Wikipedia:Reference desk/Archives/Science/2010 May 21

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May 21

Absorbent glass mat (AGM)

can i get a Absorbent glass mat (AGM) battery 4 my honda accord —Preceding unsigned comment added by Tom12350 (talk • contribs) 04:16, 21 May 2010 (UTC)

It would probably be more expensive than an ordinary one, and you won't see much difference if it isn't a hybrid. --Chemicalinterest (talk) 15:37, 21 May 2010 (UTC)

Actually, quite a few modern non-hybrid cars use these batteries - they last longer and are both smaller and lighter than a regular car battery. So even on a non-electric car, they save energy just by making the car lighter. My car has one - on previous MINI Cooper'S models there was no space under the hood for the battery - so it had to go into the trunk - I was surprised not to find it there...and doubly surprised when I couldn't see it anywhere under the hood either! On my car, the amazingly tiny AGM battery is tucked away in an extremely hard-to-reach place (against the fire-wall right beneath the lower-right edge of the windscreen, hidden in the ducting and air-vent metalwork). The claim is that they are reliable enough to never need replacing...We'll see!

I don't see any reason why you shouldn't be able to replace a conventional battery with one of these gizmo's - weight savings are always a good thing! But it might be wise to check with your local Honda dealer before you do it...you never know what weird stuff is different with them. SteveBaker (talk) 20:19, 21 May 2010 (UTC)

Photon clocks in GR

According to the equivalence principal, in a uniform graviational field, clocks at different heights will run at different speeds. If two identical photon clocks (a clock consisting of two mirrors seperated by a fixed distance with a photon reflecting between the two to measure time) are placed at different heights, then it seems that they too must run at different rates. But this would seemingly imply that the speed of light changes with height, which is obviously not true. So what gives? 173.179.59.66 (talk) 08:02, 21 May 2010 (UTC)

I assume you mean at different heights in a spherically symmetric gravitational field (which is not uniform as the direction and strength of the field both vary). If you have two photon clocks at different heights then the curvature of spacetime is different and light travels along different geodesic paths within the two clocks - so you need to think about what you mean by "identical" photon clocks. Parallel transport may be a useful concept here. Gandalf61 (talk) 08:21, 21 May 2010 (UTC)

No, I meant a uniform gravitational field. 173.179.59.66 (talk) 16:58, 21 May 2010 (UTC)

Along with time dilation, you also have length contraction. So the distance between the two mirrors will be shorter in the clock in the stronger gravitational field. Ariel. (talk) 20:07, 21 May 2010 (UTC)

Are you sure you got that the right way round? Would not the stronger field be time dilated, compared to the weaker field, and therefore have a bigger mirror spacing to get the same time delay. You could get the same idea by measuring wavelength. Graeme Bartlett (talk) 20:55, 21 May 2010 (UTC)

And the gravitational fields are the same for both clocks, no? Unless you meant potential...173.179.59.66 (talk) 21:19, 21 May 2010 (UTC)

If you really do mean a uniform gravitational field then the two clocks are indeed equivalent and run at the same rate from the point of view of any observer. You will only get a difference in time dilation rates if the gravitational field is non-uniform. Gandalf61 (talk) 21:28, 21 May 2010 (UTC)

I have to disagree with you on that one. Replace the uniform gravitational field with a uniformly accelerating rocket, with a clock A at its front and a clock B at its tail. If clock A emits light pulses at a uniform rate, then the time it takes for any pulse to reach point B from point A will L/(c+v), were L is the distance between the two clocks (which remains constant). Because v increases , the pulses will take increasingly less and less time to reach clock B, so an observer at B will conclude that the clock at A is running faster. Equivalently, redshift still occurs in a uniform graviational field, so relative time dilation must necessarily still occur. 173.179.59.66 (talk) 05:32, 22 May 2010 (UTC)

You need to be clear about what you mean by "uniform acceleration". If your ship is a solid object with a rocket that accelerates the back at a constant rate, then the (constant) acceleration of the front of the ship is not the same as the (constant) acceleration of the back. This situation is often called "uniform acceleration", but the acceleration is not actually uniform, if "uniform" means "the same everywhere". You also have to be careful when you say that the distance between the clocks is constant. It's not constant with respect to any inertial reference frame. It's constant with respect to a Rindler frame, but that doesn't match the usual special relativistic definition of length. -- BenRG (talk) 01:45, 25 May 2010 (UTC)

It's not clear to me what a "uniform gravitational field" would be in general relativity. The closest thing is flat spacetime in Rindler coordinates, which is not uniform inasmuch as the acceleration needed to stay at the same (x,y,z) position varies as a function of x. Trying to apply length contraction and other concepts from special relativity to this problem is hopeless. (Length contraction barely makes sense in special relativity, in my opinion, but that's a different question. It makes even less sense in general relativity.) There's no difficulty in the definition of identical clocks as long as they're small compared to the scale of variation in the gravitational field (which is the usual assumption).

