Enthalpy

No difference. A varying magnetic field needs conductivity along the path it is to induce a current, that's all. In nearly any electric machine, the magnets or electromagnets are carefully insulated from the circuit where current is induced. That is, a magnetic field needs no material medium at all and works in vacuum as well. A static or slow magnetic field decreases steeply over distance so the magnet should be kept near. An electromagntic field - which varies quickly - can cross huge distances, like billions of light-years for light coming from stars. Hi Anders, nice to see you! The expression "eddy current" is more commonly used for a very real macroscopic current, resulting from a varying magnetic field, that flows in a closed loop within some conductor and not through an external circuit, and does have power to produce heat: unwanted losses in transformer cores, desired braking force... But if the magnetic field is static, you get only effects like para and diamagnetism as you describe. Plus, in the case of type 1 superconductors, induced currents (see Meissner effect) which are macroscopic but give no power for free, sure. Fun: in the Meissner effect, currents repel the induction out of the material even if the induction existed before the superconductor was cooled, so this is not a simple consequence of zero resistance and induced voltage. I've seen no single piece of evidence that MHD has been used to propel a submarine. I've only heard of the Yamato, which did navigate, but had the very bad efficiency expected from any MHD boat, which must discourage any submarine designer. Plus: submarine design puts a huge effort in minimizing any magnetic field, and there MHD is the wrong direction. Add the detectable trailing heat produced by huge losses: my bet is that designers improve screw propellers, or have non-screw mechanical propellers. Anyway, pictures of recent submarines are public, and they hide only the propellers, whose position and size are compatible with screws.

It has been done through the rate at which a couple of pulsars loses its orbital energy, and rewarded with a Nobel prize. This experiment, where the orbital speed modulates the pulsar's emission frequency through Doppler effect, indicates some orbital energy loss through the radiation of gravity waves, and the rate of loss is consistent with gravity propagating at the speed of light. Though, you have plenty of room to improve the accuracy! Older experiments involved Webber bars, but as far as I understand (=little) these experiments produced and detected only near-field waves, which don't even need propagating waves (with a finite speed) to exist. A serious difficulty: gravity waves haven't been detected up to now (I believe), which won't help measure their speed.

The rounded leading edge is one more common misconception in fluid dynamics. A sharp leading edge performs better, and the swordfish has one, as well as the wing profiles used at not-very-old air gliders. I suggest to look at an air glider not older than 40 years: its wing differs radically from the old "water drop" tale. In addition to its sharp leading edge, it has its maximum thickness behing the half chord, its lower face is very convex at half chord and gets concave only behind - which also means that the lower face is longer than the upper face. Rounded leading edges are necessary (and aren't very bad) when the fluid's direction isn't precisely known, which is the case of a boat's keel which drifts unless its profile can be made unsymmetric. Unless you have much freedom (much length) to design a smooth trailing edge, you better cut it completely flat instead of making it too steep. This is done at many cars (for which lift and stability is one more worry). You get a clear dead zone after the flat cut and a not-too-bad stream around this zone, while a too steep trailing edge would induce irregular turbulence, noisier and lossier.

Researchers at the Amsterdam University use time-resolved Raman spectroscopy to image objects embedded in non-transparent materials. They target explosives hidden in clothes or luggage (an obsession these days). Time-resolution (read: short time) allows to discriminate at the detector between light diffused by the non-transparent material and by the embedded target, whose signal is fainter but strikes the detector from a different distance hence at a different time. Raman effect, as it scatters light back at a different wavelength not produced by the surrounding medium, allows further discrimination. Under "non-transparent" I expect things like cloth or leather or plastic, not aluminium sheet. A brief description in Spectroscopy Europe, vol 24 No 1 of February-March 2012. I wish this nice technique be developed for purposes I feel more useful, especially imaging the brain's cortex within the skull. This is done by MRI or X-ray tomography up to now, which needs huge and costly apparatus to give not very clear images. Pictures of broken bones, clearer than by X-Rays, would be useful as well. Beyond diffusion, the surround medium (the skull) also deforms the picture by refraction, but corrective software already exists, especially if several pictures are taken from different angles. ================================================= A different way to discriminate light from the target and from the surrounding medium would have directional sensors AND illumination source. By putting them apart, light scattered from different depths arrives at the sensor from different directions, which for instance a CMOS focal plane can discriminate. High contrast imaging is known from astronomers for instance, as they image planets around remote stars. The directional source is swept to acquire a complete image. If the detector is a 2D plane, the source beam can be flat and scan only one angle, saving time. If the beam is narrow and scans two angles, the detector can be a line. Again, pictures from different angles of view can allow software to compensate for refraction by the surrounding medium. Raman scattering would further help discriminate the target from the surrounding medium and offer useful information about the materials, and make constrast products more efficient. Marc Schaefer, aka Enthalpy =================================== The method with directional sensor and source might be the one used by secret services for their mind-reading machines, in combination with software to compensate for head movements and some automatic learning software that identifies each area of the cortex. Though, as such machines existed 30 years ago, I suppose they used an imaging radar, since this was available technology then. The necessary angular resolution looks feasible from few metres. Time-resolved Raman is even less plausible in the late 70's. Since the worst possible people have already this technology, I ask researchers to develop it for useful purposes as well, like medicine. Four layers of space blanket, which are opaque from LW to visible, protect against such machines anyway.

