Whoo-hoo. This post will take me into the 1000+ category.

I just couldn't stand seeing the counter stay at 999. Back to your regularly scheduled programming before I launch into one of my infamous long-winded explanations. [img]tongue.gif[/img]



Tom / PM

[ April 16, 2003, 11:56 AM: Message edited by: PhysicsMan ]

KeeTov & ExtrVet> "...can't stand the symmetry of 999..."

Nah, I was just worried that someone would turn it upside down and accuse me of having occult interests. [img]smile.gif[/img]
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BobB & others - Thanks for your kind words.

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Rusty and Bonni - Don't worry about getting the lead out... When I woke up this morning, I didn't feel any different now that I'm on this side of the 1000 mark. [img]smile.gif[/img]

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Fox - You always ask "real good ones", don't ya. For the full story, put Feynman, Lectures in Physics v. II on your bedside table for a month or two and browse through it a few pages at a time when the spirit moves you. If you have never looked through this book, its a great read. Altho its supposed to be pitched at 2nd term Physics majors, anyone from a seriously interested layman all the way up to grad students in Physics can read, enjoy, and profit from it.

If you don't want to take this route, try this explanation for size. Imagine a strange country, Foamland, in which every object is surrounded by a sphere of completely transparent foam rubber. Nobody knows why this is so, or who put these spheres in place, but this is just the way it happens to be in these parts.

In Foamland, when one object starts getting close to another they start pushing each other apart - gradually at first, and more forcefully as they get closer together. The best scientists and philosophers in Foamland got together to try to "understand" this. They formulated lots of detailed mathematical rules about how objects in Foamland interact, and can make wonderful predictions about the behavior of objects in Foamland, but they can never answer the question, "why does it *really* work this way".

To a large extent, this is what we are fundamentally faced with in trying to achieve a deep understanding of electromagnitism. You asked, "...where does the energy come from to move the object towards the magnet...". In the mythical country of Foamland, the corresponding question would be, "Where does the energy come from that pushes other objects away?" You know perfectly well that the foam is simply acting like a spring, and if you force one object closer to another you store energy in this spring, but YOU were the one putting the energy into the spring. The spring is just sitting there and can store the energy you are putting in. It doesn't need to have "latent energy" at the start to make it work.

It's pretty much the same thing with E/M. Charged or magnetized objects interact with each other (even in a vacuum) according to well known rules, and if you push one positively charged object towards another positively charged object, they will act much like the objects in Foamland where the foam is completely invisible to the inhabitants, and can only be sensed by its effects on how objects interact. Magnetized objects follow a slightly different set of rules, but the underlying story is still the same: "This is the way it is!".

Around the late 1800's scientists in Foamland started hypothesizing the existence of "foam", an invisible field surrounding all objects. Around the same time, in our universe, scientists started to generate mechanistic concepts like fields, field lines, exchange of virtual particles, etc. to explain the behavior of objects in our universe, but these aren't "reality", anymore than invisible foam is. They are just ways that help us get our heads around such oddball fundamental behavior of objects.

Anyway, to answer your specific questions, Fox:

1) "Where does the energy come from to move the object towards the magnet?" - Ans: From whatever is moving the magnet around. Think of it as a stretched spring that unlike normal springs, gets stronger the closer the objects are to each other.

2) "Does the magnet 'transmit' energy to the object, thus enabling it to move?" - Ans: The magnet transmits a force to the object, almost exactly like a spring would. Does a spring 'transmit' energy? No, it stores and releases energy, and transmits forces.

3) "Does the earth suck?" - Ans: It depends on my mood.

HTH. The world is quite something when you examine it in detail, isn't it?

Tom / PM

[ April 16, 2003, 05:53 PM: Message edited by: PhysicsMan ]

Heh-heH. I'll take care of that, too!

Tom / PM

A big CONGRATS, gal !!! [img]graemlins/thumbsup.gif[/img]

BTW, did you receive your ticket to the free dinner for 1000+ posters yet?
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Funny thing, neither did I, and to make matters worse, I still had to go into work today.

Tom / PM

Magnetism itself does not decay in the sense that if you keep 1 amp running through a loop of wire, it will induce exactly the same amount of magnetic field now as in the future.

OTOH, the fields from magnetic materials (ie, permanent magnets) can decay. This can occur at rates ranging from completely negligible to almost instantaneous. Magnetic materials are composed of a bunch of atoms in which some fraction of of the spins are pointing in the same direction. These spins are constantly being jostled by the everpresent thermal vibrations of the atoms. If a spin polarized atom suffers a big enough hit, its orientation can be changed and the field of the magnet decreases by a small amount. As you increase the temperature this happens more often, and above a certain temperature, the material will not be able to retain "permanent" magnetism at all.

Similarly, if you impress a random external field on a permanent magnet, you can scramble the spins and cause it to become demagnetized. This is exactly how tape head demagnetizers work.

Good question. Hope this helped.

Tom / PM

[ April 28, 2003, 11:53 AM: Message edited by: PhysicsMan ]

> ...Does this mean that any object could be magnetic if its
> atoms were spinning in the right way with stability?
> theoretical e.g. If Nitrogen were solidified, and the atoms
> arranged to spin in the right way, would it display magnetic
> properties?...