When people say that the clocks run at different rates, what they mean is that if you send periodic signals ("ticks") from the clocks to a common location, the received signal rate at that location will be different for the two clocks. This is the same as saying that there is gravitational redshift. Light clocks essentially define what time means at a given location, so you can't say that they run fast or slow in any absolute sense. If you do a local speed-of-light measuring experiment with a light clock and a meterstick, you'll get c. -- BenRG (talk) 22:23, 21 May 2010 (UTC)

I'm not very well aquainted with general relativity, but I'm using as an approximation a gravitational field equivalent to a uniformly accelerating rocket, which is simple enough so that I can understand with my current level of knowledge. In that case, two observers at different altitudes will the other's clock run slower or faster depending on the relative height of the observers.173.179.59.66 (talk) 05:37, 22 May 2010 (UTC)

Time dilation due to constant acceleration depends on rate of acceleration and initial velocity, but not on initial separation - see Time dilation#Time dilation at constant acceleration. Therefore, if your definition of "uniform gravitational field" is "equivalent to constant acceleration", then the time dilation in your "uniform gravitational field" will not depend on separation either. So all (stationary) clocks in this "uniform gravitational field" will run at the same rate. What makes you think they will not ? Gandalf61 (talk) 14:18, 22 May 2010 (UTC)

In the wikipedia article you linked to, the observer is outside the rocket, and so the time dilation in that case is just a special relativistic effect. In my scenario the observers are both inside the rocket. The thought experiment that I mentioned above shows how two clocks at different heights in an accelerating rocket will run at different rates. If you don't trust me, then here's a more comprehensive article: http://www.peaceone.net/basic/Feynman/V2%20Ch42.pdf173.179.59.66 (talk) 18:10, 22 May 2010 (UTC)

Feynman's observer is outside of the rocket too. For an accelerated observer inside the rocket, the lengths L1 and L2 would be the same. You need to have an observer outside of the rocket, in an inertial frame of reference, to establish a baseline time against which you measure proper time and hence calculate time dilation. Gandalf61 (talk) 16:11, 23 May 2010 (UTC)

Yeah, and this outside observer will see both clocks run at the same rate. But the observers inside the rocket will notice a discrepancy. 173.179.59.66 (talk) 04:27, 24 May 2010 (UTC)

Then you seem to have answered your own question. The true time dilation relative to an inertial frame of reference is the same at all points in the uniform gravitational field. The apparent time dilation observed by two stationary observers within the uniform gravitational field comparing their proper times, does depend on their separation, but this is a consequence of the fact that these observers are not using an inertial frame of reference. Gandalf61 (talk) 09:01, 24 May 2010 (UTC)

There's no "true" or "apparent" time dilation. There is only the results of various experiments. I'm still not sure what a "uniform gravitational field" is. You (Gandalf) seem to be talking about two ships accelerated by independent, identical rockets with one clock aboard each one. The anon is talking about one ship accelerated by a single rocket with two clocks on board. These are different physical situations. In Newtonian physics they would be equivalent, but relativistically they aren't. See also my reply above (the one dated today). -- BenRG (talk) 01:45, 25 May 2010 (UTC)

Note that in weak gravitational fields, the metric in natural units is approximately given by:

$ds^{2}=(1+2V)dt^{2}-(dx^{2}+dy^{2}+dz^{2})$

where V is the Newtonian gravitational potential. Count Iblis (talk) 17:08, 23 May 2010 (UTC)

Wind Turbine and surface drag

I read an article on a new paint that reduces wind resistance [1], and one of the applications mentioned was wind turbines. My first thought was that you would not want to reduce wind resistance on a turbine or the air would just slip by without turning it. Then I began to wonder whether the turbine is turned by a pressure difference rather than friction with the air, and if so does the creation of the pressure difference rely on friction? Even if the wind resistance is not responsible for the effect, is it in any way detrimental? I am now completely confused as to what actually makes a wind turbine turn! -- Q Chris (talk) 09:29, 21 May 2010 (UTC)

It depends on the type. Some wind turbines work by redirecting the air flow sideways, creating torque via Newton's third law. These usually have flat vanes angled into the wind. Most modern high-performance turbines have an airfoil profile and generate torque by the pressure difference caused by the different speeds of air travelling over the more curved leading side and the straighter trailing side. In either case, energy lost to friction just warms the blade and does not contribute to the generating capacity. --Stephan Schulz (talk) 09:40, 21 May 2010 (UTC)