Hello everybody! I've spent some time comparing rocket propellants suitable for pressure-feed at a launcher, and have gone away from oxygen-methane and cyclopropane to room-storable fuels (and liquid oxygen). Though they may offer a lower specific impulse, the tanks they need are lighter, resulting in slightly better stages - and safer if the fuel leaks and ignites less easily than a gas does. Amines outperform hydrocarbons as well. The heaviest tank stores helium, and cold liquid methane needs more helium than a room-temperature fuel does. Also, a reasonable layer of microballoons-filled polymer can insulate the fuel tank from the colder helium tank and keep a simple and sturdy construction for the booster, of thick Maraging steel sheet all welded together. More detailed rationale and figures there: Link removed As an illustration, here's a launcher whose side boosters burn pressure-fed Pmdeta (pentamethyl-diethylene-triamine), and whose central core burns hydrogen-oxygen in Vinci engines ignited after separation. The boosters - whose number fits the payload and mission - are "cross-fed", where all engines consume propellants from the tanks of one boosters pair at a time which is separated early as it goes empty to save inert mass. I'll describe later how the boosters shall sail back for reuse; this fits pressure-feed nicely. Or see Link removed if you don't want to wait. Marc Schaefer, aka Enthalpy

An idea that may break through! I suggested there (but there were already developments, at least for disabled persons, if not for every computer user) http://saposjoint.ne...php?f=66&t=1477 to track with a camera the eye position of a computer user and let software deduce where the user looks at. Then, special keys - I'd like them at the keyboard - would tell what action to start with the object looked at. This is meant as a pointing device to replace mices, touchpads and other trackballs. Meanwhile (or had they started before?), the Swedish company Tobii develops such a pointing device http://www.futura-sc...les-yeux_28435/ resembling my description. And now Tobii's development could be integrated to Windows 8, as has just been made public at the 2012 International Consumer Electronics Show in Las Vegas: http://www.tobii.com...-for-windows-8/ (developer company) (or search the Web for Tobii Windows in your preferred language) at Youtube : 3MoGzTdQnX8 (demo video by Tobii) I'd like to have such a possibility on the computer I use, if it's quick and accurate. But above all, I need some credible proof that the camera can only send data to that piece of the computer. Best wishes! Marc Schaefer, aka Enthalpy

Thermoplastics are not really possible to cast, because they remain very viscous. This leaves bubbles and voids in the mould, and you don't get the desired shape. They also react with air when warm. That's why they use to be injected at tremendous pressure, by very strong pumps, in very resistant moulds - expensive, and for bigger series. It's much easier to cast alloys like aluminium-silicon or aluminium-magnesium-silicon (AA6000 series). Dry (oven for long) plaster works as a mould. With plastics, the accessible technology are thermosetting ones, like epoxy, which you can cast in a mould, or rather over a form, and reinforce with varied materials like glass fibres.

Figures about Jupiter's depths there http://www.futura-sciences.com/fr/news/t/astronomie/d/le-coeur-de-jupiter-pourrait-etre-en-train-de-disparaitre_35400/ like 16,000K and 4TPa. Nobody went there to measure, but at least it can be present-day consensus supposition. The paper at Futura-Sciences tells "solid" and "liquid" but this must be a simplification for "dense gas and plasma". Notice taken, though, that said liquids aren't the ordinary form - hydrogen being a metal there instead. And in 2016, Juno shall orbit Jupiter to make a precise gravity map and give clues to the planet's depth.

