Wikipedia:Reference desk/Archives/Science/2011 May 29

= May 29 =

What velocity in terms of c (light speed) needed to cause fusion in deuterium - tritium fusion?
Hi. How would I go about finding the required speed of collision in terms of c (light speed) for a deuterium - tritium fusion? Thanks--InverseSubstance (talk) 02:40, 29 May 2011 (UTC)


 * If you assume that the hydrogen atoms are free (unbound), then mean kinetic energy is (1/2)kT or $$(1/2) m v^2 = (1/2) k T$$. Use whatever units for velocity you want -- fraction of c.  70.116.11.171 (talk) 04:06, 29 May 2011 (UTC)


 * So that means v = √ kT / m ? Where k = Boltzmann's constant, T = 1 billion kelvin (say), and m = mass of proton ? --InverseSubstance (talk) 05:54, 29 May 2011 (UTC)


 * You can solve the abstract problem of the minimum relative velocity at which two objects with rest masses $$m_1$$ and $$m_2$$ can collide and produce products with total rest mass $$m_f$$ like this: choose to work in the rest frame of the object of mass $$m_1$$. Then its four-momentum is $$(m_1, 0)$$ and the four-momentum of the other object is $$(\gamma m_2, \beta \gamma m_2)$$, where β is the relative speed as a fraction of c, and γ is the corresponding gamma factor. The total momentum (before and after) is $$(m_1 + \gamma m_2, \beta \gamma m_2)$$. The length of this momentum vector must be larger than $$m_f$$. This gives $$(m_1 + \gamma m_2)^2 - (\beta \gamma m_2)^2 > m_f$$, or $$\gamma > \frac{m_f^2 - m_1^2 - m_2^2}{2 m_1 m_2}$$. The equality can also be written $$m_f^2 = m_1^2 + m_2^2 + 2 m_1 m_2 \cosh \alpha$$, where α is the rapidity. It's the Lorentzian version of the law of cosines. -- BenRG (talk) 06:11, 29 May 2011 (UTC)


 * Ok, so $$m_f$$ = rest mass of a deuterium atom (say), $$m_1$$ & $$m_2$$ = mass of a proton. I just realized now its a proton proton fusion. Is that all right?  --InverseSubstance (talk) 06:35, 29 May 2011 (UTC)


 * I just noticed that my answer is irrelevant because proton-proton fusion is exothermic. Sorry. There is technically no minimum relative speed for reactions that release energy. They will happen (slowly) by tunneling at arbitrarily low relative speeds. The classical threshold energy (without tunneling) is given by the height of the Coulomb barrier. Even classically, the reaction will happen in a gas of arbitrarily low temperature because some particles will have larger-than-average speeds. I don't know the height of the Coulomb barrier for proton-proton fusion. -- BenRG (talk) 18:34, 29 May 2011 (UTC)


 * That's interesting because I was thinking the answer would be something like 'it takes 3.5Mev (say) to over come the Coulomb barrier, and 3.5Mev is enough energy to accelerate a proton of mass p to a velocity of v km/sec.  This v is then a fraction of the speed of light.  So this approach isn't helpful?  — Preceding unsigned comment added by InverseSubstance (talk • contribs) 22:19, 29 May 2011 (UTC)
 * That's the approach ignoring quantum mechanics. Quantum tunnelling allows fusion to occur even when the energy isn't enough to overcome the Coulomb barrier - the nuclei can "tunnel" through the barrier. The closer they are to getting over the barrier anyway, the more likely they are the tunnel, but they can do it at any energy level. Converting the energy into a speed is also not particularly useful. It is correct, but usually we don't have two nuclei that we want to fuse, we have a whole cloud of nuclei. In that case, it's better to talk in terms of the temperature of the cloud (which is essentially the average speed of the nuclei) rather than the speed of individual nuclei. --Tango (talk) 23:00, 29 May 2011 (UTC)

low resolution image of Lincoln


I've tried the Google, and failed.

I remember a photograph of Abraham Lincoln at the Boston Museum of Science. It was a low-resolution image -- 16 x 16 maybe -- that demonstrated that faces are recognizable with a minimum of detail or information. I'm fairly certain the image is just of his face, and he's wearing a top hat; the image shows the hat, his face, and his goatee.

Anyone know where I can find a copy of this image (short of going to the MOS and snapping a picture)?

Thanks in advance, 70.116.11.171 (talk) 03:55, 29 May 2011 (UTC)

PS. I first posted in "Computing", but after looking through the mostly technology-related questions there, thought I'd try here too. Sorry for any duplication.


 * If you Google for "Julesz Lincoln picture", you will get a bunch of relevant hits, including versions of the image. Looie496 (talk) 04:04, 29 May 2011 (UTC)


 * Thanks for the tip. I saw that one in my search, but I'm almost 100 percent sure that's not the one in the MOS.  The one I remember has his signature top hat.  It's also straight on to his face, not from the side.  Any others?  TIA, 70.116.11.171 (talk) 04:09, 29 May 2011 (UTC)


 * The museum may have created a new example from a public-domain Lincoln image just to avoid Harmon and Julesz's copyright. The alternative, that there's a second pixelated Lincoln image in wide distribution, seems unlikely since Julesz's is so famous (and I can't remember ever seeing another). This page says that Harmon and Julesz published "block portraits of Abraham Lincoln", plural. I don't have online access to the original article. -- BenRG (talk) 17:17, 29 May 2011 (UTC)


 * I don't think it's recognizable by face, but only because there are few well known examples of people with both beards and top hats. Wnt (talk) 18:43, 30 May 2011 (UTC)

