details from the World Health Organisation for the whole world and says that 23% of people worldwide die from infectious and parasitic diseases. I guess that influenza, tuberculosis and other lower respiratory tract infections and HIV must account for a considerable number. I guess that the Office of National Statistics will details about this in the UK somewhere, but in my experience their website is impossible to find anything on. Smartse (talk) 21:42, 23 September 2010 (UTC)[reply]

I found this which in Table 2 says that ~1% of people in the UK died of Chapter 1 infectious diseases and ~10% died from respiratory diseases, of which I assume many will be infectious since cancers are categorised as neoplasms. Table 2.1 lists the exact number of people who died of each disease. Maybe someone with more knowledge of the ICD-10 classifications can confirm this, but it looks to me as though the death rate due to infectious diseases is therefore probably similar in London as in New York. Smartse (talk) 22:00, 23 September 2010 (UTC)[reply]

(edit conflict) The U.S. CDC provides this list of causes of death, by U.S. state (2006). New York state is listed as having 3.3% of deaths caused by "influenza and pneumonia" alone, and there are some other infectious causes listed though I don't see it adding up to 9% (for the State; it seems about ball-park right for the city though maybe?). WikiDao ☯ (talk) 22:03, 23 September 2010 (UTC)[reply]

(ec) Here are the 2008 vital statistics for the city of New York. "Influenza and pneumonia" ranks very high, almost exclusively amongst the elderly, while HIV is pretty high amongst certain age groups. Tuberculosis has a negligible effect, interestingly enough. --Mr.98 (talk) 22:06, 23 September 2010 (UTC)[reply]

Looking at my numbers more closely, there were in 2008, in NYC, 54,193 deaths. Of these: Tuberculosis: 18; Viral hepatitis: 324; HIV: 1,073; "All other infective and parasitic diseases": 260; Influenza and pneumonia: 2,300. Total: 3,975, or 7.3%. So it's plausible that in 2009 or 2010 (so far) it could be up to 9% or so if the overall death rate dipped or the overall infection rate went up a tiny bit or both simultaneously. Also, I didn't count neoplasms because it wasn't clear to me how many of them were infectious, but if you include things like ovarian cancer or mouth cancers (which I believe some kinds are transmissible, right?) it bumps the number up to around 9-10%. --Mr.98 (talk) 00:31, 24 September 2010 (UTC)[reply]

Ovarian cancer is definitely not thought to be caused by infectious agents. You are probably thinking about cervical cancer and human papilloma virus. VERY different. Also, the majority of oral cancers are probably related to smoking and other tobacco use, although again HPV can be a risk factor. However, the death rate from these types of cancers is dwarfed by the other causes of death. --- Medical geneticist (talk) 13:02, 24 September 2010 (UTC)[reply]

Yes, indeed, that was what I was thinking of, hazily. --Mr.98 (talk) 14:03, 24 September 2010 (UTC)[reply]

I am not an expert on this but I believe that the final cause of death for very weak people such as very old persons often is an infectious disease, often they had died for other reasons very soon anyhow. --Gr8xoz (talk) 07:30, 24 September 2010 (UTC)[reply]

On the other hand elderly people sometimes get pneumonia and recover from it. 92.15.27.8 (talk) 22:34, 24 September 2010 (UTC)[reply]

Oripavin structure: wrong?[edit]

Hello.

I think the structure of oripavin has the CH3 bound to the phenolic OH not to the alchoholic as it is right now. I don't know if I'm right: the notes of my lecture says so and I don't have found any unique answer on the web.

Here is the link of the image: http://en.wikipedia.org/wiki/File:Oripavin.svg

I don't know how to edit and modify things, I just wanted to point out this small issue.

