Wikipedia:Reference desk/Archives/Science/2014 November 11

= November 11 =

Copper in homopolar motors
I've noticed that most simple homopolar motors use primarily copper wires in their structures. Is there something specific about copper or is it just because it does not attract magnets? Will a zynk or an aluminium wire work in a homopolar motor then? 128.68.216.15 (talk) 00:19, 11 November 2014 (UTC)


 * Copper is an excellent electrical conductor. Plasmic Physics (talk) 02:19, 11 November 2014 (UTC)


 * Yes. I believe silver is a better electrical conductor, but that would cost too much. StuRat (talk) 04:41, 11 November 2014 (UTC)


 * Copper is also quite flexible, so it survives vibrations better than other metals like silver and aluminium. Silver actually conducts electricity a little better than copper - and for something like an electric motor, the slightly worse conductivity of aluminium would hardly be noticeable.  Price is not a small matter - and historically, copper was cheaper than aluminium...but since the world is rapidly running out of copper ore - we'll be seeing aluminium wiring used a lot more than it is right now. SteveBaker (talk) 04:57, 11 November 2014 (UTC)
 * Copper-clad aluminium wire is also used for some purposes, although often controversially when used for twisted pair data cabling purposes . (It's more established in the high frequency coaxial market.) Also Copper-clad steel although that's not exactly new. I think these examples demonstrate some other advantages of copper such as relating to corrosion or oxidation, although some of them are probably not relevant to the motor case, e.g. Galvanic corrosion, but see our articles also also  or other discussions. (As an aside, copper clad wire has some disadvantages too such as the current difficulty separating the metals meaning recycling is difficult .) Nil Einne (talk) 11:55, 11 November 2014 (UTC)

How fast would a spacecraft be travelling?
In our article Taurus Molecular Cloud 1, there are a couple of videos that zoom in from a more distant perspective. If a spacecraft were able to travel as the speed depicted in the video, how fast would the craft actually be travelling in the depicted timeframe and distance? 99.250.118.116 (talk) 01:03, 11 November 2014 (UTC)
 * I don't think the viewpoint is changing at all. It's like the zoom lens on a camera - you're changing the field-of-view of the lens - not physically moving the camera through space. If you were moving from earth to a place where things looked that big without zooming, you'd have to cover most of the the 430 lightyear distance in about one minute.  Let's call that 400 lightyears per minute...which in "classical" terms would be about 200 million times faster than the speed of light.  Unfortunately, you can't travel faster than light...so that can't happen.  But the problem is a bit worse than that.  Because of relativity, the faster you travel, the more compressed distances seem to you.  So you wouldn't have to travel faster than light to get there that quickly...going somewhere quite close to the speed of light would be enough to get you from here to there within a minute...from your perspective.  However, if you were sending those photos back to earth via telemetry, it would take over 400 years, no matter what. SteveBaker (talk) 04:03, 11 November 2014 (UTC)
 * Note that in any "cloudy" environment, a ship would see changes as obscuring dust moved across the field of view that aren't replicated in the video. Also, in practice moving so close to the speed of light would impose really severe distortions on the field of view. Wnt (talk) 15:49, 11 November 2014 (UTC)

The end of Rosetta (spacecraft)
Rosetta (spacecraft) is in orbit around a comet and it is planned that it will land a probe on the comet tomorrow. The article says the mission will end Dec 2015. At first I assumed that the comet would go near the sun and the Rosetta would burn up, but a bit of research shows that it will only approach to 1.2 AU and will only go out as far as 5.7 AU. We have seen Mars surface explorers keep going in a hostile environment for many years past the original planned mission. At the closest approach there should be no problem with "too much sun." . So why couldn't the mission continue a lot longer than the projected Dec 2015 ending? Is it just a matter of having answered the set questions, and other projects needing attention? It should not need propellant to stay in orbit around the comet. Would there be too little solar power after Dec 2015 to run the reaction wheels from time to time for orientation correction and to keep the computers and heaters going? How about out at 5.7 AU? Edison (talk) 19:14, 11 November 2014 (UTC)
 * I note that Solar panels on spacecraft says "The Rosetta space probe, launched March 2, 2004, will use solar panels as far as the orbit of Jupiter (5.25 AU)..." How far out from the Sun will Rosetta be in Dec 2015? Edison (talk) 19:27, 11 November 2014 (UTC)