Thanks Stephan. I can see that the energy that just heats the blades will not aid generation, but will it be detrimental to the generation? As far as I can see the air will be slipping off in the same direction that the blades rotate, so it won't make much difference either way. -- Q Chris (talk) 10:03, 21 May 2010 (UTC)

What friction will do is slow down the airflow. Thus, somewhat simplified, you have a reduced "effective wind". In the case of simple vanes, for example, the induced sideways motion of the downwind air will be less (part of the energy is lost to friction), and hence the opposing force acting to turn the vanes will be less. A similar argument can be made for the airfoil. --Stephan Schulz (talk) 11:33, 21 May 2010 (UTC)

Also, friction can result in turbulence, which can wiggle the blades and cause heating and wear and tear on the bearings. Thus, laminar flow is preferred. The blades themselves may also tend to get pitted more if grains of sand are dashed against them, due to turbulence. This could in turn cause more friction and turbulence, and also unbalance the blades. StuRat (talk) 15:05, 21 May 2010 (UTC)

What is this weird creature?

Opossum

Opossum

River otters

weird creature Kittybrewster ☎ 11:21, 21 May 2010 (UTC)

Here's some more pictures: [2]. Most of the comments (i.e. here) I've seen are saying otter or possum, or possibly a nutria or groundhog. Buddy431 (talk) 12:35, 21 May 2010 (UTC)

I vote for "exceptionally ugly/drowned looking opossum." Compare with this beauty queen. Either than or an otter whose lost a lot of fur on the snout. (The skull is very otter-like.) --Mr.98 (talk) 12:37, 21 May 2010 (UTC)

If it's 'playing possum' it's overdone it.. Really appears to 100% dead possum. cf image seach for "possum skull" - note the lower 'canine' - not otter as far as I know.

Does anyone know what happens to the hair - is it common for it to fall out of dead animals in water? eg this [3] 77.86.115.45 (talk) 16:29, 21 May 2010 (UTC)

The face looks like a wolverine's, but the article says "The creature's tail is like a rat's tail and it is a foot long.", which suggests possum. Maybe a hybrid? Wolverines don't have rat-like tails do they? Vranak (talk) 17:59, 21 May 2010 (UTC)

I'd vote for otter - perhaps with mange, perhaps something else - decomposition? - causing the fur loss. The face looks very otter-like to me, though I'm curious about the tail. Of course, if hair loss also occurred on the tail, it could well look like a rat tail. Matt Deres (talk) 19:39, 21 May 2010 (UTC)

I don't think anyone would look at an otter tail and say it was rat-like though. Vranak (talk) 19:51, 21 May 2010 (UTC)

Comparing to the pictures to the right (feel free to adjust the formatting, I can't figure it out), I think the snout is too short for an opossum, and the ears too small. Also, the place where the whiskers would be doesn't seem quite right either. I'm voting tentatively for the river otter. Perhaps without fur on the tail, someone who has not seen a rat recently would say it looked like a rat tail. The river otter's ears look closer to me, as does the snout and the whiskers. Falconus^{p t c} 21:03, 21 May 2010 (UTC)

Note: I revised inaccurate captions on the pictures. I am fairly sure, for example, that the bottom picture is not a badger, but rather a river otter.Falconus^{p t c} 22:06, 21 May 2010 (UTC)

Bovine Tubersulosis versus the Human Form.

During my research into my family history I've discovered that my grandfather lost three of his siblings over a 5 year period at the turn of the 20th. Century. I am wondering if occupation conveyed a part of the other siblings immunity? I know, for example the Jenner discovered that immunising someone with Cow Pox gave them a marked immunity to Smallpox, hence my curiousity regarding T.B.11:47, 21 May 2010 (UTC)~ —Preceding unsigned comment added by 2.102.26.59 (talk)

Assuming that contact with Mycobacterium bovis can generate resistance to Mycobacterium tuberculosis (the more typical cause of tuberculosis in humans), it is important to take a major difference between M. bovis and the cowpox virus into account: the latter doesn't cause a typically life-threatening disease, while the former does. This is what made inoculation so useful. – ClockworkSoul 16:55, 22 May 2010 (UTC)

0t,1t 2t bend test / galvanized/colour coated coils

Does anyone recognise what is being asked here Wikipedia:Reference_desk/Miscellaneous#Bend_test ? 77.86.115.45 (talk) 11:57, 21 May 2010 (UTC)

Oil spill - quick drying cement?