Thank you.

matitaccia

Posted on RD/M by Matitaccia (talk · contribs) 20:43, 23 September 2010, moved here by Chzz ► 20:50, 23 September 2010 (UTC) [reply]

The structures in the databases PubChem, KEGG, ChemID and ChemSpider all show the same structure as File:Oripavin.svg. --Leyo 21:02, 23 September 2010 (UTC)[reply]

The compound with no methyl on the lower-ring alcohol would not be stable...that enol would quickly change to a ketone form, with migration of the other alkene there to form an enone (in this case, the result is morphinone). DMacks (talk) 21:07, 23 September 2010 (UTC)[reply]

Okay, thanks for the answers. I'll change my notes then. :-) matitaccia —Preceding undated comment added 13:55, 24 September 2010 (UTC).[reply]

Right/Left Placement of Gas Tanks on Cars[edit]

Hey All!

Who decides on whether a car model's gas tank is on the right or left side of the vehicle? I would assume that the ratio of right:left should be kept somewhat even in order to make the most efficient use of pumps at gas stations. Is there some sort of regulatory system for this in the US or elsewhere?

Thanks! Tewner (talk) 21:28, 23 September 2010 (UTC)[reply]

Regarding gas station efficiency, tank side is irrelevant. Cars can pull through from either direction, allowing full use of pumps even if all the tank fills are exclusively on one side. As for who decides -- the design team. I don't believe there's any regulatory system for left/right (or rear, though this is increasingly deprecated) in the US, and I have no reason to expect one elsewhere. — Lomn 21:36, 23 September 2010 (UTC)[reply]

This must depend on locale. At most petrol stations I've been to, autos are expected to approach the pumps from a particular direction; doing otherwise would interfere with the queue. -- 58.147.60.48 (talk) 02:11, 24 September 2010 (UTC)[reply]

It would strike me, in an intuitive sense, that you'd want the tank to be on the same side as the driver, as it would make it a little bit easier to align the car up with the pump and not grind up against those concrete poles (which I have unfortunately done at least once!). But my general experience is that they can be on either side. The recent cars I have driven (all rentals) have had it on the left side almost exclusively, though. --Mr.98 (talk) 21:59, 23 September 2010 (UTC)[reply]

I know of nothing to suggest there is a design rule to ensure even distribution of left/right tank access among motor cars. I own a Toyota and a Mitsubishi, both right-hand drive. The Toyota has its tank access on the left, and the Mitsubishi on the right. Dolphin (t) 02:43, 24 September 2010 (UTC)[reply]

This question has come up before both here and at The Straight Dope. Dismas|^(talk) 03:08, 24 September 2010 (UTC)[reply]

The Mini Cooper in the 1960's, which enjoyed great success in the 1960's when driven by Paddy Hopkirk etc, had twin petrol-tanks - and I believe this was to help the balance in such a lightweight car. It became quite an identifying feature of this sportier mini, and some standard minis have been modified to have a fake second filler-cap. Chzz ► 03:15, 24 September 2010 (UTC)[reply]

Twin petrol-tanks would not help maintain good balance. If the contents of one tank is significantly different to the other you have a rolling moment. In contrast, the center of gravity of the contents of a single tank will never move much. Some British cars have two tank access points and I suspect they both lead to the same tank. Dolphin (t) 06:24, 24 September 2010 (UTC)[reply]

... (restored response strangely deleted? - perhaps through edit conflict?).. :From a very limited experience of different cars in the UK, I have assumed that manufacturers place the fuel filler on the driver's side in the country of origin (of that manufacturer), but I don't have enough data to check whether my guess is correct. The actual tank is often central. Dbfirs 21:34, 23 September 2010 (UTC) Dbfirs 09:18, 24 September 2010 (UTC)[reply]

Yes probably [4] Nil Einne (talk) 10:09, 24 September 2010 (UTC)[reply]

In a couple of decades of driving in the UK, I have never once encountered a petrol pump whose hose could not easily reach a fuel filler on the far side of the car. 87.81.230.195 (talk) 16:43, 24 September 2010 (UTC)[reply]

