Wikipedia:Reference desk/Archives/Science/2019 January 19

= January 19 =

Surgical masks
The article says they stop others from getting sick if the wearer is sick. But, do they protect the wearer from sick people? Many thanks. Anna Frodesiak (talk) 04:53, 19 January 2019 (UTC)
 * Yes they do, by the same mechanism -- they filter out aerosol particles which carry viruses and harmful bacteria. 2601:646:8A00:A0B3:0:0:0:ECBD (talk) 04:57, 19 January 2019 (UTC)
 * Thank you! :) Anna Frodesiak (talk) 11:11, 19 January 2019 (UTC)
 * Careful. The article directly states - with a citation - "A surgical mask is not to be confused with a respirator and is not certified as such. Surgical masks are not designed to protect the wearer from inhaling airborne bacteria or virus particles and are less effective than respirators, which are designed for this purpose." Matt Deres (talk) 14:48, 21 January 2019 (UTC)


 * Not as much. A surgical mask is a layer of thin fabric and is effective against a spray of droplets (i.e. what's coming from the wearer's mouth and nose). However they're not effective against airborne fine particulates, such as bacteria or viruses.  The risk of infection from a patient is usually lower than the risk of infecting a patient (for conditions which aren't spraying fluids everywhere, and ignoring the fack that patients tend to be more ill), which is why the facemask was so generally worn.
 * If a medic wants to avoid an infection themselves, they're more likely to wear a clear plastic face shield than just a mask. Andy Dingley (talk) 15:05, 21 January 2019 (UTC)
 * I planned to write a better answer but to be honest, I can't really be bothered digging up the sources. As others have said and others mentioned, it's unclear whether face masks provide any real protection to the wearer again aerosolised viruses. They do provide some protection from the wearer coughing and sneezing. For example, for control of season influenza, in healthcare settings the US CDC recommends they be worn by possibly infected persons until they are isolated. They also recommend either a surgical mask, or a respirator "when antiviral medication supplies are expected to be limited and influenza vaccine is not available, e.g., during a pandemic", for healthcare workers getting within 6 feet of a person who is infected or likely infected. This is along with standard and droplet precautions which I think will generally include eye protection (either goggles or a face shield [//www.cdc.gov/infectioncontrol/pdf/guidelines/isolation-guidelines-H.pdf]) They recommend infected people wear a mask in some circumstances. Outside healthcare settings, they make no recommendations for mask use, even for people who are unvaccinated and at high risk of complications. (They do suggest if they are going to wear a mask they should do it all the time.) [//www.cdc.gov/flu/professionals/infectioncontrol/maskguidance.htm]  During the 2009 H1N1 pandemic, they did recommend a face mask or respirator for those at high risk of complications where H1N1 was presented in the community in crowded settings and for caregivers of those infected. (They also said they weren't recommended could be considered by those both high risk and not high risk in non healthcare occupational settings where people were coming into with those with symptoms.) BTW, the guidelines also suggests that although there is agreement that respirators are better, it's not clear if this applies if they don't fit properly or aren't used properly. (Maybe see also [//academic.oup.com/jid/article/204/2/193/833588] and the earlier CDC guidelines PDF which does note there seemed to be no differences in Hong Kong during the SARS outbreak.) I think this is one reason why they were mentioned together. (For healthcare workers, you also have to consider time taken etc and how these affect compliance rates and other issues.) [//www.cdc.gov/h1n1flu/masks.htm].  It's my understanding that even for those who suspect they may be of benefit when used by non infected persons who aren't knowingly coming into close contact with infected persons, one of the biggest hypothesis advantages is actually in limiting them touching their faces. [//www.ncbi.nlm.nih.gov/pmc/articles/PMC2662657/] [//www.sciencedirect.com/science/article/pii/S1473309911701426]  Nil Einne (talk) 04:55, 22 January 2019 (UTC)
 * Actually I noticed that our article does mention and link to [//www.bmj.com/content/350/bmj.h694] which seems to be a WP:MEDRS compliant source. I do not have access to the full article but the abstract does say "" So it's possible there is now evidence they are useful for non infected people in certain circumstances. That said, we still don't know why are are of use. Nil Einne (talk) 05:21, 22 January 2019 (UTC)
 * Well said. I'll add parenthetically that in talking with experts during the H1N1 prep that it's not well known how much of a factor it is that a face mask simply reminds the wearer not to touch their face. Putting your fingers into your nose/mouth/eyes is a great method for viruses to bypass several levels of our immune defense and a mask provides a literal barrier to prevent you from doing so idly. If we did that without the mask, their impact might appear to be even less. Matt Deres (talk) 18:27, 22 January 2019 (UTC)

