Enthalpy

Big objects conduct heat badly. Imagine that their power, or the heat they content, increases as R3. Their heat conductance increases as the area but decreases as the length, that is R2/R1=R. That's slower than R3, so the temperature difference is bigger. The same holds for Earth. Its whole internal heat source is as tiny as the whose energy used by Mankind, and though the core is hot. Earth would take a time in billion years to cool down if it had no internal heat source - so that whether the heat source exists was long unclear. That was for conductance. Radiation can help big objects, but it doesn't so much at the Sun, because a star consists of plasma (ionized gas) which absorbs and emits light instead of letting it through.

Transformation between two orbitals is just one possibility for an electron to radiate.

Copper sulphate is used in multi-kilogram amounts on trees. I wouldn't worry too much. About the acid neither.

They tell about the charge per mass unit. Without the voltage, any comparison with lithium is (intentionally) biassed. There is no platinum in lithium batteries. In some fuel cells.

What about an experiment kit? It doesn't suffice to become a chemist, but (1) it's fun (2) it gives some practical sense (3) you can build book knowledge over that.

I don't understand "particle" as "point". We need particles to remember that some quantities appear, disappear, are exchanged by fixed amounts. For instance electricity comes in multiples of q. Or angular momentum is exchanged in multiples of h/2pi. Or light energy appears and disappears in multiples of hF. Then we say "the photon was absorbed" or "the electron has passed", and the idea of a particle is useful. Even the absorption of a photon does not need to be very local. If a semiconductor absorbs a photon, a quite de-localized valence electron becomes a quite de-localized conduction electron, and the absorption occurs over a wide area. Here the photon is a particle, in that its hF energy suffices to give the electron the necessary energy - and this happens if the frequency suffices even if the power density of light is too small: it just happens less often, hence the useful idea of a light quantum. I wrote "a volume as small as humans can do with present technology" because, as far as I know, we have no means to observe down to a point. Since we measure a particle using an other one, which itself has some volume, I understand that a "point-like" particle can be as small as the interaction needs.

There is no strict need to ignite lighter petrol distillates with a spark and heavier ones by air compression, at least when designing a new engine, but it helps a lot. Once the engine is designed, there is little choice. Heavier distillates like Diesel oil have a lower autoignition temperature, which means that compressed air ignites them more easily. Gasoline would demand a higher temperature, hence a stronger compression, as Danijel wrote. For gasoline, a spark is easier. Why not always a spark? Because Diesel oil already injected in the cylinder would self-ignite during the compression. A purposely-designed engine would have a very low compression ratio and efficiency. So the stronger compression and efficiency - with the costs associated with oil pumps and sturdy engines - fits the Diesel ignition scheme and the heavier distillates, and the smaller compression with spark ignition fits gasoline. Traditionally, Diesel engines were also less responsive and heavier. Combined with higher cost but efficiency, they equipped lorries and taxis. This situation evolved 30+ years ago in Europe where improved Diesel engines equip small cars. This was with a Citroën Cx bought in 1987. A very nice turbo-Diesel, but of older design, with a pre-chamber where the mix keeps much fuel proportion even at idle. Rapeseed oil destroys a modern Diesel engine within days. Zero modification to the car. I put 1/3 rapeseed oil in 2/3 Diesel oil one after the other in the tank. Car like older Mercedes with a stronger Bosch in-line injection pump run with pure rapeseed oil; mine can't cope with the higher viscosity. Maybe kerosene or gasoline would allow more rapeseed oil. No worry in winter; I had -15°C there and the engine started smoothly. I've had no worry with the fuel filter neither, despite the car had run 240Mm on Diesel oil before, then 110Mm with the mix. An unmodified older Mercedes ran >450Mm on pure rapeseed oil. The seals knowingly resisted rapeseed oil, while biodiesel needs adequate ones. I've observed the same consumption by volume, which means little more by mass - only 6L/100km at 160km/h on montainous highways, for an old big car, excellent design. The exhaust gas smells distinctly; one sees less fumes (allegedly because vegetable oil brings oxygen atoms in the molecule, more possibly because it contains no carbon rings), and the Mercedes driver has observed perfectly clean cylinders. The noise got quieter. All clac-clac by the old engine disappeared with 1/3 rapeseed oil, sounding more blob-blob. Possibly the higher viscosity dampens some movements, say at the rings - only a hypothesis. For sure, the engine ran cooler; I noticed it as the cooler was nearly destroyed, and the oil has 20°C less than with pure Diesel oil. Some excellent websites existed in German, where users shared their experience; search for Pflanzenoel Diesel. If one wishes to try: your car, your risk. I tried with mine, that's enough. Do check at least the pre-chamber (needs pre-heating before start) and the injection pump.