Nor have I, but it does require careful positioning of the vehicle in some cases. Dbfirs 21:12, 24 September 2010 (UTC)[reply]

I disagree, maybe we have shorter hoses up north (no dirty innuendo please!) but in north east England I've sometimes pulled up to a pump on 'the wrong side' and then had to go to another pump after pulling the hose to its full length, despite being inches away from the pump and the filler (on the wrong side) being opposite the pump.Spoonfulsofsheep (talk) —Preceding undated comment added 21:44, 24 September 2010 (UTC).[reply]

I've had no problem in North-West England, with several different vehicles, but I agree that some fuel hoses are shorter than others, and only just reach, and then only if the vehicle is in exactly the correct position (filler lined up with hose). Sometimes it requires a considerable force to pull the hose fully out. Dbfirs 07:38, 25 September 2010 (UTC)[reply]

Thank you all for all the insights! Tewner (talk) 18:12, 26 September 2010 (UTC)[reply]

latex primer[edit]

what will happen if i just use a regular latex primer over melamine —Preceding unsigned comment added by Kj650 (talk • contribs) 21:57, 23 September 2010 (UTC)[reply]

My experience, from painting over formica, is that it will bubble and peel. If you rough-sand the surface before putting on primer, it will stick well. -- kainaw ™ 02:12, 24 September 2010 (UTC)[reply]

Paint companies recommend a suitable primer be applied on a surface before your finish coat. Some websites recommend Kilz or Bullseye primer or special melamine primer [5] over melamine rather than latex primer. The primer's job is to adhere to the surface and provide a base for the finish coat. The finish coat's job is to be tough, look pretty and resist UV and abrasion. Edison (talk) 04:29, 24 September 2010 (UTC)[reply]

There are special paints for painting over laminate, melamine, etc. Get that and follow its directions for surface preparation. People like to then add several coats of polyurethane for toughness (again, follow instructions). PЄTЄRS J VЄСRUМВА ►TALK 16:59, 24 September 2010 (UTC)[reply]

Basic magnetism question[edit]

What makes unmagnetised steel attracted to both poles? Are there any equations? I need them really bad. Is it magnetic polarization -- the repelling poles move away and the attracting poles move closer? John Riemann Soong (talk) 22:59, 23 September 2010 (UTC)[reply]

In the near-contact limit (i.e. when the magnet is very close to the steel), the force will be of order

F={B^{2}A \over 2\mu _{0}}

where B is the magnetic field intensity, A is the area of the contact surface, and the μ₀ is the permeability of free space. For real metals (as opposed to ideal ones) the actual force is usually somewhat less than this. Dragons flight (talk) 00:56, 24 September 2010 (UTC)[reply]

Ferromagnetic materials like steel comprise a large number of tiny bipolar magnets called magnetic domains, all aligned at random. When you bring the north pole of a permanent magnet close to a piece of unmagnetised steel some of the magnetic domains cease to be aligned at random and instead align so that their south poles face the north pole of the permanent magnet. (The opposite happens when you bring the south pole of a permanent magnet close to the unmagnetised steel.) The stronger the permanent magnet, and the closer the permanent magnet comes, the more magnetic domains are aligned by the permanent magnet so that the piece of unmagnetised steel progressively becomes a magnet. When the permanent magnet is taken away, most of the magnetic domains in the steel resume their random alignment so most of the magnetism disappears from the steel. However, there will be a small amount of residual magnetism due to magnetic hysteresis. Dolphin (t) 02:53, 24 September 2010 (UTC)[reply]

The force required to change the air-gap in a closed magnetic circuit is $F={\frac {1}{2}}J{\frac {d\phi }{dx}}$ where $d\phi$ is change in magnetic flux, dx is distance of the movement and J is the bond current or the current in the coil times the number of turns. Equations for some other cases can be found at Force between magnets, this article maybe should include the case with closed magnetic circuits.--Gr8xoz (talk) 07:22, 24 September 2010 (UTC)[reply]