Preventing another Balvano
I've recently read the General Code of Operating Rules, and one of the things it says is that if a diesel train gets stuck in a tunnel, the crew must shut down the engine at once (to prevent carbon monoxide poisoning, obviously). Which raises the question, what is the emergency procedure if a steam train gets stuck in a tunnel??? Because that would be a dire emergency indeed -- a steam train generates far more carbon monoxide (along with other toxic gases) than a diesel train, and also I don't see any way that the smoke production can be shut down instantly! 2601:646:8A00:A0B3:0:0:0:ECBD (talk) 05:03, 19 January 2019 (UTC)
 * Over 500 people died in a coal-burning freight train in the 1944 Italian Balvano train disaster from carbon monoxide poisoning during a protracted stall in a tunnel. Preventive regulations were subsequently enforced and were repealed in 1996 when the line was electrified. It is difficult and dangerous to shut down a steam boiler rapidly e.g. by dousing the grate with boiler water. DroneB (talk) 14:07, 19 January 2019 (UTC)
 * It depends a lot on the tunnel. Some tunnels are better ventilated than others, some even have mechanical forced ventilation by fans.  Also the exhaust from a loco depends enormously on how hard the loco is working at the time.  As well as Balvano, a similar accident had happened a little earlier at the Swan View Tunnel in Australia, although in that case a purely freight train was affected and so there were only the two engine crews. These cases are rare though: in all the ones I know of, they're the combination of a single track tunnel, a steep uphill gradient, heavy trains, slow movement (at best) and the train stalling or slowing right down.   There are many cases though, especially in the 1890s, where particular tunnels were known to leave engine crews 'half choked' as a matter of regular occurrence (although not measured at the time, many tunnels, such as Swan View, were later measured at increasing crews' blood carbon monoxide levels "by 10%"   (I have no idea what that means, as baseline CO levels should be near enough zero). Several of these tunnels were early targets for the replacement of steam haulage, although this was done by electric locomotives, not diesel (electric locos appeared 30 years before diesels). Newcastle Quayside's infamous horseshoe tunnel gave rise to the ES1 electric locos. Baltimore's Howard Street Tunnel of 1902 and New York's S-Motors of 1904 were early electrification schemes for approach tunnels to large urban terminii, after the Park Avenue Tunnel accident, caused by poor visibility from steam locos obscuring signals.
 * In some cases though, steam locos can stop in a tunnel without problems. During WWII, a train heading to South Wales was strafed by enemy aircraft and 'hid' in the Severn Tunnel for some time. There were no problems of suffocation, as this tunnel is double track, ventilated by a large fan at Sudbrook, and although the loco had been working hard (at excessive speed) to reach the tunnel, its fire could be quietened down somewhat once within it, as the train stopped.
 * The greatest risk in tunnels though is fire. In the confined space, escape is difficult and the fire is usually left to burn itself out. The Summit Tunnel fire and the shorter but equally fierce Howard Street Tunnel fire are good examples. Only in cases like the 1996 and 2008 Channel Tunnel fires can such fires really be fought, as there is an isolated service tunnel giving access.
 * Diesel locomotives (unlike petrol engines) generate carbon dioxide rather than carbon monoxide. This is still an asphyxiant, but not toxic as the monoxide is, and so the fume hazard is much lower. Again, we're back to the question of the tunnel. It's not a big problem for the train to stop in the tunnel and to shut down the engine, a greater risk would be the locomotive was still moving slowly and working hard uphill. Diesels do need ventilation though; the Cascade Tunnel was built for steam and had such problems working uphill that the trains had to be spaced much further apart in one direction than the other. Very soon it was another site for early electrification, just to avoid this. The whole tunnel was eventually relocated (to reduce the effects of bad weather) and the new electric tunnel was much longer. When that tunnel was dieselised in the 1950s, electric ventilation fans (1,600 bhp, the size of a small diesel locomotive) had to be fitted.
 * To the original question though, steam locomotives are recognised to rarely 'break down' as diesels can do. Even diesels rarely stop dead in one place, and so crews of either type would generally make great efforts to at least reach the end of the tunnel (and wouldn't enter it with a failing engine).  The greater risk is when they struggle through the tunnel slowly, with the engine working flat out.  There are fewer failures for a steam loco which stop it dead on the spot, and most of those would count as "accidents", bringing their own problems and likely injury.  If a steam loco was stopped though, it's possible to reduce the strength of the fire fairly quickly (the fire does respond to the load on the engine, owing to the draughting effects of the blastpipe). 'Dropping the fire' would be the usual next step, throwing the burning coal onto the ground and away from the firebox and boiler.  This is normally done in a situation such as when the boiler is about to overheat through lack of water, and it still leaves the fire burning.  It would be possible to quench the fire (steam locomotives have plenty of water handy), but the risk then is that quenching the fire produces water gas, much as some old forms of gasworks produced, and that's mostly carbon monoxide!  It's a fire risk too.
 * So, in general, the advice has always been 'keep moving'. But in a handful of excessive causes, that led to accidents such as Swan View and Balvano. Andy Dingley (talk) 14:41, 19 January 2019 (UTC)
 * So, in general, what would be the right course of action if a steam train, working hard, stalled in a tunnel on a steep uphill grade (same situation as Balvano): (1) shovel on more coal, build up as much steam as possible, sand the rails if need be, put the reverser full forward, and try to get going uphill again; (2) put the engine in full reverse, open the throttle and back out of the tunnel; or (3) drop the fire, set the handbrakes, and tell the pax to run for their lives? (And if (3), should the pax evacuate uphill, past the engine, or downhill away from the engine?) 2601:646:8A00:A0B3:0:0:0:ECBD (talk) 08:49, 20 January 2019 (UTC)
 * The right course of action is to not get into that situation. So run trains with adequate motive power for the load and gradient.  Limit the size of trains going through such tunnels.  Double head them with two locos. Double head with a diesel loco as well as steam.  Design cab-forward locos, such as the Southern Pacific class AC-12, for use in such tunnels. Provide oxygen masks (either regular or emergency) for the crews.
 * These were very rare accidents. They happened during pressured times (the two here were both wartime) when trains were overloaded and motive power in poor condition.
 * Certainly the idea of rolling back downhill has been used in some cases, but that would have to depend on the driver and where in the tunnel they were. Andy Dingley (talk) 11:01, 20 January 2019 (UTC)