Very interesting! Because Earth consists of little compressible solids or liquids, allegedly with a uniform Ni-Fe composition over much of its radius. In contrast, Jupiter is commonly considered as a ball of gas, more compressible than rock. A perfect gas would differentiate more the density, making Jupiter more spherical than the extrapolation from Earth. A first answer is that gas near to liquid density isn't very compressible - but still much more than a liquid. (In contrast, a plasma is nearly a perfect gas) A second reason would be a high temperature gradient, consistent with the excessive heat radiated by Jupiter. But my gut feeling is that 1/15 versus 1/12.3 doesn't allow a heavy kernel of metal or ceramic. If you could put some numerical constraint on the mass of a solid kernel, it would be worth a paper on arXiv. With some nearly-reasonable assumptions, like an adiabatic convective "equilibrium" over altitude, as in Earth's atmosphere. Alas, both Van der Waals' and virial's equations are very inaccurate near liquid density. Bad joke. Better equations must exist. Segregation by gravitation over the molar mass makes things more complicated. The upper atmosphere does it on Earth, despite the gravity well isn't as deep. Maybe Jupiter's flattening can put a numerical constraint on molar mass segregation, telling that convection is efficient - the planet's aspect speaks in favour. ----- Could an analytic nearly-solution be just little more complicated than the model with an equatorial ring? Like: the altitude of equal potential differs from a sphere by thatmuch*cosine(2*latitude), and if really needed by thatlittle*cosine(6*latitude)? I may have read somewhere that the field is then a Bessel function. For sure, it has already been done. Less usable for a simpler book.

That mere heat separates H2 from O2 is perfectly true. Solar heat is enough for it, and our best materials would suffice. It's so true that you'll advance human technology by proposing a solution to the separation of H2 and O2. The question you ask was already around... Sorry for the disappointment. In the Pdf you link, they use two steps. Mg decomposes H2O, producing H2 there; and Sunlight heat decomposes MgO, producing O2 there. Elegant, probably workable. Some energy is lost at the Mg+H2O reaction, leaving theoretical room for improvement. By the way, I'm not sure we shall produce hydrogen first. It's nice for fuel cells but nasty for everything else. If you can produce a hydrocarbon, or ethanol, or something similar, you'll find a market sooner.

It would be interesting if you could determine through its bulge if Jupiter has a big dense kernel. Its bulge is huge and well documented. No liquid nor solid surface has been observed but the upper atmosphere's properties are rather well known. As you developed at the linked site, mass distributed at the outer shells (upper atmosphere there) creates a higher bulge than mass concentrated in a dense kernel. Jupiter has a low average density, so a rock or metal kernel would be small as compared with the atmosphere's radius. What is not known (or wasn't): - If such a kernel exists (astronomers bet on "yes" presently), if it's metallic hydrogen, rock, metal... - What the atmospheric density and temperature profile is (the big red spot is colder) - What kind of gas composes the deeper atmosphere (molar mass depends on altitude on Earth) - Where the very strong magnetic field (1T) originates. People desire a metal kernel for that. - Where the excess heat originates. Jupiter radiates seriously more infrared than it gets Sunlight. Well, I can't tell you much more... Maybe someone has already investigated that. I'm a dilettante for astronomy as well, and astronomy improves quickly. Good luck!

Faraday effect wouldn't need an experiment nowadays, at its name implies... In vacuum of course. But I didn't find again the address quickly.

The beams at LHC contain enough energy, power, impulse, and are concentrated enough, that they bore a hole in the metal tube if steered badly. Targets intended to absorb such a beam are difficult to design because of heating, among others. By the way, electron beams are an industrial means to weld alloys, and these machines don't have the LHC's power.

Oops, exactly.

