Wikipedia:Reference desk/Archives/Science/2019 November 16

Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is a transcluded archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

November 16

RF scanners and cards.

I recently had a card blanked out. It wasn't a credit/debit card, but 1 used for public transportation, with a magnetic strip. I recently started a part-time job where I go through security every day, a metal detector and use of RF scanners, like the ones used in airports. Could that interfere with the magnetic strip of cards? 67.175.224.138 (talk) 14:52, 16 November 2019 (UTC).

It's all about distance, but also frequency. In general, no. It is very hard to wipe magnetic tape with either an RF field, or a permanent magnet. This changed somewhat when the far more powerful rare earth magnets appeared. Magnetic key cards in the 1980s often had small magnets embedded in them, but these disappeared from use (in favour of mag stripes or RFID) because they would wipe stripe cards if placed alongside them in a wallet. Your most likely "everyday" field of interest is some piece of power mains equipment, such as a transformer or an electric motor. Some metro trains have been known to do this if you put a bag on the floor above the traction motors on the wheels.

I'd ask around. If the scanner is wiping cards, you won't be the first to suffer. Andy Dingley (talk) 15:20, 16 November 2019 (UTC)

Re: "such as a transformer or an electric motor." Does the electric motor have to be turned on? 67.175.224.138 (talk) 15:45, 16 November 2019 (UTC).

Yes.

TBH, cards are fragile, they fail for many reasons and without walking through your workplace - maybe even carrying a field strength meter - it's impossible to say, "Yes, the coffee machine did it". But RF scanners aren't noted for doing this and the magnetic devices which might do it are only powerful enough to do this at short range.

Sadly I know of no published and citeable work on this, since an IBM paper in the 1960s on the safe transport of magnetic tapes. Which was a little surprising for finding that permanent magnets just didn't have any effect so long as they were outside the carrying case. Andy Dingley (talk) 16:31, 16 November 2019 (UTC)

I've always wandered why the cards slot in wallets are not coated with metal to produce Faraday cage. I made my own from aluminium foil after several cards were wiped out simultaneously. אילן שמעוני (talk) 18:20, 17 November 2019 (UTC)

• A Faraday cage is a shield against electrostatics, somewhat against RF fields. It has almost no effect against a magnetic field. If you make a shield of mu-metal, that would help against magnetism, but it's likely to need to be very thick. Andy Dingley (talk) 18:42, 17 November 2019 (UTC)

(sigh) Thanks. It seems I just have to avoid putting my wallet mindlessly on electric motors. אילן שמעוני (talk) 19:38, 17 November 2019 (UTC)

As alluded to above, physical damage to the card is a more likely culprit. I had a credit card a while back that got scratched up and intermittently started not working when swiped. As it still worked sometimes, the magstripe wasn't blanked. --47.146.63.87 (talk) 06:32, 21 November 2019 (UTC)

CO2 2018 engine cars

On a recent documentary I watched it said that a 2018 petrol engine car on a 35Km journey would usually produce 4.8 Tons of CO2. How many Tons of CO2 would a 2018 diesel engine car on a 35Km journey usually produce? I hope this is the right place to ask. (78.16.154.120 (talk) 17:16, 16 November 2019 (UTC))

• Two things.

First of all, that's nonsense. How much fuel does a car burn in that distance? Maybe a gallon? Now, how do you produce "4.8 tons of CO₂" with just a gallon of fuel? Let's say (wild assumptions, not looking up anything) and that's 5kg of fuel, which we'll call "pure carbon", which is then reacted with twice as many moles of oxygen from the air. I can't remember the atomic masses, so assume they're the same, and now you have 15kg of CO₂ produced. Which is so far from "4.8 tons" that I'm just going to stop there.

Next thing is the actuality. It turns out that cars vary a lot in how much CO₂ they produce. This varies with their engine design, how they're driven, but perhaps most significantly how new they are, and which set of emissions standards they meet. Because that has a large influence on how much they produce, and it has fallen significantly in recent years. This [1] is brief, readable and worth a look.

