Enthalpy

I suggest to compare the masses of neutral atoms of hydrogen and helium, without detailing the reactions in between, since this is the net effect of fusion. The only imprecision is between neutral atoms and plasma, approximately 4*13,6eV versus 24,6+54,4eV: only 24,6eV as compared to MeV, and the kinetic energy of the species, 1keV at 10MK. 4 neutral hydrogen atoms of 1.00795u become 1 helium of 4.0026u, releasing 0.029u of 1.66e-27kg, that's 4.36e-12J per helium atom. Agreed. 3.9e26W need more than (because neutrinos evacuate a part) 1.5e14mol/s helium creation. That's 1.9e28kg/Gyr (billion years). All from Wiki, the core weighing 34% of our Sun's 2.0e30kg being composed of 60% He instead of initial 27.4% has created 2.2e29kg of helium. This would have needed 12Gyr, not 4.6Gyr, agreed as well. ---------- Explanation attempts: Neutrinos evacuate a significant portion of the fusion energy? 60% He is a proportion at the center, not a mean value over the core? The 60% and 27% at Wiki are wrong or undetailed? Nobody measured them. I suppose their difference results from computations like yours, just more differentiated over the depth.

Are all alternatives to new particles abandoned to explain dark matter? I haven't followed this recently. Two decades ago planet- and star-sized objects were ruled out by the low frequency of microgravitational lensing. But objects of different mass? All we need is that galactic dark matter can float where normal matter isn't after a galactic collision, and that it's little observable.

In the case of an electrostatic field, the divergence is the density of charge (with permittivity and signs), so it's zero without charges and nonzero with charges. In both cases, the electrostatic field has zero curl. So curl and divergence are independent. Outside electrostatics, a varying induction creates a curl in the electric field. This creates our electricity in generators at the power plant, where the rotating magnetic field induces a voltage in a closed loop (the electric circuit), hence with curl.

For the vacuum vessel of a hydrogen tank, cast shell elements make naturally a single wall. I prefer sandwich double-walled shell elements, as they resist shocks and deformations far better and offer redundant airtightness. Diffusion bonding permits it; not as cheap as casting, but aeroplanes afford it. Among other providers, a doc with nice examples, sandwiches on page 9 and 10: http://www.formtech.de/download/formtech-short-presentation.pdf and here is how the vacuum vessel may look like: The external skin with stiffeners in several direction, for instance as an isogrid, is cast or rather machined. It can provide the smoothly curved outside aerodynamic shape, while its inner face is machined flat for bonding. The inner skin is a sheet here, though ribs are possible. Diffusion bonding makes a sandwich shell element of both, with manageable size and shape. Then the elements are assembled by Tig, Mig or other welding to make a shell. Milled keys bring accurate positions when welding. Alternately, two thin skins and a (bidirectionally?) corrugated spacer could be superplastically formed before diffusion bonding, if the shells are accurate enough for diffusion. Seemingly, magnesium alloys still don't resist deep bumps. This would leave aluminium (AA5083) and titanium (little alloyed). Cylindrical tanks at rockets can keep my cheaper extruded profiles. Marc Schaefer, aka Enthalpy

Hello nice people! Some older or disabled persons can't reach the ground but would like to continue gardening. I imagine we could put at the proper height smaller plants (redcurrant, strawberry, raspberry, tomato, blueberry...), example: Have you already seen a similar item for sale? I think at it for my old mum. If it doesn't exist yet, someone should produce it. The soil would better be around hip height, so dumping it right on the ground would need an excessive amount. I'd like a mobile item to adapt the sunlight exposure to the plant, to move it when mowing the lawn, and to get rid of it without damage if the attempt ceases. The capacity of a wheelbarrow seems a good start, though narrower and deeper. Details: The materials must resist sunlight (beware plastics), stagnant water, plant acidity. The container should have drains, for instance at the stiffeners. Its bottom can be V-shaped. The item should permit to assemble tight rows. The container could accommodate poles that hold a net against birds. The frame should stop slugs and mice if feasible. Several unassembled items should fit in a car's trunk. Marc Schaefer, aka Enthalpy