We see shooting stars linked with comets, so they also release some coarse dust, which Rosetta's operators suspect can harm a probe. After some time, dust's speed must differ from the main body, due to Sunlight pressure, hence have a speed relative to the probe. Ice sublimation by Solar heat releases vapour, and lost ice releases dust... Maybe UV light can evaporate water molecules directly, without heating ice first. All these objects must be rather loose aggregates, especially the smaller ones. They're supposed to re-arrange from time to time, and this must release more gas and dust. Holmes 17P may be one such case.

These were open questions decades ago, as the possibility remained that the Universe were finite and small enough. Presently it's considered probably flat and definitely bigger than our horizon. One suggestion: have more recent books, this saves time. You don't have to learn and de-learn abandoned theories.

A photon is much like other particles in that the wave appears and disappears in small packets, that some quantities are conserved (charge if any, angular momentum and more). Because of that, we like to count in a unit called particle. At the interaction (the absorption if applicable), the wave can reduce ("collapse") its diversity. It can be very broad, say many light-years if coming from a star, but be absorbed (with the correspondingly tiny probability) at a small place, say one camera pixel at a telescope. This does not need the particle to be a point before its position gets better known. I prefer to say (as others do) that the photon is as big as the wave. Some particles (photon, electron and more) can be detected in a volume as small as humans can do with present technology, and still carry all their properties; then we can call them "point-like". Take a two-slit experiment: The photon is broad before the screen. There it can be absorbed or not; if it passes through, it has the shape of the slits just downstream the screen. Further, the interference gives it the shape of lines - but at a camera, it takes the size of one pixel if it's detected. I do not feel the need to say "the photon can pass through either slit", because I don't imagine a point photon; I say instead "it passes through both". Similarly, a valence electron has the size of an atom - but more accurate tools can detect it in a smaller volume. Other properties, not only the size or position, can have a big diversity but collapse when the particle interaction needs it. For instance the polarization of a photon, an electron (the magnetic momentum then), and more. Some properties can vary continuously for a free particle, others (like the angular and magnetic momentum) have a finite set of possible values.

If the final state isn't quantized, that is for an ionization, light need only a minimum frequency... Easy, and not your question I suppose. Even if the initial and final states are quantized, there is some frequency tolerance. To my understanding, it means that the photon absorption must be quick if the mismatch is big - or reversed, the photon has less time to be absorbed hence has less chances. This can be seen on gasses. A higher gas density reduces the mean time between molecules' shocks; then the light absorption line gets wider, that is more tolerant to frequency mismatch. Though, I believe absorption gets less efficient then; the center (=frequency accurate) of the absorption peak only gets cut away by the too short time, and the less efficient sides (frequency mismatched) remain. As an extreme case, liquids have much broader absorption lines; and this width in Hz due to the mean time between collisions is the same for all frequencies, so that no resonance (=small relative width) is observed at radio frequencies. For instance the 2.45GHz resonance is sharp in low-pressure vapour but flat in liquid water. An other contribution for line width is that the excited state isn't eternal. In addition to shocks among molecules, the excited states returns to fundamental in a finite time, which can be shorter or longer than the free flight. Absorption couldn't take much longer than the de-excitation, so the absorption peak is limited in strength and narrowness. Still an other contribution is the Döppler effect. The absorbing object (molecule...) has a speed which de-tunes its resonance versus the light source, so if one object doesn't absorb a wavelength, an other may. These three contributions (did I forget some?) compete, often with similar importance.