 * Probably the money that was allocated to keep the ground station operative runs out at the formal end of mission. This has spelled the death of many vehicles.  NASA sometimes extends missions - but only when they are generating new science...and it may be that the instruments on Rosetta will have extracted about all they can by December.  You only need just so many photographs of a comet.  But also, as the craft gets further from the sun, the solar panels produce less and less energy.  The specifications say they produce 850 watts at 3.4 AU.  When you double the distance, you get one quarter the amount of power...so at 5.7 AU they are probably only generating about 300 watts.  It doesn't take much extra distance to make a huge difference to the power availability.  As the amount of sunlight reduces, the need to heat the spacecraft to keep the electronics alive increases - so you have increasing demand and decreasing supply.  I also wonder whether the spacecraft's orbit is as stable as you imagine - given that the asteroid doesn't have a nice spherical shape - and its' mass is reducing all the time and at unpredictable rates.  SteveBaker (talk) 19:59, 11 November 2014 (UTC)


 * There might be new information as the comet moves away and cools. Will its weak gravity draw back at least a portion of the outgassed vapor and emitted particles, so that they redeposit? If I recall correctly, Rosetta was 19 miles or so from the comet, so its orbit should be less effected by the duck or potato shape of the comet than if it were close by. The comet is putting out huge amounts of water vapor presently (a liter every 15 seconds if I recall correctly). I wonder if passing through the vapor will push Rosetta farther away, or degrade its orbit and bring it in until it hits the comet. Is there any reason Rosetta would not follow the comet indefinitely? Edison (talk) 22:39, 11 November 2014 (UTC)


 * As Steve suggested, given 67P's small, spatially- and time-varying gravitational field, Rosetta surely needs to expend fuel to maintain even a fixed-distance orbit around it. And this fuel is a factor limiting the mission's lifetime, along with funding. Rosetta FAQ says, Rosetta's nominal mission will end in December 2015 after a total lifetime of 12 years. There could be a six-month extension provided there is fuel remaining, nominal activities are completed by the end of 2015 and additional funds are made available.... A decision on this will be taken in late 2014. And just as an aside: Philae's mission ends in March 2015, because "lander will become too hot to operate" at that distance from the Sun (1.2AU?). Abecedare (talk) 01:10, 12 November 2014 (UTC)


 * According to WolframAlpha, the target comet 67P/Churyumov–Gerasimenko is currently (12 Nov 2014) just under 3 AU away. On on 31 December 2015 it will be just over 2 AU away, so it is still approaching Earth  CS Miller (talk) 11:16, 12 November 2014 (UTC)

Magnets and heat
I've heard that enough heat can ruin a magnet. How much heat are we talking about exactly? For example is it possible to ruin a neodymium magnet by boiling it in a cooking pot? 89.178.237.122 (talk) 20:27, 11 November 2014 (UTC)


 * This page on a magnet supplier's website says that some neodymium magnets can be be demagnetised permanently at a temperature of 80 °C, so yes, boiling one in a cooking pot might do it, though it is dependant on multiple factors - precise makeup, shape, and exposure to reversed magnetic fields. AndyTheGrump (talk) 20:41, 11 November 2014 (UTC)
 * In short, no, boiling the magnet won't ruin it. Curie temperature is what you need to reach, i can't see a reference in that article for neodymium but you might be able to google it.Vespine (talk) 22:06, 11 November 2014 (UTC)
 * Depends on the exact composition of the material. Some Rare-earth magnets are build specifically to withstand high operating temperatures, such as those used in electric motors (e.g. ). Mihaister (talk) 23:47, 11 November 2014 (UTC)
 * 3 more magnet suppliers that all state that heating some neodymium-containing magnets above 80 °C may lead to a permanent loss of magnetism: I very much doubt that they would state this if it weren't correct - why should they invent limitations that don't apply? AndyTheGrump (talk) 00:01, 12 November 2014 (UTC)
 * Reading the sources, a perhaps important point of clarification is that as I understand it, the loss is only of part of the magnetism (unless you reach the Currie temperature) and that although it is irreversible it sounds like it depends mostly on the temperature, not how long it's held there. By which I mean even if you boil the magnet for 24 hours, I don't think the magnet is going to have no strength left, in fact, it may not be much different from one boiled for 1 hour. The is IMO quite useful here. It ofters some quantification on the irreversible loss for certain magnets above 80 °C. It doesn't really explain why, but perhaps a search for "permeance coefficient" would give more details. Note that the Curie Temperature of these neodymium magnet with maximum operating temperature of 80 °C, may be 310 °C . In both cases (i.e. even if heated past the Curie temperature and all ferromagnetism is lost), as I understand it these losses are irreversible but not permanent. By which I mean you can remagnetise the magnet if you have the necessary equipment like that used to magnetise it in the first place. You need to heat it a lot higher before you damage the magnet such that it can't be remagnetised (without recycling and resintering). Nil Einne (talk) 06:32, 12 November 2014 (UTC)