It goes without saying, but maybe better if I say it: concentrated sunlight, split over wavelength bands, has uses not only at Europa. Here electricity production for a geosynchronous satellite: As usual, the satellite's body revolve once a day around the North-South axis to face the Earth, and at its North and South faces, some parts revolve once a year to face the Sun - but here the solar cells can turn with the body. It needs no slipring to transmit the electricity, only a Nasmyth path or similar. Solar cells of varied bandgap can be separated. No difficult new technology, and we can have more different materials. That's much cheaper and efficient. They get their best band through filters. The smaller cell area saves costly semiconductor and work. Concentrated light improves the efficiency further. Heat stays bearable thanks to the good conversion. Fluid cooling is an option. The unconverted light, typically mid-infrared, may power a turbine, or be dumped. The concentrators should be lighter than solar panels of same area, and more so of same power. At 50% efficiency, two D=4.57m concentrators (to fit flat in the fairing) provide 22kW electricity. Marc Schaefer, aka Enthalpy

Hi Konstantin95, no worry with your English, I'm happy that we share a language. But are discussions about explosives manufacture permitted on ScienceForums?

Is the Lightest Supersymmetric Particle a candidate as dark matter within the galaxies as well? http://en.wikipedia.org/wiki/Lightest_Supersymmetric_Particle suggests a rest mass like 1eV, so if such light particles have an average energy of 3K=260µeV, their mean speed is 0.023*c or 6.8Mm/s, well beyond the galaxy's escape speed. So, if I get it properly: - Either they are colder than 3K - Or they can make the dark matter at bigger scales only, not the dark matter needed in galaxies - Or "something" holds them within galaxies, so they aren't very dark. Maybe they make heavy clumps among themselves but interact little with normal matter?

If some particles are fundamental - which the electron seems to be in every experiment up to now - then they are not explained by constituents. This is equivalent to say that they are (or bear, who cares) a set of properties, among others the charge. Mass and charge are independent up to the self-energy of the electric field, which means a minimum rest mass for a charged particle. Some theories tried to reduce the electron's rest mass to this electric self-energy, I believe they're abandoned.

The composition of extrasolar planets is unknown very accurately. A handful of teams claim to have observed atmospheres and even a composition, but this relies on a few received photons, and to my knowledge, has not been observed by two independent teams. So while we may have this knowledge in the future, presently it's more hype and buzz for popular science and the general press.

Dark matter candidates would better be heavy enough to stay trapped in galactic gravitation wells, since we need at least some dark matter concentrated there. Ah? Not necessary. Neutrinos are already difficult enough to catch and common models hosts them in three big spatial dimensions.

Several observers or several sources of sound that the target reflects. This is done to know the position of a submarine from the reflection of the noise created at the Ocean's surface. The hunter measures the surface noise over many directions and correlates it with the directions at depth; reflections, if any, reveal the target. The signal processing includes Doppler measure and compensation, and this indicates the target's 3D speed in addition to its position. You guessed:submarines are built to reflect as little noise as possible, and they hide in terrains that reflect noise, preferably behind mountains.

It wouldn't worry me if the new photon pair had a momentum different from the original photon, because the gravitational field introduces one more actor that interacts with the already mentioned particles. The mass that creates the field would equal the momentum difference out. This in addition to the nucleus that provides the electric field, usually - which would also absorb momentum. Some experiments achieve to produce pairs from two photons in vacuum, far from any nucleus, in rare events.

If an engine flows the oxygen instead of the fuel in the jacket to cool the engine wall, for instance if this oxygen rotates the turbine, then the engine can burn ethylene or other fuels unsuited for high temperature. Ethylene is flammable (detonation speed between methane and acetylene) but it brings performance second to cubane only. Here a comparison at the pressures estimated on 30 March, 2014 for a first and a second stage. Marc Schaefer, aka Enthalpy

Trying to estimate a minimum size that brakes little through the atmosphere: The hypersonic drag is less than density*V2 The path is longer than a vertical fall Both shall compensate an other, ah. Then I can compare directly the kg/m2 of the object and the atmosphere, to deduce if much air mass brakes the object mass. The atmosphere weighs ~100,000Pa or 10,000kg/m2, so an object with density 5,000kg/m3 must be 2m long to punch unbraked through the atmosphere. This favours your doubt a lot. A 2m rock would have made a bigger crater. That's only a rough estimation though. ----- Now, if the object does brake before the impact: I take the opposite situation, where its speed results from aerodynamic fall only. 5,000kg/m3 * pi*D3/6 * 10m/s2 = 0.3 * 1kg/m3 * pi*D2/4 * V2/2 200,000*D = V2 still 150m/s for 0.1m, 260m/s for 0.3m, 470m/s for 1m Somewhere between the two last figures would fit the crater size, intuitively. Meteorites use to melt a few surface mm.

