Magnets: How do they work?

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lunchstealer
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Magnets: How do they work?

Post by lunchstealer » 09 Feb 2018, 16:04

Questions of science for the Gryllihive.
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lunchstealer
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Re: Magnets: How do they work?

Post by lunchstealer » 09 Feb 2018, 16:20

So my understanding of general relativity is super tiny, but one thing is bothering me.

One of the thought experiments used by Einstein in expanding special relativity into general relativity was the 'photon in an elevator'. The idea is that acceleration due to gravity and acceleration due to propulsion of some form were indistinguishable. If you are in an elevator (or other closed space) and throw a ball, it will travel in a straight line, unless it is under the influence of gravity or if the elevator is being accelerated at a uniform rate. Then a photon would ALSO follow a curved path regardless of whether it was gravity or outside acceleration that did it.

The thought experiment hinges, if I understood it correctly and recall my understanding correctly now, on the fact that it is impossible to distinguish between gravitational acceleration or propulsive acceleration in a non-gravitational space.

However, while it may be that the instantaneous acceleration at any given point cannot be distinguished from gravitation, gravitational acceleration is not constant and linear through any euclidean path. I think maybe there could be a horizontally linear gravitational acceleration due to a slab of uniform density and thickness extending infinitely in all directions, but even then gravity would increase as one approached the slab, so there would be a vertical gradient. So a particle following a curved path in a box under linear thrust would follow a different path than a particle following a curved path due to gravitational acceleration.

So does anyone with familiarity with special relativity (non-Thoreau people are free to chime in) know if it is just the case of the instantaneous acceleration that is being discussed in Einstein's thought experiment? If not, is the fact that gravity always has some spatial rate of change a problem for the thought experiment?
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Aresen
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Re: Magnets: How do they work?

Post by Aresen » 09 Feb 2018, 18:00

The Postulate of Equivalence basically says that there is no way to distinguish between uniform acceleration through the application of an outside force and the force exerted by a uniform gravitational field.

Note the word 'uniform'. Basically, it is an 'ideal' situation, since 'uniform gravitational field' is something that can be described but does not really exist. It's kind of like the "Ideal Gas Laws" that way.

The implication is that inertial mass and gravitational mass are the same thing. (Which kind of rules out those 'inertialess drives' science fiction is so fond of.)
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Re: Magnets: How do they work?

Post by Fin Fang Foom » 09 Feb 2018, 18:29

Magnets don't work. They are lazy bums. They stick together though, which is something to admire, I guess.
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Re: Magnets: How do they work?

Post by thoreau » 09 Feb 2018, 18:43

My understanding of the Principle of Equivalence is that the laws of physics have to work out in such a way so that you cannot distinguish between accelerating frames and gravitational fields via purely local measurements, i.e. measurements conducted in a "small" region of space-time, i.e. a region defined by very small distances (where our criterion for "very small" is "distances much less than (c^2)/g or (c^2)/a, where g or a is the gravitational field or the acceleration of your reference frame, however, you choose to describe it.").

There's nothing in the laws of physics that says that you can't look out the window and notice that there's a giant rock below you (i.e. a planet producing a gravitational field) or that there's a rocket firing (i.e. something causing you to accelerate), but if you only do measurements over infinitesimal distances, over which g is constant, you will not be able to tell whether you are accelerating or in gravity, because the laws of physics have to give the same predictions for both cases.

But I'm really not the guy you want to be posing General Relativity questions to. I get relativistic electromagnetism, but gravity is a whole other beast.
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Re: Magnets: How do they work?

Post by Warren » 09 Feb 2018, 19:05

I think of the Principle of Equivalence as something to be embraced as opposed to thwarted. If you can cleverly find a means of distinguishing between gravity and acceleration, good on you, you're very clever. But for me, the point is to help me understand how the general theory works. If I'm trying to understand how gravity bends space-time in the general theory, I think about how acceleration bends space-time in the special theory.
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Re: Magnets: How do they work?

