Wikipedia:Reference desk/Archives/Science/2020 July 7

= July 7 =

Magnetic field intensity: measured in amperes per meter or ampere-turns per meter?
The first reference in the flyback transformer article says the the SI unit for magnetic field intensity is ampere-turns per meter, but the Wikipedia article on magnetic fields says it is amperes per meter. Is the former in error? ZFT (talk) 04:55, 7 July 2020 (UTC)


 * Neither is in error, because neither says that the International System (SI) unit of field intensity for magnetic fields is anything other than Tesla (T).


 * A search of the first reference in the flyback transformer article did not find the word "meter"


 * Our article on Magnetic field says:


 * "In the International System of Units, $H$, magnetic field strength, is measured in the SI base units of ampere per meter (A/m). $B$, magnetic flux density, is measured in tesla (in SI base units: kilogram per second2 per ampere), which is equivalent to newton per meter per ampere. $H$ and $B$ differ in how they account for magnetization. In a vacuum, $B$/$$\mu_0$$ and $H$ are the same; but in a magnetized material, $B$/$$\mu_0$$ and $H$ differ by the magnetization $M$ of the material at that point in the material."


 * --Guy Macon (talk) 05:15, 7 July 2020 (UTC)


 * I mean the first article (Magnetics Design Handbook, Section 1) that is in the "References" section. ZFT (talk) 16:32, 7 July 2020 (UTC)


 * That page says:
 * "Old-time magnetics designers in the U.S. are acclimated to the CGS system, and may prefer magnetics data expressed in Gauss and Oersteds. But newcomers to magnetics design, as well as experienced designers outside the U.S. prefer the internationally accepted SI system - Tesla and Ampere-Turns."
 * The page makes no mention of ampere-turns per meter. That being said, Dixon was not being precise in his language. Ampere-turns do not convert to Gauss.
 * So:
 * Tesla / Gauss measure magnetic flux density,
 * Ampere-turn / Gilbert measure magnetomotive force, and
 * Ampere per meter / Oersted measure magnetization.
 * To confuse things further, there are informal terms. Ampere-turn should actually be Ampere (See Magnetomotive force) but nobody calls it that for obvious reasons. --Guy Macon (talk) 22:37, 7 July 2020 (UTC)


 * Turns is dimensionless and not considered to be a unit, and so ampere-turns and amperes are equivalent as far as SI is concerned. catslash (talk) 22:52, 7 July 2020 (UTC). Not quite the same, but Ohms per square is considered to be the same as Ohms. On the other hand radians are SI units despite being dimensionless, whereas apparently whole cycles are not a unit. catslash (talk) 23:42, 7 July 2020 (UTC)


 * In Table 1 the SI unit for field intensity is A-T/m. Does the "T" stand for "tesla"? ZFT (talk) 18:34, 8 July 2020 (UTC)


 * It's not clear what "Table 1" you refer to, but no, magnetic field intensity is the same thing as magnetic field strength (commonly denoted $H$) and has SI units of A/m. In the context of a coil or winding, the unit is often called ampere-turns per metre, because the total current circulating around the core is the current in the wire times the number of turns.  So "A-T" here clearly means ampere-turns. catslash (talk) 08:33, 9 July 2020 (UTC)

Vent shaft construction method
In this vent shaft build (https://www.youtube.com/watch?v=8xu8d7POPZ4), how is the ground supported as excavation takes place and how is the concrete poured? I don’t see any shuttering. Clover345 (talk) 09:09, 7 July 2020 (UTC)
 * , that link is giving me a "404 not found". {The poster formerly known as 87.81.230.195} 2.122.56.20 (talk) 13:37, 7 July 2020 (UTC)
 * Oops, Wikipedia doesn’t let me post you tube links but it should come up if you copy and paste 8xu8d7POPZ4 into the you tube search bar. Clover345 (talk) 16:59, 7 July 2020 (UTC)
 * Try Timelapse - Fisher Street shaft completion. It's from the Crossrail project in London. Alansplodge (talk) 18:10, 7 July 2020 (UTC)

Cluster containment methodology
How does the cluster containment contact tracing which Japan describes here work? What’s different from the prospective contact tracing also described in the presentation? Clover345 (talk) 10:19, 7 July 2020 (UTC)

Guidance on using a headphone port as line out
I'm aware that there can be a difference between the two: line out ports are low power and expect and infinite load. But, is there any practical issue with using a headphone port as line out? In particular, if a headphone port's all that's available, which components, if any, risk damage?--Leon (talk) 19:58, 7 July 2020 (UTC)


 * [ https://www.sweetwater.com/insync/headphone-outputs-used-line-outputs-for-line-level-gear/ ] --Guy Macon (talk) 20:56, 7 July 2020 (UTC)
 * Okay, thanks!--Leon (talk) 15:29, 8 July 2020 (UTC)