Wikipedia:Reference desk/Archives/Science/2024 January 16

= January 16 =

Total arable land
Does arable land exclude forests? As land where trees can grow, can also be cultivated by farmers?

Land use statistics by country

If USA has so much cultivated land(1,681,826 km sq), then how most Americans are urban citizens?

And Canada has less arable land. Then they have excluded forest area in this list. 2409:40E1:1061:5E7A:99F:C56B:E37A:8102 (talk) 13:12, 16 January 2024 (UTC)


 * Arable land is land that is plowable and can be used for crops. Trees are not considered a crop in this sense.
 * There is ebb and flow, but most people in the United States live in urban and suburban areas. They do not live in rural areas. In urban areas, there is high density housing. In suburban, it is dense with apartments and townhomes, but you will also find a lot of subdivisions with single family homes on small lots.
 * They do exclude forests from arable land. It is not plowed and not available for crops. 12.116.29.106 (talk) 16:52, 16 January 2024 (UTC)


 * Oxford English Dictionary: Arable...L. arabilis, f. ara-re to plough...Capable of being ploughed, fit for tillage; opposed to pasture- or wood-land. — Preceding unsigned comment added by 78.146.96.26 (talk) 17:06, 16 January 2024 (UTC)
 * How do you say land that can grow crop(s) in the soil without importing any of soil, water, light, CO2 or fertilizer but not necessarily without first cutting down trees? Would olive trees count as crops? What about olive trees in non-native places like California? What about Christmas tree farms? At some point it becomes so close to forests virgin and left alone except for being lumberjacked every few decades or so that it doesn't seem like crop anymore. Sagittarian Milky Way (talk) 19:15, 16 January 2024 (UTC)
 * The terminology is used for census and taxes. If the administration in charge of the census and/or taxes wants to consider an olive orchard to be arable, it will be arable. If they don't, it won't. Overall, arable is a subset of agricultural used to designate land prepared for crops. It isn't covered in forest. It isn't all gravel. It isn't under water. It is agricultural land that is ready for crops. My experience is in state census and taxes in Hawaii. There are a lot of rules and waivers for declaring your land to be arable and get a tax discount, which is important because land is very limited in Hawaii, so it is highly taxed, so you want as much of it as possible to be legally arable for a tax discount. 12.116.29.106 (talk) 19:31, 16 January 2024 (UTC)
 * The WP article on arable land gives a precise definition (via FAO) for land use stats. Note in particular: "Data for 'Arable land' are not meant to indicate the amount of land that is potentially cultivable." So no forests. The World Bank data uses the FAO definition. For comparison, the EU definition: "land worked (ploughed or tilled) regularly". (Note this is much more restrictive than the FAO def, which includes pasture etc.) SamuelRiv (talk) 19:38, 16 January 2024 (UTC)
 * I'm a little confused as to your question regarding most Americans being urban given the large amount of cultivated land. If you read the article that you yourself linked to, you'll see that it isn't that much land compared to the total area of the US. Only 17.1% is cultivated, and 82.9% is not. --OuroborosCobra (talk) 20:01, 16 January 2024 (UTC)


 * Arable land is land that is tilled ... by Arabs. (Peter Cook). -- Jack of Oz   [pleasantries]  20:33, 16 January 2024 (UTC)
 * To your question about population, let's take as an example the US state Iowa: population 3.2m, land area 0.14m km2, pop density 22.1/km2. Nearly 90% of the land area is farmland (though most of that is no longer actively farmed) (Des Moines Register 2018-06-29), so we'll use Iowa as a best/worst-case example in this exercise (since Iowa has actually a lot of urbanization). If all of the US's 1.68m km2 of arable land (the rest being either cities, which have relatively little area, or non-arable terrain like mountains and forests, which are mostly uninhabited) had the population density of Iowa, then the US would have 37m people (it actually has almost 336m now). By contrast, just adding the 3 largest US metro areas (NYC, LA, and Chicago) would give about 42m people. You could also compare how many US cities proper you have to add up to be equal (or try if you sort by density instead). Of course, if you excluded Iowa's cities, then the farmland and farm towns won't have a density anywhere near as high as 22/kn2. (Not even in the days before mechanized farming, where large workforces may be needed year-round in some areas. The reason is that there'd be less productivity to sustain an even larger workforce, so large farm towns wouldn't grow or be sustainable.) SamuelRiv (talk) 06:42, 17 January 2024 (UTC)

At what point would a faraday cage stop working?
If you hang a small AM/FM receiver in the middle of a meter-radius sphere and wrap the sphere it wouldn't work anymore, at least if the string's non-conductive and wrap is enough layers of aluminum foil (I don't know if 1 is always enough). If the wave's big enough like 540 AM then if you cut a circular hole in the sphere the hole is much smaller than the wave no matter how big it is but I can't possibly believe a shield with a 180° hole in it would block the signal except possibly if the wave would have to diffract or reflect off the room or something to reach the antenna and probably not even that. So how many degrees wide would the hole have to be before the cage would stop working if the hole was pointed upstream?Sagittarian Milky Way (talk) 18:47, 16 January 2024 (UTC)
 * It would not make any difference which direction the hole was facing, as the electric currents induced in the sphere would flow to where the hole was and some energy could come in. Graeme Bartlett (talk) 21:33, 16 January 2024 (UTC)
 * Do you know why the photons don't just quantum tunnel through Faraday shields and fine metal meshes? Sagittarian Milky Way (talk) 23:38, 16 January 2024 (UTC)
 * A Faraday cage operates when an external electrical field causes the electric charges within the cage's conducting material to redistribute so that they cancel the field's effect in the cage's interior. Full shielding is achieved only by a continuous unbroken cage made of a perfect conductor. That is near impossible to create; aluminium has significant ohmic resistance and any conductor has an effective skin depth that decreases with radio frequency. See the article Faraday cage. The rf power leakage allowed by cages made of metal mesh or perforated material is complex to calculate but the subject is covered in Electromagnetic shielding. There are too many variables for anyone here to calculate for you the actual isolation of a particular spherical cage with a particular size hole cutout at 540 kHz or 555 meters wavelength. Philvoids (talk) 01:54, 17 January 2024 (UTC)
 * So when they say holes much smaller than the wavelength, that's a big oversimplification. Allegedly aluminum foil is conductive enough to block a RFID tag and cellphone reception though. And walls with steel in them sometimes block reception. Since minimum usable signal is non-zero. Sagittarian Milky Way (talk) 03:31, 20 January 2024 (UTC)
 * Any gap in the wall of a Faraday cage allows the enclosed volume to behave as a cavity resonator. Every cavity has numerous resonant frequencies that correspond to electromagnetic field modes (see Standing waves) satisfying necessary boundary conditions on the walls of the cavity. One resonant mode will be at the wavelength equal to the largest internal dimension of the cavity, which we may compare to the air resonance in a Helmholtz resonator, but sound pressure wave patterns give an inadequate analogy to the various electrical and magnetic modes (TEM, TE, TM, HEM) of radio energy. Examples of analysis and practical testing are in the referenced articles. A problem description like "a 180° hole...pointed upstream" is too simplistic for comment. Philvoids (talk) 14:44, 20 January 2024 (UTC)