It's also noteworthy that being diesel or petrol doesn't make a significant difference in CO₂ emissions. It does, but it's dwarfed by the greater reduction imposed by the emission standards. This is why the "terrible, filthy, lying Volkswagens" are actually still some of the cleanest and lowest emission cars around. The emission standards are 130 g/km at present, 95 g/km in a couple of years. About 50% more for light vans. So that's a required emission level of 4.55 kg for your 35 km journey (and I think we can see what the error was now) for a "new" car, reducing to 3.325 kg if you buy a 2021 car. This is the same for petrol or diesel. They might vary, but they both have to get under this limit. Andy Dingley (talk) 17:29, 16 November 2019 (UTC)

• Yes, the strange combo of metric "Km" (abbreviated that way instead of the standard "km") and non-metric "tons" makes me suspect that some unit must be wrong. Does Km mean something like "thousand miles" in this abbrev ?

• What the original poster heard as "tons" may have been metric tons, also known as tonnes. (This does not change the argument that the statement is nonsense if "35Km" does indeed mean 35 kilometers.) --76.69.116.4 (talk) 21:13, 16 November 2019 (UTC)

• Carbon dioxide produced will be proportional to the amount of either gasoline or diesel fuel used, so just look at the MPG to figure out which pollutes more. You may have heard that diesel pollutes more, but that is not carbon dioxide they are talking about, it's other pollutants, like sulfur compounds. Perhaps a chemist can comment on if there is any difference at all between gasoline and diesel, in carbon dioxide production. That is, does a vehicle which uses 20 MPG of gasoline produce the same amount of carbon dioxide as one that gets 20 MPG on diesel ? SinisterLefty (talk) 19:11, 16 November 2019 (UTC)

I've driven a few thousand mile journeys. I've never driven a 35 thousand mile journey. Certainly not on one tank. Andy Dingley (talk) 19:58, 16 November 2019 (UTC)

Yes, it would be quite a journey, something like visiting every city in Eurasia. SinisterLefty (talk) 20:34, 16 November 2019 (UTC)

The Rutan Voyager traveled around the entire Earth without stopping for fuel, and they only made it to 26,366 statute miles. Nimur (talk) 16:51, 18 November 2019 (UTC)

diesel pollutes more

Diesel pollutes less. Also less CO₂. But what it does produce are particulates. Which are the fashionable demons of popular paranoia this week. They're a problem, but also they're fixable by filtration. For NO_x there's AdBlue. Andy Dingley (talk) 20:06, 16 November 2019 (UTC)

How much less carbon dioxide ? As for diesel being clean, before they switched to the low sulfur formulation in the US, the clouds of black smoke pouring from diesel truck engines would make me choke and make my eyes water, so it's not just paranoia. They are better now, but by no means clean. SinisterLefty (talk) 20:36, 16 November 2019 (UTC)

"Clouds of black smoke" has nothing to do with fuel, that's a fault in either the fuel pump or the injectors. Either wear with age, or it's a smal engined Ford car. See rolling coal, BTW.

Sulphur is why Volvos smelled of eggs in 1990 (their particular catalysts, combined with certain fuels like the high-sulphur semi-synthetic VIP petrol in NE England), but that's a petrol issue. Andy Dingley (talk) 20:45, 16 November 2019 (UTC)

I'm talking about big trucks when I say clouds of black smoke. Apparently making them burn cleaner robs them of horsepower, so if laws don't require it, they don't bother. See [2] and note that the Class A (and B ?) trucks in that race put out lots of black smoke. SinisterLefty (talk) 21:04, 16 November 2019 (UTC)

Diesels get a bum rap because diesels are cheap to maintain so cheap people and cheap companies buy diesels. They can also take a harder beating, you'll run a poorly maintained gasoline car to the chopping block in no time. Their advantages also get bigger when it comes to hauling heavy loads. That's why when a transportation company is aching for that extra % in savings (and which one isn't), using legal minimum catalysts, doing poor maintenance and MacGyvering broken parts, the smoke they'll be blowing in your radiator can only come from diesel fuel. 93.136.94.213 (talk) 05:33, 17 November 2019 (UTC)

I feel I must add an important reservation to Andy Dingley answer. While true that diesel engines produce less CO2, they produce way more poisonous particles than gasoline engines. There is a going dispute how to evaluate both pollution types against each other. אילן שמעוני (talk) 18:12, 17 November 2019 (UTC)

Particulate matter is toxic (it certainly is not the "fad of the month" to study that effect and try to alleviate it), but not "poisonous" as they do not act by chemical reaction.