Why cannot the source of the oil spill just be covered with a large amount of quick drying cement. After the first batch has hardened, another batch, then another. This is not a silly question - cement does set under water. 92.28.253.142 (talk) 14:32, 21 May 2010 (UTC)

I believe the force of the oil, which is coming out under pressure, would push the concrete out of position before it could harden. StuRat (talk) 14:58, 21 May 2010 (UTC)

As do many polymers, going back to the wet-climate-set Chinese lacquer. My guess is that the big question is how to put a big heavy block onto a big hole spewing crud at very high pressure and not just having it get knocked to the side by the spray. Consider how hard it is to stop the water from coming out of a hose with your thumb. SamuelRiv (talk) 15:07, 21 May 2010 (UTC)

As an extension to the above idea - first cover the flow with large chunks of broken stone - so that the oil is diffused out between the gaps in the stone - the concrete would be less likely to be swept away before setting due to the reduced local flow rate.. In practice once completed the flow would probably just exit below the stone - and scour the (loose?) sea bed, creating its own channel.

I wonder if a rheopectic substance or Dilatant would help here (probably not).77.86.115.45 (talk) 16:13, 21 May 2010 (UTC)

Along these lines, Google for: bp oil "junk shot". DMacks (talk) 16:29, 21 May 2010 (UTC)

I wonder if they've considered using (hydraulic?) clamps to 'crimp' the end of the pipes to restrict the flow? - are the pipes made of a workable steel or a brittle steel?77.86.62.107 (talk) 16:57, 21 May 2010 (UTC)

Not a good idea, it could break the pipes. Even workable steel has a limit to which it could be cold-worked before it starts to crack (generally no more than 20% deformation), so "crimping" the pipes won't work. 67.170.215.166 (talk) 05:05, 22 May 2010 (UTC)

And is it possible that it is impossible to close this oil spill until all the oil has run out? Mr.K. (talk) 16:00, 21 May 2010 (UTC)

No, just until the pressure drops. Are they not planning to drill nearby to relieve the pressure? Dbfirs 17:19, 21 May 2010 (UTC)

"They" (we) are planning to do just that, but it could take up to several weeks due to the great depth of the well required and also the difficult rock formation. (Well, I don't know all the particulars -- I work on the refining end, not the extraction end -- but that sums up what I've heard from the company management.) FWiW 67.170.215.166 (talk) 05:00, 22 May 2010 (UTC)

I believe the amount of oil shooting out of the broken pipe would prohibit this sort of remedy. And it's a little sloppy too, just pouring heaps of cement over a leak. Reminds me of Chernobyl. And just getting the cement out in the middle of ocean, and then down miles beneath the surface, is rather more problematic than, say, trying to plug Old Faithful in the same manner. Which, come to think of it, seems rather problematic in itself. Mind you one is a pipe, the other, a natural geyser -- apples and oranges and all that. But the point remains, pouring cement at those depths, that far from land -- not entirely trivial. But ultimately, I feel that the force of the oil is the critical factor. From all I have read, pumping drilling mud into the aperture is the way to go. Vranak (talk) 17:50, 21 May 2010 (UTC)

If I understand it correctly, the advantage of injecting heavy mud is that it can flow down the borehole against the oil counterflow. As the depth of the borehole fills with mud it creates a backpressure to gradually reduce the oil flow while also narrowing the part of the borehole through which the oil is flowing. The other approach, of capping the pipe, can only work if the pressure inside the cap is kept below the surrounding water pressure. Given the amount of dissolved gas from this well, that's not a trivial thing either. As the gas rises and expands it undergoes adiabatic cooling (like the working fluid in a refrigerator or air conditioner). On mixing with water this cold gas can form ice crystals that clog the pipe to the surface platform. This appears to be what happened to the first attempted cap. LeadSongDog come howl! 19:31, 21 May 2010 (UTC)

Not exactly ice crystals but methane clathrate, which is less dense than pure water ice. The mud (it's actually synthetic) is made extra dense by adding the mineral barite to it, which is how highly pressured oil and gas are controlled when they are first encountered when drilling. Mikenorton (talk) 19:55, 21 May 2010 (UTC)

The key problem here is three-fold. One is pressure - the oil isn't just slowly washing out - it's coming out at some ungodly pressure. Below the seafloor, there is about 18,000 feet of solid rock pressing down on the oil reservoir - that's an incredible amount of pressure. Nothing like wet cement is going to be able to resist that. Second is temperature. This oil is incredibly hot. Third is that there is a lot of debris there - for starters, there is 5,000 feet of crumpled up pipe that fell from the rig onto the well-head. Then there is debris from the rig itself. So wet concrete is out of the question. Even dropping a very large concrete block onto the well-head won't work because the oil is under such pressure, it would just channel a route through the sand and stuff underneath it and squirt out around the edges. So you'd have not one neat little pipe to cap - but the entire perimeter of the concrete block. The various domes and such they've tried (and failed) with would have to have the oil continually sucked out of them - with a suction more powerful than the pressure of the oil - so as to 'suck' them onto the sea floor. These are all very difficult solutions.

