1851: Magnetohydrodynamics

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Revision as of 08:29, 16 June 2017 by (talk) (Transcript)
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Magnetohydrodyanmics combines the intuitive nature of Maxwell's equations with the easy solvability of the Navier-Stokes equations. It's so straightforward physicists add "relativistic" or "quantum" just to keep it from getting boring.
Title text: Magnetohydrodyanmics combines the intuitive nature of Maxwell's equations with the easy solvability of the Navier-Stokes equations. It's so straightforward physicists add "relativistic" or "quantum" just to keep it from getting boring.


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In this comic, Ponytail says that the Sun's atmosphere is a super hot plasma controlled by "Magnetohydrodynamics", the study of magnetic properties of electrically conducting fluids. This is true, as the Sun's atmosphere is controlled by the magnetic forces following laws of electrodynamics and also by properties of fluids, which follow laws of hydrodynamics. This subject is probably too difficult for Cueball and he mentally replaces it with "magic" and says, "Ah yes, of course."

The subject of Magnetohydrodynamics is extremely difficult as Maxwell's equations are quite difficult to understand intuitively, and the Navier–Stokes equations are notoriously difficult to solve. (The solutions of Navier–Stokes equations are on the Millennium Problems list) . The title text sarcastically makes fun of this and says that its too boring for physicists so they add "Quantum" or "Relativistic" to it to make it further more difficult.


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[Ponytail explains to Cueball and Megan]
Ponytail: The Sun's atmosphere is a superhot plasma gverned by Magnetohydrodynamic forces...
Cueball: Ah, yes, of course.
[Caption below the panel:]
Whenever I hear the word "Magnetohydrodynamic" my brain just replaces it with "magic".

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Hey, how the hell am I seeing this comic here when I can't see it yet on xkcd??!!?! And I'm not even trying to explain this one, I feel my complete unfamiliarity with Magnetohydrodynamics, except for being able to parse the word, will greatly hinder my ability to sufficiently explain it. :) (And that any explanation should and will include explaining what it is). I'm just rather amused at the concept that he's hearing the word often enough to make a "whenever I..." statement about it, like he's hearing it daily or more. Especially since his clear unfamiliarity suggests he isn't in the field or anything. And hey, leave my comment first this time, huh? LOL! NiceGuy1 (talk) 04:49, 16 June 2017 (UTC)

Figured it out. I last loaded the page with the previous comic before midnight EST, then came here to comment a lot on it, but after midnight, after the comic was posted, the link was still dead, I had to Refresh to get the real link. :) NiceGuy1 (talk) 05:11, 16 June 2017 (UTC)

""...I replace it with "magic" "" magic has 5 letters, one more than the average anglospeaker is able to understand. 13:10, 16 June 2017 (UTC)

Not necessarily. "Troll" has five letters and it's meaning and present applicability are easily understood. These Are Not The Comments You Are Looking For (talk) 01:48, 18 June 2017 (UTC)

Possible interpretation: It could also be that Cueball is simply unable to comprehend such a large word, and so compresses it into it's beginning and ending letters. Therefore, magnetohydrodynamic becomes mag...ic. (talk) (please sign your comments with ~~~~)

Randall is downplaying it. I used to work with magnetohydrodynamics when building systems for studying the solar wind. It's really hard because you've got masses of local and non-local (at sane timescales) interactions and the non-linearities don't cancel out nicely. Doing the same within the sun, especially at the boundary between the radiation zone and the convective zone, would require relativistic quantum magnetohydrodynamics, and that's got like every sort of hard in modern physics. I think I'll stick with simple things like trying to build a true AI. 17:10, 16 June 2017 (UTC)

There is a typo in the caption: "magnetohydrodyanmics" :-) (talk) (please sign your comments with ~~~~)

That's because we have a trivia section.--Dgbrt (talk) 22:10, 16 June 2017 (UTC)
You meant to say "That's why we have a trivia section." :) NiceGuy1 (talk) 04:25, 27 June 2017 (UTC)

"Magnetohydrodynamics". I mean, the basic concept of magnets influencing water doesn't sound so bad, but the word itself sounds like a mad scientist having a stroke. 19:23, 16 June 2017 (UTC)

Hydrodynamics is the study of liquids in motion - not only water. The term hydraulics is an other example not much related to water nowadays. --Dgbrt (talk) 22:10, 16 June 2017 (UTC)
I thought of that after I posted... The fact I didn't go right back and change it and didn't expect to be corrected suggests I forgot where I was, lol. All the same, thank you. 22:16, 16 June 2017 (UTC)

I think he just calls it "magic" for the obvious reasons: the word is very long, and the concept denoted by it is really confusing. 02:05, 17 June 2017 (UTC)

Hydrodynamics is the study of fluids, not just liquids, as it includes solids (though usually plain hydrodynamics or radiation-hydrodynamics is used in explosion simulations, e.g. dynamite or something) as well as liquids and gases (again rad-hydro is used here lots in stellar explosions) and things that are sort of in-between or not really SL or G. Hydrodynamics is regularly treated numerically using simulations, you can't solve it analytically (unless someone claims that prize) so you integrate it bit by tiny bit, loads of times. MHD adds a layer of difficulty as you might expect, it's used to describe interstellar gas where the magnetic bits gets important (e.g. stellar winds, the top few layers of the sun (Solar Vortex Tubes!)), MHD is widely used in the Magnetic Confinement Fusion community to simulate plasma flow/turbulence/horror in Tokamak or Stellerator (and weirder) devices, it's pretty difficult to get right, hence very little fusion yet in these devices. Though it is worth noting that it is actually a simplification of particle kinetic behaviour and that it can't describe a lot of really fast time dynamics, or some of the more odd instabilities, so some poor sods (me sadly) have to use the particle kinetic codes to simulate their plasmas.|20:39, 17 June 2017 (UTC) -- Xoanon (talk) (please sign your comments with ~~~~)

To make things more unpleasant, several of the assumptions MHD uses don't apply to most fusion plasmas (particularly the collisionality).

Yes, there is a typo in the title text: "...-dyanmics".