Post by Aresen » 09 Feb 2018, 19:36

Fin Fang Foom wrote:
09 Feb 2018, 18:29
Magnets don't work. They are lazy bums. They stick together though, which is something to admire, I guess.
Magnets are alt-right?
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Re: Magnets: How do they work?

Post by Sandy » 10 Feb 2018, 16:21

Aresen wrote:
09 Feb 2018, 19:36
Fin Fang Foom wrote:
09 Feb 2018, 18:29
Magnets don't work. They are lazy bums. They stick together though, which is something to admire, I guess.
Magnets are alt-right?
I thought the kids were alt-right.
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thoreau
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Re: Magnets: How do they work?

Post by thoreau » 10 Feb 2018, 16:24

Magnetic fields obey the right-hand rule: Curl your right hand around a wire, with your thumb pointing in the direction of current, and your fingers will wrap in the same direction as the magnetic field.
"ike Wile E. Coyote salivating over a "4000 Ways To Prepare Roadrunner" cookbook without watching his surroundings, the Road Runner of Societal Inertia snuck up on them both and beepbeeped them off the mesa."
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Re: Magnets: How do they work?

Post by Warren » 10 Feb 2018, 16:35

thoreau wrote:
10 Feb 2018, 16:24
Magnetic fields obey the right-hand rule: Curl your right hand around a wire, with your thumb pointing in the direction of current, and your fingers will wrap in the same direction as the magnetic field.
Haw haw haw. He thinks the protons are the charge carriers. The current runs the other way André-Marie. Use the left-hand rule. And you call yourself a physicist.
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thoreau
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Re: Magnets: How do they work?

Post by thoreau » 10 Feb 2018, 16:37

Warren wrote:
10 Feb 2018, 16:35
thoreau wrote:
10 Feb 2018, 16:24
Magnetic fields obey the right-hand rule: Curl your right hand around a wire, with your thumb pointing in the direction of current, and your fingers will wrap in the same direction as the magnetic field.
Haw haw haw. He thinks the protons are the charge carriers. The current runs the other way André-Marie. Use the left-hand rule. And you call yourself a physicist.
I defer to your superior experience as a STEM professional.
"ike Wile E. Coyote salivating over a "4000 Ways To Prepare Roadrunner" cookbook without watching his surroundings, the Road Runner of Societal Inertia snuck up on them both and beepbeeped them off the mesa."
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Andrew
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Re: Magnets: How do they work?

Post by Andrew » 10 Feb 2018, 18:29

A chemical engineering professor of mine, who was nearly 80, told a story about when he was in undergrad. "I was in my first electrical engineering course. They said electron flow was this way, but current was defined as that way. That's how I ended up with a degree in chemical engineering."
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Warren
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Re: Magnets: How do they work?

Post by Warren » 10 Feb 2018, 19:26

Andrew wrote:
10 Feb 2018, 18:29
A chemical engineering professor of mine, who was nearly 80, told a story about when he was in undergrad. "I was in my first electrical engineering course. They said electron flow was this way, but current was defined as that way. That's how I ended up with a degree in chemical engineering."
You can thank Benjamin Franklin for that.
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Kolohe
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Re: Magnets: How do they work?

Post by Kolohe » 10 Feb 2018, 20:57

thoreau wrote:
10 Feb 2018, 16:37
Warren wrote:
10 Feb 2018, 16:35
thoreau wrote:
10 Feb 2018, 16:24
Magnetic fields obey the right-hand rule: Curl your right hand around a wire, with your thumb pointing in the direction of current, and your fingers will wrap in the same direction as the magnetic field.
Haw haw haw. He thinks the protons are the charge carriers. The current runs the other way André-Marie. Use the left-hand rule. And you call yourself a physicist.
I defer to your superior experience as a STEM professional.

Actually i am confused now because EE's believe in pos to neg flow & j is complex while evil fisiks believe in electron flow & i is complex.

Right?

(I think everyone comes together about holes in semiconductors)
when you wake up as the queen of the n=1 kingdom and mount your steed non sequiturius, do you look out upon all you survey and think “damn, it feels good to be a green idea sleeping furiously?" - dhex

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