The one I could imagine working would be to construct four vast rolls of flexible water/oil-proof fabric or plastic sheeting - each 5,000 feet long by (let's say) 100 feet wide - and to unroll these from the sides of ships to create four curtains around the well-head - then to heat-seal or 'zipper' the edges together as they are unrolled to make a vast, square-cross-section, floppy tube that could eventually rest on the seafloor far enough from the well head to avoid the debris field and the immediate heat/pressure of the oil. (You'd have to weight-down the bottom edge of each strip). This container would start to fill up with oil - which could then be pumped out from ships on the surface. The large width of the tube would ensure that the oil had room to expand and to relieve the pressure. Since oil is less dense than water, and we're pumping out the inside of this giant tube - any small leaks would have water flowing in - not oil flowing out. The technical difficulty is in handling such enormous amounts of fabric - and how well the stuff performs at the very low temperatures at the bottom of the ocean - also, how the pressure of ocean currents would work to push the thing out of shape.

SteveBaker (talk) 20:03, 21 May 2010 (UTC)

You're hired. Get your ass to the Gulf of Mexico. Comet Tuttle (talk) 20:16, 21 May 2010 (UTC)

Well, there's theory, then there's practice. I should think that Red Adair was so good at capping blowouts because of his experience in the field, not so much a sound theoretical background. I also imagine that too much theory and not enough experience was one of the chief causes for this mess in the first place. Vranak (talk) 20:20, 21 May 2010 (UTC)

The oil coming out now is actually at low pressure. It's being dumped into the ocean as quickly as the weight of the rock is able to force the oil into the well, and so there is very little accumulation of pressure. (Even given the large weight of the sea floor, oil will move slowly through the pore spaces in oil bearing rock.) If you are losing "only" 100,000 barrels per day through a 50 cm pipe then the average flow rate is only about 1 m/s (2.2 mph), which would imply the pressure difference of only about 500 Pa between the well and sea floor. Any person with a stout piece of plywood could actually block that off. The problem is that the weight of the sea floor will continue forcing more oil and gas into the well space even after you block the opening, so pressure will buildup and unless your obstruction is very sturdy the well will blowout again. Dragons flight (talk) 00:08, 22 May 2010 (UTC)

Why can't they just use a wider (4 -5 ft) pipe from the source to the surface and that would no doubt solve the problem of slushing up the pipe with frozen methane? —Preceding unsigned comment added by 98.221.254.154 (talk) 02:05, 22 May 2010 (UTC)

Actually the pipe would still slush up, it would just take longer. 67.170.215.166 (talk) 05:21, 22 May 2010 (UTC)

I would like to clear up a scientific inaccuracy that seems to be floating around in the above posts. The reservoir pressure is not exactly caused by the weight of the 18,000 feet of rock overhead. Rock is not a fluid (not a good one, at least); it does not behave like a hydraulic head; although the weight of the rock does contribute to the pressure, the real defining equations for pore pressure are a lot more complicated. The pore pressure contributes to, but must be converted into, an effective reservoir pressure. Finally, this can be used to estimate a pressure at the wellhead on the seafloor. The Terzaghi equation, in conjunction with reservoir characterization and geomechanical measurements, can be used to calculate the reservoir pore pressure; or an empirical law relating pore pressure to some other geomechanical property or observable from a borehole logging measurement. Finally, the fluid seep into the borehole needs to be modeled and eventually a hydraulic head can be calculated to determine the pressure at the exit point of the wellhead (on the seafloor) or at the ocean surface. To do all this, you need to know a lot of details about the fluid composition in the bore (probably drilling mud and other debris at this point), as well as details about the reservoir that only a privileged member of the BP E&P team would have access to. I just want to make sure nobody is thinking of applying the old "P = ρ g h" static pressure-head equation with the density of rock in a feeble effort to calculate the fluid pressure - it is unfortunately much more complicated than that. The short answer is, we don't know what pressure the oil is under as it rises to the well-head - the fact that there was a blowout indicates that neither did BP or its subcontractors. Nimur (talk) 02:44, 22 May 2010 (UTC)

We do know that at the time when it started to flow that the pressure in the reservoir was somewhere between the hydrostatic pressure (the pressure at the base of a column of water equal to the depth) and the lithostatic pressure (the pressure at the base of a column of rock equal to the depth). Most reservoirs are 'normally pressured' and lie close to the hydrostatic pressure, taking into account the column height of the hydrocarbons involved. Some reservoirs are 'overpressured' and lie closer to the lithostatic gradient but never reach it because high pressures will cause fracturing of the top seal (the impermeable layer that helps form the top of the 'trap') and this is known as the fracture gradient, so again we know that it will be less than that. After flow has started the pressure will start to reduce, something known as depletion, but the amount of depletion depends on the extent to which water flows into the reservoir as the hydrocarbon is removed, something known as 'aquifer support'. As to the blowout, the gas may have come from a shallower formation (unless this is associated gas i.e. coming out of solution from the oil), which should have been held back by the cement liner or casing in the wellbore above the reservoir. Mikenorton (talk) 13:12, 22 May 2010 (UTC)