Hi Hannes, willkommen here! OK for the maximum stress and propagation speed - more subtleties later. The stress propagates backwards in the rod at sound speed; before the wave front passes (hence further from the impact) the material isn't stressed but keeps its speed; after the front (nearer to the impact) it is immobile, compressed and stays compressed. Once the front reaches the far end of the rod (at this time the whole rod is immobile and compressed), the compression converts into speed, and a new front propagates forward: before the second front the matter is immobile and compressed, after the front the matter has backward speed and no stress. One the second front reaches the impact point, all matter has backward speed and no stress; the rod just detaches from the target and continues bouncing back. Subtleties: A target much stiffer than the rod is uncommon. More usually in solids, as in acoustics, the constrast between materials is small, so the impact spreads its effect into both objects. The ratio of acoustic impedances determines much IF both waves are (were) plane. A flat shock is quite difficult to achieve. But a spherical contact, computed with Hertz's (static) formula, and the metal's static yield strength, gave me good results. Young's modulus E isn't always enough - as in acoustics. If the objects are wide and the shock has fast time components, then matter has no time to move sideways. Though, E supposes a free side move. When this side move isn't possible at all, one should replace E with E/(1-2µ2) where µ is Poisson's ratio, as in acoustics [or maybe E/(1-2µ2/(1+µ)), please check]. This means also that short compression shocks and sounds propagate a bit faster than long ones, depending on the rod's diameter. Most metals and ceramics have E constant over a very wide frequency range, at least many 100MHz. Other materials vital to shock engineering, especially elastomers, depend fundamentally (E varies by 100 or *1000) on the frequency (coupled strongy with the temperature). This is a consequence of the desired damping. Check viscoelasticity, glass transition temperature. Data for elastomers is scarce and difficult to use (big deformations make E meaningless), so most designs are experimental. A rod is the simple case. Parts use to have complicated shapes, and in metals and ceramics, all echoes, impedance mismatches and the like apply. For instance, I had an aluminium part hit frontally at 30m/s a massive steel target. A small ring was milled (single part) at the rear. The front kept intact as expected, but the concentrated wave at the narrower rear broke the ring and ejected the bits backwards. Shocks and acoustics aren't very developed theories. Since waves and their propagation in shapes, materials, impedances... are essentially the same as an electric signal in a stripline, and theory is much more advanced there, radiocomm engineering is a better formation for shock and acoustics engineering; just pick their books.

In planar epitaxial transistors, the base current is essentially injected by the base into the emitter. The collector current is injected by the emitter in the base but sucked by the collector and doesn't exit at the base. Because these two currents depend on the base-emitter voltage roughly in the same way, they are more or less proportional to an other. So the base current sets a base-emitter voltage which defines a collector current. In bipolars of allied germanium technology, recombination in the base was the main contribution to base current. This isn't the case with planar epitaxial transistors. Many books are outdated. Many bipolar have an emitter/base heterojunction now, which reduces the injection in the emitter. I'm not sure this injection is still the main contribution for their base current. For planar epitaxial transistors, the gradient of doping in the base determines the speed of the carriers injected there, much more than thermal diffusion does. With a heterojunction, or if a thin oxide separates the polysilicon emitter from the base, the carriers are shot in (if possible through) the base with a high initial speed. Things may have changed further with gallium nitride.