Armor Penetration
Hello, I wonder when I read about armor resistance against penetration, that for any type of armor,  resistance against HEAT is greater than APFSDS. 46.32.122.165 (talk) 06:28, 19 January 2019 (UTC)
 * APFSDS = Armour-piercing fin-stabilized discarding sabot
 * HEAT = High-explosive anti-tank warhead


 * HEAT round technology dates from the 1940's German Panzerschreck and Panzerfaust that are fired at low velocity and rely on focusing the blast energy of their inner charge against a tank armour, see Monroe effect. HEAT warheads have since become less effective against tanks and other armored vehicles due to the use of composite armor, explosive-reactive armor, and active protection systems which destroy the HEAT warhead before it hits the tank. The APFSDS response is to launch at high velocity a long, thin round that maximises the kinetic energy in a smaller area. This development employs a rod of special deep-penetrating material such as Tungsten Heavy Alloy (WA) and Depleted Uranium Alloy (DU) and incurs a danger to nearby troops and vehicles from sabot petals that are discarded at high velocity. DroneB (talk) 13:33, 19 January 2019 (UTC)
 * Ahem, I think you'll find that the British PIAT was a squeak earlier than those German contraptions, a creation of Winston Churchill's Toyshop. The actual charge was invented in Switzerland by Henry Mohaupt before the war, but making it fly through the air was a challenge for both sides. Alansplodge (talk) 17:41, 20 January 2019 (UTC)