Reprocessing plants for nuclear fuel, like La Hague, emit also tritium, which they dilute in the Ocean. Here's data (for 1999 and in French, sorry, but figures read the same in English): http://www.irsn.fr/EN/Research/publications-documentation/radionuclides-sheets/Documents/Tritium_H3_v1.pdf The amounts are small, and I couldn't consistently estimate the risks for humans; it seems that significant risks would only result from a non-uniform dilution, for instance if the food chain absorbs tritium before it's fully diluted - and that's hard to model. ---------------------- Anyway, if tritium had to be stored, it looks rather easy. Fine powder of dessicated alumina is sold like plaster, to be mixed with water and moulded in form by the user to make ceramic objects - I suppose the reaction produces Al2O3°3H2O, maybe with a hydroxide fraction. That's a stable, inert compound. Make dilute superheavy water with the tritium. Let this water react with the dessicated alumina powder to obtain a ceramic, maybe the size of a tennis ball. Then, use normal water to add a 20mm cap of hydrated alumina around the ball, to absorb any Bremsstrahlung from the weak betas. La Hague emitted 1.3*1016 Bq/yr of tritium in 1999; over 17.7yr this cumulates only 640g of pure tritium. If ten Al2O3 incorporate one tritium atom (for mechanical stability and against proliferation) it takes 340kg of ceramic balls which cumulate only 210W (5.7keV mean electron energy), emit no betas and very little gammas absorbed by shallow water.

Depending on the temperature and the desired insulation... A few rags; Polyurethane foam, fitting the shape; Glass wool, with a paper back at the low temperature; A Dewar (usually not needed!) Multi Layer Insulation in a Dewar (even less necessary) =============================================================== One funny idea with heat exchangers is that only technology limits their size. That is, smaller exchange elements (tubes...) put the same area in a smaller volume, and decrease the distance heat crosses: more effective. The pressure drop would increase with smaller elements, but you can make the individual flows shorter and connect them all in parallel, keeping the flow as easy. This is what a lung does for instance: veins divide in smaller and smaller ones, down to short veinules and arterioles, which converge to arteries back. One limit is the cleanliness of your fluids. Easier in a closed circuit! The other limit is how you manufacture the exchanger. There I describe a tiny exchanger made of thin electrolytic of catalytic nickel deposited on a tiny frame later dissolved: http://saposjoint.ne...6&t=2051#p23419 interestingly, a research team in California recently used a similar process to make a "microlattice" http://www.sciencefo...-microlattices/ Puzzling.

Here an elementary example of a Krypton-85 storage, to give a general sense of its difficulty. I didn't and won't check how to catch 85Kr from the fuel rods; boiling nitrogen? A tube of ID=0.17m and L=6m stores the 85Kr extracted during one mean week at La Hague, at absolute 0.95b if it were to reach +100°C. Bent in a U shape and immersed, it holds the gas even if a seal leaks. Aluminium AA5083 is one interesting choice. 20mm walls and two thick welded plugs let it sink head up. Betas of mean 251keV produce 223W directly in the wall, very easy to cool. The dose at the walls is several magnitudes below a fission reactor, and betas damage alloys little. 85Kr produces a 514keV gamma in 0.43% of the disintegrations. Bremsstrahlung in aluminium adds little. 20mm walls shield the gammas by a factor >60, so a human standing along the tube in air would get 0.1Sv in >20min. The produced Rubidium-85 melts readily and the liquid might dissolve aluminium. Water in the tube would catch Rb but the alkali corrodes aluminium. A mild acid instead would still produce hydrogen, not so nice. I imagine a metal with varied valences solves that, like Iron in FePO4 becoming RbFePO4, or Prussian blue, or potassium ferricyanide - ask a chemist. Vacuum doesn't buckle the tube. A pool of 12m*12m would store the 85Kr extracted during 15.4yr; this is enough if older, emptier tubes are re-filled with newer 85Kr, but I'd prefer a bigger pool and fewer operations. Some 3m water shield the gammas and cool the 180kW by natural convection of few dm3/s. An exchanger at one side suffices, easing crane operations. A direct exchanger with some 15m3/s air would be challenging, but heat pipes between the pool and the blown fins make it easy. Marc Schaefer, aka Enthalpy

When opening the fuel rod, you wouldn't first introduce atmospheric air in the gas going to be stored. Sure.