Most research papers (still a bit early for applications, but coming) deal about electrical systems. More precisely, electro-optical. Generally to produce light from electricity. So the query is still vague.

I doubt very much about any link with 2014RC. Or at least, about Earth's gravity splitting a meteorite from 2014RC. This would have happened near Earth, so the fragment wouldn't have already deviated enough when passing by Earth. To the very least, it demands an other planet to split 2014RC much before. In addition, I can't imagine a single rock detaching from 2014RC; much more, tidal forces disintegrate small celestial bodies, as was seen near Jupiter. Was it a meteorite impact? No trail was observed, but maybe the impact was vertical. I considered a hypervelocity weapon, since a standard explosive shell making such a crater would have blown away much more vegetals at the surface. Though, witnesses report the fall of a liquid together with dust - the soil doesn't look like the source of this liquid, which would then have fallen with the object. What liquid? A celestial body can bring ice - I expected that only farther from our Sun. A satellite carries propellants, which are usually nitrogen tetroxide and some hydrazine; both stink, and I have not read witnesses telling it. A hypervelocity weapon may carry some rests of propellants, more if the launch failed, and usually as stinky as onboard a satellite. If a satellite had fallen, I'd expect some more parts to be discovered over time around the impact. This resembles the impact in an Andine country few years ago, but there the inhabitants reported the bad smell and cattle got sick.

The energy of an electron on the 2s orbital of ground-state lithium and excited hydrogen match imprecisely: http://www.webelements.com/hydrogen/atoms.html 1312kJ/mol for the 1s electron in hydrogen, hence 1312/22=328kJ/mol as a 2s electron. http://www.webelements.com/lithium/atoms.html Compare with 520kJ/mol for lithium. These are ionization energies, that is potential energy minus kinetic one, but the kinetic energy compensates half the potential, so you can compare the ionization energy just as well. The energy difference tells that our representation of heavier atoms is an approximation. A lithium nucleus plus two 1s electrons would behave as a hydrogen nucleus if experienced from distance, but lithium's 2s electron is close to the nucleus and the 1s electrons, so all electrons interact and find a common behaviour that is more favourable, hence the bigger 520kJ/mol.

May I suggest specifically http://en.wikipedia.org/wiki/Quantum_dot

To prototype a vacuum-insulated hydrogen tank as described here on 14 April 2013, one doesn't have to pay the special aluminium profile right from the beginning. Existing aluminium profile is heavier but demonstrates the balloon in an evacuated shell. One example of existing profile, among many from varied suppliers: http://www.bso-gmbh.de/pdf/produkte/alu-profil-nut-8-40x160-4n-leicht.pdf its flat face is easy to weld and 160mm width limits the weld work. ---------- The production shell, sphere-like or more ellipsoidal, can also consist of cast elements, for instance two shell parts with seals and bolts, or 20 triangles welded as an icosahedron, possibly subdivided in smaller triangles, and the many variants http://en.wikipedia.org/wiki/Icosahedron http://en.wikipedia.org/wiki/Pentakis_dodecahedron http://en.wikipedia.org/wiki/Truncated_icosahedron cast elements commonly have integral stiffeners. Marc Schaefer, aka Enthalpy