Its complicated. Every material in the universe has magnetic properties (ie, "is magnetic"), some are weak magnetic materials, some strong, some permanent, some temporary, some where the fields point in the usual direction, some in the opposite, etc.. So, I'm going to interpret your "...could be magnetic..." phrase the way I think you probably meant it, namely, as "...could be permanent magnets". Then, it gets a bit easier to answer your question.

To be a permanent magnet, not only do the spins of the electrons in each atom have to be aligned with each other, but they must interact with the spins in other atoms in a cooperative way so that when one flips, it tends to take its neighbors with it and they then stay that way (ie, the relaxation time is long and not too much thermal jostling). This puts severe constraints on the density, temperature, microcrystaline structure (ie, definitely not liquid or gas), as well as the atomic composition of the material itself.

So, the bottom line is that being a permanent magnet is a pretty special state of affairs, so that merely solidifying N2, won't do it. Your next statement, "and the atoms arranged to spin the right way...", is the real killer. Usually this sort of "arranging" is in the realm of the guy that directed Bill Cosby (and Noah), "I WANT YOU TO BUILD AN ARK", not us normal mortals (although the situation is changing - [img]tongue.gif[/img] )



Tom / PM

> ...So, magnets do not obey conventional laws of physics?

WTH said that? They operate under some of the most conventional and well tested laws of physics.
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>...divided by the square of the distance between them...

Sorry. The force goes down faster than the square, is dependent on angles, etc. 'Taint the same thing.
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>...chaos, universal gravitation, etc.

Lets talk about that over a beer some day.

Tom / PM

[ April 29, 2003, 09:38 AM: Message edited by: PhysicsMan ]

re Dating, I never actually read that book. Does it go into why he had to leave Cornell so abruptly? [img]graemlins/evilgrin.gif[/img]

P-man, my knowledge of a Magnetic Resonance Imaging machine is that it works by aligning the "spins" of the water molecules and then switching them somehow, getting them to release em waves which can be picked up. I assume that incredibly huge "donut" magnet is what aligns the spins. Ok, here's the questions:

You described it just about perfectly.

Does the donut have a fixed direction magnetic field or does that vary? ... Why does the machine make the "clanking" noise? It sounds like there's a little guy in there with a hammer banging on the magnet. (He's a bit annoying so can't they replace him with electronics?) ...

The main magnet puts out a field which points in one direction but slowly (ie, like over a fraction of a second) increases in time from a low to a high value, and then suddenly drops back to its starting value. This abrupt change in field makes anything magnetic that is in the field (including the wires themselves) suddenly move by a tiny ammt. This mechanical motion is what makes the periodic "clanging" that you hear.

One way to think about how MRI works is to imagine that there are also other quasi steady (lets call it DC) electromagnets of lower strength that cause the overall DC field to be slightly higher at one end vs the other, higher on one side than the other, and higher on the above vs below the patient. The net effect of these "gradient" magnets is that only a very small small region of space is at exactly the correct magnetic field to be tuned into the high frequency RF excitation (which gets the spins lined up - see below).

How do they switch the spins precisely so they get useful info?

They actually don't get them aligned in a static sense, but they start them rotating at a very precise frequency around the axis formed by the big DC field, sort of like a gyroscope with one tip on the ground and the other tip going in circles above that point. They do this by repeatedly pumping radio frequency energy into them at a very precisely defined frequency and duration. When they turn the external source of RF off, the spins keep precessing, and put out their own weak signal. This signal quickly decays with a very signature that is characteristic of the thermal jostling and each spin interacting with its neighbors. The overall frequency tells you what type of atom you are listening to, and its decay over time (ie, microseconds to millisec) tells you what sort of environment that atom is in (eg, aqueous, lipid, etc.).

Then, on top of all of this chemical info is the spatial info. So, if you only want to visualize carbon atoms in an unusual lipid environment, its quite possible to do this.

Since pocket change has almost no ferrous materials in it why do you have to remove it? Are all metals suceptible to being attracted (magnetized?) given a high enough magnetic field?

Even if the coins are not ferromagnetic, they are like little loops of wire, and act like the secondary windings of a bunch of little transformers, wildly perturbing the fields in their vicinity, and royally mucking up the image.

What's the impressive thing about the machine technically? Is it the detector of the em radiation and the processing of it?

Not to be ga-ga over it, but the whole thing impresses the hell out of me.

Did this evolve out of someone sort of playing around in the lab?

Not really. The non-imaging version was used in chemistry labs for decades for sophisticated chemical analysis before somebody thought up the imaging version. It took a major effort to get the first imaging machine up and running, so it definitely wasn't somebody just playing around.

Does the machine and the results support quantum theory or is it "too close to tell" and can be explained with classical yada yada?

Its a fully quantum effect since spins themselves are quantum properties. The basic magnetic resonance effect was first experimentally observed well over a half century ago as part of the fundamental studies of matter which helped quantum mechanics become fully accepted. I'm pretty sure theorists predicted it before the experimentalists set out to look for it.

When the subject is taught in upper level undergrad and grad school to physicists, they get the full, unabridged quantum version. When you are teaching it to chemists, they usually start with a highly watered down classical (ie, "little spinning magnets) approximation of the quantum effects. When you teach it to medical students / medical technologists, they just get the "black box" approach - ie, "here's what it does & its limitations, don't worry about what's on the inside".

HTH,

Tom / PM