I am not sure what "going dispute" you refer to. Pretty much everyone knows that CO2 emissions are a global problem whereas PM are a local problem - on the one hand, high concentrations of vehicles in densely-populated cities cause a uptick in respiratory diseases even when said vehicles are equipped with countermeasures; on the other hand, container ships all use low-grade diesel with an engine optimized for consumption and not PM minimization, but nobody cares because their emissions gets diluted over large distances before they reach the population. If there is a dispute, it's political (what levels of PM are deemed acceptable in a given city/country), not technical. Tigraan^{Click here to contact me} 10:06, 18 November 2019 (UTC)

The dispute I mentioned is from the public health point of view. Naturaly, you are right to point that ocean going ships are out of this debate. What is within the debate is the classification of emission ratings for cars. If you will insist I will try to fetch a source or three. אילן שמעוני (talk) 15:23, 18 November 2019 (UTC)

The pollution from ships is a concern from multiple POVs. There's the effect on the workers aboard the ships. And there's the effect on ports, especially when they run their engines while in port, to provide electricity and heat. This can be solved by providing electrical cables from the port to the ship, but not all ports are so equipped. Also note that dilution alone doesn't actually solve the problem, since, if it stayed in the air forever, it would just accumulate until it was a danger worldwide. But dilution followed by natural cleaning of the air, from precipitation, for example, can solve the problem, or at least convert it into a problem of water pollution (acid rain and such). SinisterLefty (talk) 16:32, 18 November 2019 (UTC)

• The answer to the question "does diesel get more miles per gallons / kilometers per liters than petrol" is "yes" but it involves three separate effects.

One is explained at Diesel_fuel#Fuel_value_and_price: a gallon (or liter, or...) of diesel is not equivalent to the same volume of gasoline because diesel is more dense. Both fuels have the same heat of combustion in joules per kilogram, but for a given volume diesel contains ~10% more mass hence energy (well, enthalpy to be precise).

The other is the difference in ratio of mechanical work recovered to fuel energy (i.e. the energy efficiency). That is due the technology difference (diesel engine vs. petrol engine have very different combustion processes) which causes efficiency to be higher for diesel than petrol - the most significant difference is due to the higher compression ratio (good article even though lacking in sources), which itself is due to complex considerations (the oversimplified summary is that a short fuel autoignition delay is good for diesel but bad for petrol so petrol needs lower chamber temperatures hence CR). In general diesel has a better energy efficiency than spark-ignition, but the variation within each combustion mode is significant depending on engine speed, load, engine design etc. - a rough order of magnitude would be an average of 35% for petrol and 40% for diesel for 2019 engines.

The third is how much mechanical work you need to push your vehicle a given distance. This depends on vehicle aerodynamics and traffic conditions (which are roughly the same for diesel and petrol cars) as well as vehicle mass (which is a tad higher for diesel, the engine being heavier). Tigraan^{Click here to contact me} 10:30, 18 November 2019 (UTC)

This is quite comprehensive, but misses a small point. Diesel fuel contains on average longer-length carbon chains, and hence a slightly higher ration of C to H in the fuel. So not only is Diesel denser, it also produced a bit more CO2 (and less water), proportionally, than petrol. On the third hand, petrol will be more highly refined, so there may be more CO2 emission during production of the fuel. --Stephan Schulz (talk) 14:31, 18 November 2019 (UTC)

Original poster. The figure is 35 Kilometres. The 4.8 Tons of CO2 was how much was produced making the documentary & the text at the end said it was equivalent to a 2018 petrol engine car for 35 Kilometres. Petrol & diesel are often lumped together & was wondering which was worse for the environment. (78.16.154.120 (talk) 16:20, 18 November 2019 (UTC))

Which one is worse for the environment depends very much on the exact details, and which factors you consider. For CO2, Diesel engines are slightly better than equivalent petrol engines, but this is not a big differences. CO2 emissions cannot reasonably be avoided - burning of fuel is the way the engine works. The other two big emissions considered are usually NOx and small particulates. As written above, these mainly affect the local area, and can cause health issues. Diesel engines normally produce more of both (more NOx because the engine works at higher temperatures and pressures, more particulates because of the higher sulphur and carbon content of the fuel), but there are filter/converter technologies for both, and a Diesel engine with modern exhaust processing is much cleaner than a petrol engine without equivalent equipment. So it really comes down to the emission standards the car was built for. And no, I don't think there is any street-legal vehicle that will produce 4.8 tons of CO2 for 35 km of driving. It would need to burn roughly 2 tons of fuel for that distance. I'm sure there are rocket engines like that, and the Queen Elizabeth-class aircraft carriers seem to burn about 1 ton of fuel per 5km, but neither of these are road-worthy. --Stephan Schulz (talk) 17:29, 18 November 2019 (UTC)