-------- Descent and ascent -------- Each prospector leaves the ferry at 1000km altitude, descends to its location on the Moon, collects samples, and joins the ferry at 1000km with its samples. With 6m/s2 acceleration, 2° orbit correction (one window every third month) and 50m/s navigation, the ascent costs 2215m/s. 3 landing trials take 2 hops to 200m distance and three hover periods of 20s, or 160m/s together: the descent costs 2375m/s. The ferry won't see the prospectors as they land. On the Moon's near side, the Deep Space Network antennas can receive some bad video for human decision. At the far side, the ferry at 100km altitude would have seen the landing prospectors but not the Earth. So: automatic landing and take-off, after training on the near side. ---------- This landing script needs simple behaviour and sensors yet brings resilience: The prospector extends its legs (A1) and fires its engine (A2) to join a landing site. Descending at moderate pace at the landing site, it touches with a first foot (B1) which optionally climbs during the brake (B2) to zero speed. Each other foot descends until it touches the ground (B3), in which case the engine stops. If the ground is too far below a foot, the prospector hops ( C) to 200m distance and resumes landing (D). This leg movement shall happen within 20s hover. 6kWe are available at that time. Tiny hydraulic dampers could save a hard landing at 2.5m/s. I imagine prospectors walking on 6 landing legs, though wheels are possible. Vertically rotating hips and knees extend the legs easily, make a smooth walk together with the feet's vertical moves, and fit well a truss body. This is not limited to our Moon. ---------- The 3.3kN engine burns diethyl-trimethyl-dipropylene-triamine (mp -100°C hopefully) http://www.chemicalforums.com/index.php?topic=56069.msg272080#msg272080 or commercial pentamethyl-dipropylene-triamine (mp-78°C), with liquid oxygen cooled actively http://saposjoint.net/Forum/viewtopic.php?f=66&t=2051 stored in a superinsulated balloon tank hold by polymer straps http://www.scienceforums.net/topic/60359-extruded-rocket-structure/page-2#entry761740 It has 11kWe electric screw pumps for 100bar in the chamber http://www.scienceforums.net/topic/73571-rocket-engine-with-electric-pumps/#entry734835 (and nearby messages) Expansion in D=0.8m to 135Pa brings Isp=3989m/s=407s. The 12.4kg Li-poly battery is recharged before the ascent and outperforms any pressure-fed solution. It's very useful for the prospector's operation and nighttime hibernation. Pumped Mon-33 would bring 1/3 less samples than oxygen and is toxic. Strained fuels like spiropentane would add only 5% samples, methane less. Hydrogen would have brought 1/2 samples more; a fuel cell wouldn't improve over batteries, except for nighttime if the water is electrolyzed during daytime, hum. ---------- Starting from orbit at arbitrary 1000kg for later scaling, each prospector lands with 551kg. More details should come. Marc Schaefer, aka Enthalpy

This is not the case. There is no simple relationship between rugosity and the coefficient of friction, and some plastics are known to glide more easily against a steel surface that isn't too smooth, and also to wear less then. Steel and chromium surfaces can be made very smooth: 0.3µm is moderately smooth for steel, and chromium at hydraulic cylinders is hugely better. They shine because they're optically smooth, simply. ---------- Friction isn't understood - or uncompletely, as a nice wording - despite a good theory would be extremely useful. Friction shouldn't be modelled as climbing hills of rugosity: this old attempt (Newton?) fails to explain low coefficients of friction unrelated with steep slopes. To the very least, such attempts must consider that elastic materials push forward at the places they descend slopes, thus recovering much of the force used to climb the hills. Some theories have partial success, but they include so many effects with so many tinkering coefficients that they would explain wrong curves as easily as good ones. Anyway, they can only interpolate curves between many measured points, and a graphical interpolation would do it about as well. A serious difficulty for any theory is that friction depends on surfaces, whose composition can't be inferred from the base material. For instance graphite glides in our atmosphere but rubs in vacuum. Many surfaces meant to glide are so smooth that minimal amounts of allogen materials fill the crevaces.

Field lines are just a graphical help for us to imagine a field. We can use more, we can use less of them. So clearly electrons won't care about. Then you can have electric field lines through an insulator, where electrons often don't flow. Or through vacuum, where electrons must first be brought, and then the inital speed may determine more the electrons' path. An added magnetic field influences electrons as well. Or gravity, but usually little. One case where electrons would flow along the direction of the electric field is a conductor or resistor. But an isotropic one: in single-crystal graphite you could have the electric field in one direction and the current shifted to the direction of easier flow.

And would the modified Nikon D50 DSLR detect thermal infrared? That's quite a special job! The detectors for 5µm or 10µm use to be extremely expensive, of low resolution, and chilled. Not the same as a silicon sensor with a different filter, which stops around 1.1µm.

Hi Kimalinoa, welcome here! 3) I haven't seen big applications of nitrocelulose for decades. It's replaced by present-day polymers as much as possible. I burned some when it was available, and the result explains easily why manufacturers avoid it. 2) One standard mechanism is that the liquid absorbs heat to evaporate. Enough liquid can prevent the dangerous substance reaching its ignition temperature, pretty much like water extinguishes fire. There may be other mechanisms, specific to each substance, which I ignore in this particular case. 1) I have no desire to spread such information on the Internet. There are already enough explosives around. Just use a safe polymer instead, as presently nitrocellulose offers very few attractive properties. If you tell what properties you seek, we may help you choose a better polymer. 0) Some people would use the answers you seek to find products that can be misused as an improvised explosive.