So the penetrating rod is more resistant to lose its kinetic energy than the penetrating jet formed due to the melting of the copper cone inside HEAT warheads. 149.200.193.163 (talk) 14:03, 19 January 2019 (UTC)
 * The HEAT warhead works only if it detonates at exactly the right distance while the APFSDS can be made as long, as thin (aerodynamic) and as fast as you like. DroneB (talk) 14:13, 19 January 2019 (UTC)


 * Yes. Resistance against HEAT is greater than APFSDS. This is because HEAT is 40 years older, and armour was then developed to resist it. APFSDS was then developed to defeat that improved armour. It's the classic arms race, where each weapon promotes a defence against it, and each defence promotes a new weapon to defeat it. Andy Dingley (talk) 14:43, 19 January 2019 (UTC)
 * I wouldn't be that general. Against passive hardened steel armour, HEAT is very effective - and indeed, I think more effective than APFSDS fired with the same muzzle energy. It's less effective against active armour technologies. But the advantage of APFSDS comes at the cost of a much more powerful launch system. Nearly all man-portable weapons effective against post WW2 tanks are HEAT. --Stephan Schulz (talk) 16:28, 20 January 2019 (UTC)


 * HEAT can be counteracted by spaced armour and slat armour.  Alansplodge (talk) 17:41, 20 January 2019 (UTC)
 * Yes, APFSFS is more foolproof if you can bring it to the battle, althought there are things like tandem-charges and even tripple charges to compensate for some of HEATs weaknesses. But the main problem is that a typical APDSFS delivery system as e.g. the Rheinmetall Rh-120 weights 4.5 tons (without carrier vehicle). A typical HEAT delivery system is the RPG-7 at about 7kg. The very best APDSFS system can be defeated by not being at the battle due to logistic difficulties. --Stephan Schulz (talk) 09:19, 21 January 2019 (UTC)

Thanks alot everyone. 80.10.51.64 (talk) 21:49, 20 January 2019 (UTC)


 * Just a note about the caption on the right; the 1918 rocket launcher lacked an effective warhead against tanks (even 1918 ones). It was not until the US produced the M10 shaped charge anti-tank grenade in the spring of 1942, that Leslie Skinner and David Uhl were able to put rocket, launcher and grenade together. The first prototype (made from a piece of scrap tubing with a wire coat-hanger for sights) wasn't demonstrated until May 1942. See There's a War to Be Won: The United States Army in World War II by Geoffrey Perret. Alansplodge (talk) 19:20, 21 January 2019 (UTC)

Enthalpy vs. internal energy: potential mistake in article
Our article standard enthalpy of reaction currently states: "From the first law of thermodynamics we have a relation, $ \Delta E = Q_v$

That is, the enthalpy of a reaction at constant volume is equal to the change in the internal energy (Δ E) of the reacting system."

I will readily admit that is one topic where I get confused all the time, but I believe this is incorrect. The first law at constant volume says the change in internal energy is equal to the heat transfer. The standard enthalpy of reaction is defined by the heat of reaction at constant pressure (since it is for everything at standard T/P), but the heat of reaction at constant pressure is not necessarily the same as the heat of reaction at constant volume, is it? Tigraan Click here to contact me 19:33, 19 January 2019 (UTC)
 * They differ by the work performed to change the volume i.e. by $$ \Delta H = \Delta E + p\Delta V$$. Ruslik_ Zero  20:51, 19 January 2019 (UTC)

A mundane web search says constant pressure of 1 atm, one bar and a specified temperature, 1 atm, 25 C, usual phase for STP, or 1 M in solution. It might be productive to keep looking, because I just took the top university-looking hits, but I think the OP has the right of it and can cite any of these or some other to make the change suggested. Just intuitively ... the other definition supposes, for example, that the enthalpy change of running a furnace reflects the heat produced when you dispose the exhaust from the furnace as high pressure CO2 and water, which might be ecologically friendly but doesn't seem very common. Wnt (talk) 22:07, 19 January 2019 (UTC)