Putting a few figures should help to imagine how the attempt looks like. Satellites and debris move at 8km/s on a low Earth orbit - a rifle bullet can have 800m/s; at such speeds, the matter's deformation properties have no importance. Their orbit inclination differ, so no two satellites would have a moderate relative speed. Any material needing a small relative speed to the debris must be in orbit itself, which demands a complicated launcher. Satellites are less strong than a wardrobe, for most of their parts. Any significant shock or deceleration breaks them. De-orbiting a satellite already very low, when acting at one end of the Pacific to target the other end, takes roughly 100m/s. If one wants to avoid creating more debris, the deceleration must be far less than 1g - to be applied very uniformly, because the outer surfaces of a satellite are weak. But even at 1g, it takes 10s to decelerate the little bit that will trigger the re-entry. Within these 10s, the satellite moves by 80km. If the relative speed with some colliding matter is 10km/s, 1g deceleration (probably too much) of a 5t satellite on 5m2 surface needs a pressure of 10kPa obtained by a density of only 0.1g/m3. Unless the colliding material is very well targeted, big amounts must be put there. Over 80km length, a D=100m trail takes 63t - to be put very evenly at exactly the right position at exactly the right time despite falling down. Not exactly easy, is it? What can be done presently is render a satellite unfunctional, but this spreads its fragments. Satellites designed accordingly and still controlled can be de-orbited properly by many means, but if poorly designed or out of control, de-orbiting is an unsolved task because it's seriously difficult.

Napalm in Syria maybe? Reportedly dropped by helicopters. Before combat gas were forbidden and collected in Syria, many people asked (on an other forum) how to produce bromine, chlorine, hydrogen sulfide, mercaptan and the like.

Through its abundance maybe? Or because the products are volatile?

No carbon could be a help against corrosion. About analysis: some methods make virtually no damage to the artifact. Modern analysis apparatus evaporate a tiny bit of the alloy using a spark that also lets the vapour emit light; the spectrum tells the composition. Works for exposed parts, to be repeated at several places because alloys are inhomogeneous. The apparatus costs a few k€ but users or manufacturers could run theirs on your artifacts.

The number of charges added or subtracted from a metal is the same as in a semiconductor (if the gate oxide layer or depleted region are as thick), but with a metal - There are usually many more charge carriers, unaffected by the gate voltage. Or equivalently, the gate influences an extremely shallow portion of a metal; - Metals have no bandgap. Even if you add or remove charge carriers, you get no situation where all the mobile charge carriers have been removed. There might be some way out - please take with caution, that's hypothetical. We can make extremely thin layers of anything presently, for instance single-atom layers. The band structure differs in thin layers, so some metals might have a bandgap as a thin layer. Also, the number of charge carriers to remove isn't that big in a monoatomic layer. Then a FET might be possible with a metal channel. The next question is: how useful could it be? We are limited by the number of charge carriers that are controlled by the gate: the same for a semiconductor and a metal - but charge carriers are more mobile in a semiconductor than in a metal (at least a bulk metal), which permits to control a bigger conductance. FET are being experimented with a graphene channel, which is allegedly interesting because charge carriers are so mobile in graphene - advantage over silicon and metal.

This depends on detailed reaction mechanisms, specific to each target. But don't expect the perodixe to decompose before reaching its target.

"Unpredictable, improve my chances": this looks like everyone's situaton... Though, academic researchers could trigger lightning and guide it to the ground, using a powerful pulsed laser that creates an ionized path for the current. I suppose a similar weapon already exists for a few years, possibly shot from above but not necessarily. The Faraday cage carries the current in its conductors. It avoid the current to flow through the persons within the cage, and (more important for other uses) also reduces the fields resulting from the curent flowing through the conductors. Hence if the Earth contact isn't perfect, it's important that the floor as well conducts electricity, otherwise the current might choose a different path to a better Earth point. The are probably standards which I ignore; for customers, you have to comply with the standards, but if designing for myself, I'd just look around and copy existing designs. I've seen plain and braided copper, zinc; lighter aluminium brings some difficulties.

You will not get atomic oxygen because energy lacks. 147kJ is 59* RT at 300K, impossible. Peroxide will react with the pollutant. The net result is water and some combination of oxygen and the atoms of the pollutant, but this reaction doesn't pass by atomic oxygen. Details of the process are necessarily complicated and depend on the exact pollutant and possible catalysts, so I feel better to ignore them.

Focus means deflect. By a different angle according to the distance to the axis and to the position around the axis, so that the deflected light converges to a point. To deflect light, you need an angle between the entering and the exiting interfaces. If both faces are parallel, light will be deflected in the plastic but recover the initial direction as it exits. When bending a part, you keep the faces roughly parallel, so light will be offset by passing through the plastic, but not deflected. It won't focus.