4.8 tons of CO2 is more like 35000 km in any case, and likely a lot more with a new small car engine. I wouldn't trust a documentary that gets things wrong by 3 orders of magnitude. The first thing that should have ticked them off is how you get 5 TONS out of half an hour's drive when your fuel tank holds 30-80 KILOGRAMS of fuel. You can create volume by physical and chemical reactions, but you can't create MASS. 93.136.31.83 (talk) 03:46, 19 November 2019 (UTC)

Combustion products have more mass than the fuel because of the oxygen atoms (for example, ${\ce {C->CO2}}$). But this is only a factor of a few, as someone already discussed earlier in this thread. DMacks (talk) 03:58, 19 November 2019 (UTC)

Indeed, I stand corrected. 93.142.92.186 (talk) 06:18, 20 November 2019 (UTC)

Remote sensing, Terrain mapping and supercomputers

Could a type of computer, like a supercomputer, map terrain without relying on input from a satelite? Furthermore, how powerful would a supercomputer have to be to map out one ace of terrain each nanosecond, creating a slideshow of maps? déhanchements (talk) 22:02, 16 November 2019 (UTC)

Are you asking whether it was possible to make terrain maps (I guess you mean topographic maps) before the deployment of satellites? Yes, the history of cartography indicates that such maps existed long before Sputnik or computers. The 19th century Great Trigonometrical Survey of India is a famous example of such a mapping project. Satellite measurements are just another form of surveying which was traditionally done with ground-based instruments. You'd take a bunch of surveyed measurements and convert them into a map. Computers help but it was historically also done without them. As for mapping one acre per nanosecond (10⁹ acres/sec or about 1.5 million square miles/sec), that depends on the resolution of the map and the measurements, of course. You have to be more clear about what you want. 67.164.113.165 (talk) 22:32, 16 November 2019 (UTC)

The techniques for managing geographic data, that may be obtained from satellites or manual land surveying, are described in the article Geographic_information_system (GIS), particularly about map making. A human typically uses minutes to examine a finished map and the source data is seldom collected in real time but comes from sources (e.g telemetry downlink, photograph scanning or data file reading) that may have low speeds. Computer speed is seldom a limiting factor in matching the source to the viewing requirement (unless a specially complex processing step is required) and large area GIS projects can make use of Parallel computing. The Nanosecond which is about the time for light to travel 1 foot is too short a time unit to be meaningful here.

See also Supercomputers will start building a 3D map of the world. DroneB (talk) 01:30, 17 November 2019 (UTC)

Mapping has become a ideal task for satellites because of the developments in sensors! A "Supercomputer" is idealy suited to do simulations or scan giant data sets for expected and/or unexpected data.

So "Supercomputers" have little to no use for the scanning/collecting geographic data, which will at best become a near real static Interpolation. Satellites can go far beyond because their sensors and filters give them not only the ability to collect real geographic data but other realtime valuable data like temperatures, moisture, movement, etc. continuously, which for example enabled a total revolution for Agricultur, Geology, Archaeology and many other scientific fields - last not least Astronomy. Satellites are the superior scientific tool at the moment. --Kharon (talk) 19:25, 18 November 2019 (UTC)

I'm sure someone like User:Nimur could elaborate further, but I think you're seriously underestimating the amount of processing of the satellite data needs to go on before it tends to be in a useful state. In other words, it's not an either/or equation. Nil Einne (talk) 12:02, 19 November 2019 (UTC)

I'm not sure that the question is very well-formed...

It sounds like there is a formal misunderstanding between the concept of sensing data and processing data. That implies a pretty big knowledge-gap ... so before we start diving in to technicalities, why don't we direct our OP to some basics, like our article section on modern cartography?

We use computers to process geographical data in a lot of different ways. Some of that data comes from satellites - including photographs - but that's not the only source of mapping data. In fact, satellite photography is probably not even the primary source of mapping data in most cases. And finally, the questions about compute-performance are a little too vague to be usefully answered: we don't measure supercomputer performance in "acres per nanosecond" - not even when we're using computers to create maps - because that's a pretty meaningless unit.

Anyway, the best thing for our OP to do will be to read some introductory material, like our articles on cartography and geographical information systems.

Nimur (talk) 20:14, 19 November 2019 (UTC)