I never told about trapping 85Kr from the atmosphere or separating it. 85Kr does get caught when the spent fuel claddings are opened, because releasing it at the plant would be deadly. It's all about storing the caught 85Kr, instead of diluting it for release. This number of moles and volume is a good estimation, and storing such a volume is easy, yes. You're perfectly right. As you all like to compare lives and costs, saving three dozen people at the price of a few bottles in a pool is very cheap. -------------------------- People who answer "I better spend my money to feed the hungry and vaccinate the poor" usually don't do it neither. -------------------------- Of course, other radioactivity sources are more intense than dilute 85Kr. But 85Kr does add its activity and its deaths. You're telling basically "let's kill 36 people because they can't prove we did it". I won't follow you there.

So little deluded that this method is used. Even before half a metre beyond surface, you get a temperature constant all the year, the reason why water pipes are just buried here in tempered Europe to protect them against icing winter. Complete houses get cool air in Summer by this method, and, err, cool air as well in Winter, as the mean temperature is like +15°C here, so a supplement of heating is needed in Winter. Alternately, some people live in caves to benefit from a constant temperature. For your network of pipes, which is compounded with a heat pump in Germany or Switzerland, you need enough soil mass to store heat (and cold) for a season, and pipes close enough to draw that heat (or cold) within a season, but the depth isn't critical if soil is used as a heat accumulator. The pipes can be horizontal (adios for some time, nice garden) or vertical (bore holes instead of digging trenches). The situation would be very different if you wanted to exploit the geothermal temperature gradient for good. This would require hundreds of metres just to get a few degrees more. A few places are exceptions, in Iceland for instance. In the Rhine valley, you'd get an important gradient, like 300°C within 3000m. Hard to exploit for a home. But to keep a greenhouse well over freezing temperature, that's the perfect use. Give your tubes a U-form, or have concentric tubes, to circulate air in them. They'll also provide fresh air in Summer, to your greenhouse or outside as you wish.

Materials with varied density compose the Earth, for instance lighter oxides float over denser metals, and lighter continents float higher than denser Oceanic floor. Beyond this equilibrium, Earth dynamics, especially the inner heat, create movements and situations outside equilibrium, like mountains.

Slightly different approach: transformation of calcium carbonate into lime and later concrete evolves big amounts of carbon dioxide in the atmosphere. The heat source could be Sunlight instead of a hydrocarbon fire, but CO2 from the carbonate is more difficult to solve. A solution for that would be highly welcome, be it as gas sequestration in geological reservoirs, or by transformation of silicates, or any sensible means.

Not to my knowledge. Not under normal engineering conditions. But an experiment was (is?) made in Italy trying to detect a similar effect, where a magnetic field would rotate the polarization of light. Did this imply the creation-annihilation of a pair of exotic particles on the trip? I don't remember. And I wonder... Imagine for instance a photon with just under 2*511keV passing by a heavy nucleus: it must create a virtual electron-positron pair that lasts for some time, and this pair is influenced by the nucleus, especially by the gradient of the electric field. Would this deviate the re-created photon? Relativistic effects of huge fields? A magnetic or electric field is energy so maybe it deviates light as mass does. One person at Physforum could answer that.

Relativity makes the assumption that information travels at most as fast as light does in vacuum. Then it can explain why Lorenz' transformations are meaningful. That's why neutrinos faster than light is a serious worry if confirmed. They're produced at will, and detected... well, some are detected. So they are a means to transport information. Other experiments with a wave guide used below its cut-off frequency, hence carrying a fading wave whose phase speed is infinite, claim to have transmitted a modulation, hence information, without delay over a limited distance. The limit must be more subtle, like: the amount of data arriving too early is limited, increasing it slightly requires impractical power losses. Just like any computer modem violates the energy-time inequality by transmitting more than pi bits per second per Hz of bandwidth, but does so by using a big signal-to-noise ratio, and hits a hard limit quickly. Intricated particles share a synchronized behaviour within a delay smaller than light needs to travel between them, but acting on one particle doesn't influence the other, so it's not a means to transmit information. Gravity waves should propagate at light speed... This is consistent with the energy and momentum loss of an observed pulsar pair. But gravity waves escape our detectors up to now, and this might indicate a flaw in present understanding. I haven't seen anything about the propagation speed of the weak nor strong forces. Present physicists would naturally assume "c at most". As these forces are observed over tiny distances, any argument about their speed must be indirect.

Nice ! Filmed for Bbc... http://www.bbc.co.uk/nature/15835014 (needs Flash Player 11) http://www.youtube.com/watch?v=LMhBuSBemRk (same without Fp11)