In the full-flow staged combustion of oxygen and methylamine (or hopefully a combination of less volatile amines), no oxygen needs to be injected in the amine pre-chamber. The recomposition of the amine produces enough heat to achieve the 312 bar permitted by the oxygen side. This needs a stable reaction of the amine in the pre-chamber; catalytic recomposition of methylamine is known to be unstable, but maybe the hot pre-chamber solves that.

Hi Nanda, welcome here! Enthalpy of formation : -188kJ/mol peroxide -286kJ/mol water +249kJ/mol atomic oxygen so I'd say atomic oxygen won't form from spontaneous decomposition. The real situation must be less direct. Decomposition must proceed around other compounds like traces of organic substances, mineral dust (this is a known condition for peroxide storage)... through multistep reactions. And even if no other compound were available, cosmic rays would act. So I'd say "Form monoatomic oxygen which then reacts" no; "React with a different compound where a single peroxide can give a single oxygen atom" yes, and it likely precedes the formation of oxygen molecules if any. Let's see what chemists say.

Any weighed sum of orbitals is a valid wave function for the electron. The orbitals are just the wave functions which don't evolve over time, that is which have one well-defined energy. If one measures an energy - which is the case with the slow absorption or emission of a photon - then the wave function reduces, because of the measure, to the energy eigenfunctions which are the orbitals. But if one measures something else, for instance the position of the electron, then the wave function will reduce its size instead of its energy choice; then one can observe a wave function which is not an orbital. Because most orbitals are separated by more energy than the usual thermal energy, electrons group to the lower energy states, so usually they have a fixed energy. Orbitals have a single energy in an isolated atom; by "overlap" I mean that different orbitals of a single atom give a good presence probability to the electron at locations that are widely the same. For instance the 1s orbital has its maximum density (per volume unit, not per radius unit) at the nucleus and decreases regularly with the distance; we can say arbitrarily it's positive everywhere. The 2s orbital has also its maximum density at the nucleus but decreases, goes through zero and changes its sign. Both have a significant density at the same locations - they only need that their product, summed over space, is zero, but they can overlap. The absorbed energy puts the electron farther from the attracting nucleus as a mean distance. In the excited state, the electron's kinetic energy is smaller. Orbitals don't look like planetary orbits; nice illustrations are available there (click on orbital names at left): http://winter.group.shef.ac.uk/orbitron/ but at least for attraction versus kinetic energy, they work the same: more kinetic energy for lower orbits. The transition from one orbital to a other takes a measureable and measured time ranging from femtoseconds to millions of years. It depends on both orbitals. During the transition, the electron's wave function is a weighed sum of both orbitals, with proportion evolving over time. If this sum lets the electric charge wobble a lot, then the emission (or absorption) of the photon is efficient, and the transition is quick, like nanoseconds. Some sums of orbitals don't let the charge wobble, and then the emission is inefficient; this is called a "forbidden transition", which lasts very long - competing transitions generally take over. An equivalent description compares some quantum numbers of two orbitals with the quantum numbers a photon can have to tell if the transition is permitted, but this is the same as checking for a dipolar wobble which is an efficient light emitter. Orbitals don't wobble at all. Their shape (the position, size, shape of the electron) is constant over time, and the charge doesn't radiate. External factors can influence the duration of the transition. A laser cavity shortens it. As opposed, an antiresonant cavity that forbids the emission of a photon can conserve an excited state for longer; this was used in some designs. Also, shocks among molecules reduce the time during which the photon can be emitted with a consistent phase; this time between collisions uses to be less than millions of years. The duration of the transition defines the duration of the emitted light, which in turn defines the precision of its frequency or wavelength. ----- If you measure the energy of the electron during the transition, you will find it in one orbital or in the other, because the energy measure lets it chose, so only the probability to find it as the old or new orbital evolves, and observing this probability needs several atoms or successive measures. A different limit is that it takes time to measure an energy accurately.