I don't udnerstand why they can't just bring another rig to that location, send a pipe down and resume normal operations. Why not?--92.251.177.211 (talk) 20:30, 22 May 2010 (UTC)

BP are currently drilling two 'relief wells' to try and intersect the original wellbore, but it takes time and the target is rather small. Mikenorton (talk) 20:36, 22 May 2010 (UTC)

Field Strength of LHC Magnets

I am told that the magnets at the 27 km circumference LHC operate at 8.3 Teslas, while the protons contained in the beam have an energy of ~7 TeV (see Wikipedia LHC article). Trying to match these data up using simple physics fails, and so I'd like to know where my mistake(s) is/are.

As protons are travelling at near the speed of light:

$mv\approx {\frac {E}{c}}$

$F={\frac {mv^{2}}{r}}=Bqv\implies B={\frac {mv}{rq}}\approx {\frac {E}{crq}}$

Substituting in values:

$B={\frac {7\times 10^{12}{\mbox{ eV}}\times 1.602\times 10^{-19}{\mbox{ J eV}}^{-1}}{3\times 10^{8}{\mbox{ m s}}^{-1}\times 4300{\mbox{ m}}\times 1.602\times 10^{-19}{\mbox{ C}}}}=5.4{\mbox{ T}}$

I presume I've missed off some relativistic effects, but even when I tried to account for some of them, I still got around 5.4 Tesla. Any suggestions? --80.229.152.246 (talk) 15:46, 21 May 2010 (UTC)

(altered/corrected your ref tags - no reference section at bottom of this page)

Is it possible that the figures you've used are maximum operational limits - eg the magnets can go up to 8.3T safely ? (but are operated at a lower figure?)77.86.115.45 (talk) 16:19, 21 May 2010 (UTC)

there doesn't seem to be a reference for the 8.3T figure to get its context from.77.86.115.45 (talk) 16:23, 21 May 2010 (UTC)

There are a few mentions of 8.3 T being the required field in LHC: The Guide. --80.229.152.246 (talk) 17:38, 21 May 2010 (UTC)

Mmmh actually - plan B - the magnets aren't actually completely circular are they?? ie it's magnet/straight/magnet/straight etc? Thus the turning circle at the magnetic turning sections must be smaller than the radius of the machine - hence a higher magnetic field required - ie the colider shape in not a circle but a n-agon (polygon) (with rounded corners) - surely this must be the case? So radius is less than 4300m. 77.86.62.107 (talk) 18:31, 21 May 2010 (UTC)

I'm pretty sure it's quite circular. But what about length contraction? The magnet slightly ahead of the current position of the proton is length contracted toward the proton, so the radius might be lower than what you calculate. But it's not totally ahead of the proton, just slightly, so the contraction is only a fraction of the total speed. Ariel. (talk) 20:16, 21 May 2010 (UTC)

Fairly certain polygonal eg [4] states 1232 15m dipole magnets - so that's 18.5 km of bending magnets - but the circumference is 27km. 77.86.62.107 (talk) 20:27, 21 May 2010 (UTC)

(ec) That's correct. The bending magnets occupy only a portion of the ring's total circumference. The remainder contains quadrupole focusing magnets, experiments, and gaps between magnets. In total, the LHC contains 1232 dipole bending magnets, each with a length of approximately 15 meters. (That PDF link contains a number of other important specs for the bending magnets, as well.) 15 meters times 1200 magnets accounts for just 18 kilometers (about two thirds) of the total tunnel length. TenOfAllTrades(talk) 20:33, 21 May 2010 (UTC)

...back of envelope attempt at getting radius of curvature of magnetic sections - since for each turn (2pi/1232 radians) angle is small I assume sin x=x and cos x=1

r_effective=4300m , L=(27000m-18500m)/1232 (length of each straight section), A = 2pi/1232 (angle radians)

r=radius of curvature of magnetic sections

I get r_effective/r = (rA+L)/rA using similarity of triangles (approx since cosA~1)

solving for r (using pi=22/7)

gives r=4300x(15/22)

Thus using the original equation with the new r I get magnetic field = 5.4 x (22/15) = 7.92 (pretty close to the 8.3 quoted)

Looks like this method is probably along the right lines.?77.86.62.107 (talk) 21:36, 21 May 2010 (UTC)