MP3 Player, Computer Circuit integration
Can any one suggest me circuit diagram for the mp3 player with memory card i want it and also audio or mp3 comparator (Ic 741 can used ah?) and how to connect a circuit to computer (output of the circuit is txt files and computer will generate mp3 files as output)? — Preceding unsigned comment added by Kanniyappan (talk • contribs) 06:21, 29 May 2011 (UTC)
 * You mention the 741 which is an Operational amplifier and here is its circuit. However it is no use in building an MP3 CD player which is a complex digital device.. TXT files are for text, not audio. Cuddlyable3 (talk) 15:03, 29 May 2011 (UTC)
 * If you do a Google search on "mp3 player project", there are several kits you can buy and circuit diagrams you can download. However, this would be a fairly complex electronics project, and I wouldn't recommend it if you don't have much experience in the area. Tevildo (talk) 21:04, 29 May 2011 (UTC)

TONSILS
Kissing tonsils is a familiar term. But is there something which is identified as non-kissing bilateral tonsillar edema, the term "non-kissing" is used and is related to tonsils. Is there something like "non-kissing tonsils"? aniketnik 08:21, 29 May 2011 (UTC) — Preceding unsigned comment added by Aniketnik (talk • contribs)
 * I thought this was a question about tonsil hockey, a game which I have played many times in the past. But alas no! --TammyMoet (talk) 10:32, 29 May 2011 (UTC)
 * "Non-kissing bilateral tonsillar edema", taken as a whole, is not a formally recognized entity. It can be translated into ordinary English, though: it means tonsils that are swollen on both sides due to fluid accumulation, but not so swollen that the tonsils on opposite sides come into contact with each other. Looie496 (talk) 17:06, 29 May 2011 (UTC)

3.5 metre Under keel clearance for the Malacca straits passage for ships
How is the figure of 3.5 meter under keel clearance arrived at ? — Preceding unsigned comment added by 202.63.224.53 (talk) 17:03, 29 May 2011 (UTC)
 * By a lengthy process of negotiation. The higher the clearance, the less chance of ships running aground but the fewer ships are able to pass.  According to this book, the Indonesians wanted 4.6 meters (to maximize safety) but Singapore wanted 2.6 meters (to maximize traffic); they settled about halfway between.  Unfortunately it took a disastrous accident to get them to come to any agreement at all. Looie496 (talk) 17:22, 29 May 2011 (UTC)

I have some questions about whales and sharks
1. How far do Humpback_whale travel? 2. Are there any sharks in Colorado or Alaska? Thanks! Neptunekh2 (talk) 21:24, 29 May 2011 (UTC)
 * In Colorado...? --Elen of the Roads (talk) 21:57, 29 May 2011 (UTC)
 * Try Downtown Aquarium, Denver --Digrpat (talk) 22:09, 29 May 2011 (UTC)
 * On your first question, see the first sentence of the second paragraph in Humpback whale. Deor (talk) 00:12, 30 May 2011 (UTC)


 * There are sharks in Alaskan waters, mainly salmon shark (see that article), though other species have been reported. --Colapeninsula (talk) 09:21, 30 May 2011 (UTC)

Physics of Liquids and Friction
Hello all. I was pondering liquids the other day, and before I had confused myself too much, I posed a question. The molecules of liquids have very little friction and they slip and slide off of each other, which makes the substance act like a liquid, right? But what if the molecules were to increase their friction? Would this make the fluid more viscous? Or would this change some other property of the fluid? And is such a feat possible? Is there some way to increase a fluid's molecules' friction? Thanks! Stripey the crab (talk) 23:29, 29 May 2011 (UTC)


 * See Temperature dependence of liquid viscosity. Very technical article but it indicates you could change viscosity by changing temperature.  You might also try the article Viscosity.
 * Wanderer57 (talk) 23:51, 29 May 2011 (UTC)


 * If you have ever heated corn syrup, you'll know that it becomes quite thin like water. Alternatively, if you put it in a refrigerator, it will become as thick as some trees' gum. Plasmic Physics (talk) 00:56, 30 May 2011 (UTC)


 * Regarding your statement that liquids (and I will extend that to all fluids) have very little friction, I'm not sure that's quite right. While solids have a very concentrated site of friction (the boundary between them), where it's easy to observe the effects of friction, like heat and wear, fluids can have friction throughout their entirety.  So, the total friction might well be more with fluids, and especially so with thicker fluids. StuRat (talk) 06:22, 30 May 2011 (UTC)
 * The OP's statement is about friction between molecules, not surfaces. The relatively high friction between molecules of solids is the reason that continuous movement occurs only at their boundaries. Cuddlyable3 (talk) 11:17, 30 May 2011 (UTC)


 * Concepts such as friction, slipping, and sliding are not meaningful at the level of molecules: all molecules can do is attract or repel each other.  Concepts such as friction only come into existence when you have unimaginably large numbers of molecules interacting with each other within a region -- literally unimaginable, quintillions or sextillions. Looie496 (talk) 22:48, 30 May 2011 (UTC)
 * No. Friction is meaningful at the atomic level. Determining the forces required to move atoms past each other is a challenge in designing nanomachines. In 2008 scientists for the first time were able to move a single atom across a surface, and measure the forces required (see above link). Cuddlyable3 (talk) 10:34, 31 May 2011 (UTC)


 * The question has already been answered - temperature, and pressure affects the internal friction of a fluid. Plasmic Physics (talk) 23:13, 30 May 2011 (UTC)


 * I know the question has already been answered however I would like to point out a practical everyday example of this. The designation of motor oil indicates how the viscosity changes.  10W30 for instance the 10 and 30 refer to the hot and cold viscosity as I understand.  HominidMachinae (talk) 22:46, 2 June 2011 (UTC)