 * Heat added to (or subtracted from) a solid, a liquid or a gas at constant volume is equal to the change in internal energy. Heat added to (or subtracted from) a gas at constant pressure is equal to the change in enthalpy. Dolphin ( t ) 06:06, 22 January 2019 (UTC)

at witch pressure morphes air to a state that looks like steam ?
I just want to know at wich pressure air morphes into a state where it is compressed so much that it looks like steam — Preceding unsigned comment added by Saludacymbals (talk • contribs) 20:26, 19 January 2019 (UTC)
 * well pure steam looks like air. Do you mean condensed steam that looks like a cloud? This depends on the humidity and whether the compression is adiabatic. If you compress the air and remove the extra heat to bring it back to the same temperature, the condensation will be inversely proportional to humidity. So if humidity is 50%, you will have to compress by a factor of 2. If humidity is 90%, then the volume only has to be reduced to 90% of the original, ie compress by 100/90 or 1.11111.. to make fog. If you compress the air fast, there is no time for heat to be removed and the air holds more water vapour, and it does not cloud up at all. Instead the air has to be expanded or decompressed to cool it so that cloud condensation occurs, that is how clouds are produced. If you compress air by an extremely huge amount it (nitrogen, oxygen, argon) will not liquify, as it is supercritical but at some pressure it will solidify. Graeme Bartlett (talk) 22:05, 19 January 2019 (UTC)

no i meeant this:https://www.youtube.com/watch?v=dKvNmftZ3tU&t=106s and this : https://www.youtube.com/watch?v=LW9gLmDo9t0&t=2s in both videos you can see that air pressure can be so high that the air somehow solidifies and i wonder at wich pressure this occures — Preceding unsigned comment added by Saludacymbals (talk • contribs) 00:20, 20 January 2019 (UTC)
 * Well that first video shows burnouts (burning rubber) and airborne dust, no compressed air visible. The second video shows compressed air driving a turbine. Where you see the condensation that would be due to adiabatic cooling. The air escaping from the turbo is close to atmospheric pressure, but is colder, cold enough to condense water vapour to fog. Graeme Bartlett (talk) 03:04, 20 January 2019 (UTC)

DNA
Why is it that sources will say chimpazees and humans share 98% of DNA and all humans share 99.7% of DNA, but two non-identical human siblings share 50% of their DNA? TFD (talk) 22:47, 19 January 2019 (UTC)
 * It's because, depending on context, those sources are comparing different things.
 * Taking the numbers in your question as being approximately correct, if you sequence the DNA of a chimp and a human and then line up those sequences side by side, 98% of the letters (nucleotide bases: a, c, g, t) in each of those lined-up sequences will match. (I'm not going to get into the precise details of what gets lined up, and the difference between expressed genes and introns, and so forth; we're just going after the general idea.)  If you sequence the DNA of two random humans, they'll match up in 99.7% of those letters.
 * All of human genetic variation, then, is due to those minute differences in that last 0.3%. (I'm again ignoring some ifs, buts, and other assorted approximations, and will continue to do so without additional disclaimers.)  So where does the 50% figure come from?  It comes from the fact that each child gets 50% of their DNA sequence from Mom, and 50% from Dad.  So if you sequence the DNA of Child One and line it up with Mom's DNA, you'll find that most of it lines up pretty well (remember, any two random humans are within about 0.3%), but 50% of it matches exactly.  Same thing when comparing Child One and Dad&mdash;pretty good matches all the way along, but 50% is exactly the same.
 * Now when we look at Child Two, they also get 50% Mom DNA and 50% Dad DNA&mdash;but it's a random assortment. Child Two isn't likely to have gotten the exact same 50% of Mom's DNA, or the exact same 50% of Dad's DNA. On average, though, Child One and Child Two get exact duplicates of the same 50% of their parents' genetic material: 25% of Mom's DNA (50% of 50%) plus 25% of Dad's DNA. So Child One and Child Two will have DNA that lines up very well all the way along – they're both human – but along about 50% of their DNA they'll match perfectly. TenOfAllTrades(talk) 01:50, 20 January 2019 (UTC)
 * Thanks. TFD (talk) 04:50, 20 January 2019 (UTC)
 * Other articles to read are identical by descent, coefficient of relatedness, and Hamilton's rule. The 50% figure for full sibs is the coefficient of relatedness, which enables us to predict/understand when selection will favour helping kin at some cost to the donor. Jmchutchinson (talk) 22:01, 20 January 2019 (UTC)