I should probably point you back to the PDF that I linked in my response — it actually has more precise values for the length of each magnet, as well as explicitly specified bending radius and angle per magnet. The real magnet length is actually a bit less than 15 m — the specs say 14.3 m — which probably just about accounts for the discrepancy between your calculated field value and the nominal. TenOfAllTrades(talk) 03:11, 22 May 2010 (UTC)

doh, silly me I should stop skimreading - yep my estimate came out at ~2930m - the pdf gives ~2800m .. giving an unsuprising 8.3T for the required field which is spot on.77.86.62.107 (talk) 03:23, 22 May 2010 (UTC)

Thanks everyone. I'm glad to see it's not down to awkward relativistic effects like length contraction and the like (I did consider that but didn't think it'd have a large enough effect, plus I have no idea how to work that out...). --80.229.152.246 (talk) 15:45, 22 May 2010 (UTC)

Iron in high oxidation state

Is there any way to create iron compounds with an oxidation state of more than +3 using household chemicals? --Chemicalinterest (talk) 20:57, 21 May 2010 (UTC)

Potassium ferrate - you would probably only have access to the sodium compounds - sodium ferrate is more unstable (see http://library.sciencemadness.org/library/ferrates.html)

In sweden you can produce it from bleach possibly http://sv.wikipedia.org/wiki/Natriumferrat ?? seems to be confirmed by youtube http://www.youtube.com/watch?v=pUvdETUQPuo (I think this is probably true) 77.86.62.107 (talk) 21:49, 21 May 2010 (UTC)

I tried adding KOH and household bleach (NaClO) to ferric oxide but it didn't react. (Oh and I boiled it).--Chemicalinterest (talk) 22:42, 21 May 2010 (UTC)

Watch the youtube video linked above - you need strong bleach, and they boiled it.77.86.62.107 (talk) 22:47, 21 May 2010 (UTC)

Oh.. and the step were they add ammonia at the end - don't try that if there is excess bleach ammonia can react with it to make hydrazine - explosions can potentially follow... alternatively the reaction produces chloramines - which are toxic.. (or nitrogen trichloride even which too is explosive) In fact I wouldn't try this at home - boiling bleach sounds like a recipy for disaster in one form or another..77.86.62.107 (talk) 22:56, 21 May 2010 (UTC)

Pushing or pulling Asteroids

Lets say we have a spaceship that's powerful enough to move asteroids. Is it easier for it to push it or to pull it? ScienceApe (talk) 21:04, 21 May 2010 (UTC)

A joule is a joule is a joule. Presently, I don't believe we even have any technology that is capable of such a feat, so it's a purely speculative issue at this point, yes? Vranak (talk) 21:07, 21 May 2010 (UTC)

It would be easier to pull using the gravity inherent in the ship's mass, because that would not use any external energy. dude❶❽❶❽ (talk) 21:11, 21 May 2010 (UTC)

You still have to use energy to keep the gravity tractor in the right place. --Tango (talk) 21:13, 21 May 2010 (UTC)

Pushing is easier. Dauto (talk) 21:17, 21 May 2010 (UTC)

From an energy point of view it's the same, but from an engineering point of view pushing is easier. For two reasons: One you can just push the asteroid, and don't need some method of attaching to it. Two: If you pull, your exhaust gases will impact on the asteroid, which will push it back - you'd need some way to reroute your exhaust around the asteroid. Ariel. (talk) 21:42, 21 May 2010 (UTC)

Ok. But think of failure. If your're gunning it full steam towards an asteroid and the 'link', whatever that may be, fails, then you're going to crash into it (unless your distance is great, or you have good retro rockets, etc). If you're pulling it, you just go in the opposite direction. Vranak (talk) 21:44, 21 May 2010 (UTC)

That assumes pushing from a distance. If the asteroid is strong enough to be pushed from a single point without breaking up, the spacecraft can be brought right up against it first (in effect a soft landing) and then start pushing. However, you would need to keep the pushing force accurately aligned with the center of mass. --Anonymous, 23:08 UTC, May 21, 2010.

Exactly. In any case, a worst-case scenario looks a lot worse when you are pointing towards the 'roid, rather than away from it. Well, if it were remotely-operated, I suppose a hull breach isn't necessarily curtains. Vranak (talk) 00:16, 22 May 2010 (UTC)

I too would pull - for using you need a structure to transmit the pushing force from the front of the spaceship - but the force propelling the spaceship is probably at the back - would need a strengthened airframe - you also can't see where you are going..

For pulling all you need is some ropes - these can be attached close to the propulsion source - ropes can be long - gas back draft shouldn't be a problem..77.86.62.107 (talk) 21:46, 21 May 2010 (UTC)

Gas back draft most certainly is a problem. Or earth if you go far away the air disperses the gas. Not so in a vacuum. The gas will go directly toward the asteroid, and will totally cancel out the effect of the propulsion. It would be like trying to move a sailboat by attaching a fan to the boat and blowing on the sail. Ariel. (talk) 23:43, 21 May 2010 (UTC)

That depends on whether the gas molecules collide after being ejected - if so the gas will disperse (in a 'cone' shape) - I think it would be hard to make a rocket nozzle that outputs molecules that do not collide with one another - would require low gas densities - though it would be the most efficient.77.86.62.107 (talk) 00:34, 22 May 2010 (UTC)

For pushing you have to locate and get exactly behind the asteroid's center of mass or the thing will start spinning instead of going the direction you expect. For pulling this is not a problem. Cuddlyable3 (talk) 22:24, 21 May 2010 (UTC)

It's exactly the same problem for pulling. If the force, whatever the direction, doesn't go through the centre of mass then you will introduce a torque. If you are using a gravity tractor, then the force is automatically through the centre of mass, but if you are pulling it using a rigid scaffolding or something, then it could be a problem. If you are pulling it using a flexible cable, then the torque will be temporary and it will soon end up going through the centre of mass. You can avoid a torque when pushing by having the engine attached to the asteroid via a gimbal - that way you can make sure the thrust is straight "up", which means it goes through the centre of mass (actually, that may only be precisely true for a spherical asteroid, but I think the principle can be generalised). --Tango (talk) 22:39, 21 May 2010 (UTC)

Also for turning - if the spaceship has weak turning thrusters and powerful main engine fixed on axis then it is easier to pull since "thrust vectoring" can be used - whereas when pushing the turning thrusters would be required to turn the entire combined mass - depends on what sort of spaceship you're using I suppose...77.86.62.107 (talk) —Preceding undated comment added 22:34, 21 May 2010 (UTC).

see: Pendulum rocket fallacy.—eric 23:10, 21 May 2010 (UTC)

I don't think that fallacy is relevant. The IP is correct - if you are pulling the asteroid using a cable then all you need to do to turn is change the attitude of the rocket. If you are fixed to the asteroid, thrusting in a fixed direction and pushing it along then you would need to turn the whole asteroid. However, if you are fixed to the asteroid but can change the thrust direction of the main engine then you can use the main engine to turn the asteroid and it becomes pretty easy (in fact, it's probably just as easy as in the pulling case, since in both cases you will use the main engine to turn the asteroid). An alternative would be to push the asteroid using a rocket than can move around on the surface (probably hopping around - there is so little gravity on a typical asteroid that the thrust required to hop would be minimal, you could probably do it with an air horn!). --Tango (talk) 23:25, 21 May 2010 (UTC)

(I was assuming that someone would be steering the ship . but it is an interesting link)77.86.62.107 (talk) 00:37, 22 May 2010 (UTC)

I like the idea of pushing with a clamp-on linear accelerator, which mines the asteroid, ionizes it, and accelerates a small portion of it to near the speed of light as a source of propulsion, all powered by nuclear reactors, or better yet, a matter-antimatter reactor. StuRat (talk) 04:15, 22 May 2010 (UTC)

If you test that device at home, you'll need a user manual. Cuddlyable3 (talk) 22:42, 22 May 2010 (UTC)

Car Mechanics...... power steering

Hi evry one ... my car power steering system breaks down .... so i tried to find out how it works , i googled it , to find this ... that the system use a pump attached to the engine (use engine power to compress hydrulic oil) in the system to maximize your hand power and make it easier to rotate the car .... so while searching i found that the oil pump compress the system faster at higher rpms (revolve per minute) .... which leads to more sensitive steering at higher speeds ... should'nt it be the other way around ...? as a safty measure the steering should have a slower response at higher speeds ...? --Mjaafreh2008 (talk) 21:47, 21 May 2010 (UTC)

Did you read in the article Power steering this: "...at high engine speeds the steering would naturally operate faster than at low engine speeds. Because this would be undesirable, a restricting orifice and flow control valve are used to direct some of the pump's output back to the hydraulic reservoir at high engine speeds." ? Cuddlyable3 (talk) 22:19, 21 May 2010 (UTC)

I don't know that it's done intentionally, higher oil pressure is a necessary side-effect of more RPMs (ergo more mechanical and thermal stress). I don't think increasing oil pressure would increase steering sensitivity, but I do think it would increase the power that goes into turning the wheels, which is better used at high speeds than at low speeds - as speed increases, so does momentum, which means changing the direction of the car requires a heck of a lot more force. I can safely drive at 20mph to the corner market lot with no power steering at all, I just have to put some muscle into turning it. But I don't want to imagine what happens if I drove unpowered at 75mph on the highway; I've done it, but at great risk - if something happened at that speed (blowout, shenanigans with other cars, etc) a gorilla couldn't control that wheel, let alone me. ZigSaw 12:14, 24 May 2010 (UTC)