Magnets!

[scrapinpeg]

So I was standing in my kitchen this morning, in my pre-coffee zombie zone, and found myself wondering what keeps the magnets stuck to the refrigerator door.

Gravity wants them to fall down. But they're holding themselves up.

If that takes energy to do, and energy is mass, then are my magnets using themselves up?

If it doesn't take energy for the little buggers to overcome the force of gravity, then what the heck is going on?

Okay, gonna drink my coffee now. Mmmmm.

[dunastor]

Quantummechanically the first question is are there really magnets at all?

The second question is, is there a fridge door at all?

The third question is, is there a difference between them?

[J.Harris]

My guess is at first there are two forces on the magnet. 1) the weight of the magnet acting straight down and 2) the magnetic force of the magnet acting into the fridge door.
The resulting force would be diagonally down and into the fridge door.

The fridge door would push diagonally up and out against the magnet (equal but opposite reaction) and this is what holds the magnet up.

Now I'll just sit here and wait for someone to tell me how wrong that all was.

[ThatReallyHurt]

Missing one important thing here - friction. If you had a perfectly smooth fridge door and a perfectly smooth magnet, then yes, the magnet would slide down to the edge of the fridge door (but not off, of course).

[InweCiryatan]

I could be very, very wrong, but I heard somewhere that scientists don't know exactly what causes magnetism, but they think it has something to do with electrical currants. Maybe it's a bit like static electricity?

[Epee_Pox]

Wow, there must be physics in the air today. I woke up with a profound feeling of "aha!" this morning, about the nature of photons. (Probably the result of strange dreams brought on by a midnight snack of oreos and scotch.)

My "aha!" was that photons could be considered, not so much things in and of themselves, but rather points of intersection between three fields at right angles to each other -- the X & Y of the electromagnetic field, and the Z axis being gravity.

This would explain how photons appear to behave as massless pointlike particles, while really behaving as waves that somehow are able to interfere with themselves.

Probably wrong, but it's good enough for Monday morning.

[latenight]

You guys are great..............

My first thought this morning was......where are my car keys.

[Have At You]

Originally Posted by Epee_Pox

Wow, there must be physics in the air today. I woke up with a profound feeling of "aha!" this morning, about the nature of photons. (Probably the result of strange dreams brought on by a midnight snack of oreos and scotch.)

My "aha!" was that photons could be considered, not so much things in and of themselves, but rather points of intersection between three fields at right angles to each other -- the X & Y of the electromagnetic field, and the Z axis being gravity.

This would explain how photons appear to behave as massless pointlike particles, while really behaving as waves that somehow are able to interfere with themselves.

Probably wrong, but it's good enough for Monday morning.

Wow, that totally... makes sense.
So it must be wrong. First rule of quantum dynamics.

Photons are great. Emit one at point A at the beginning of the universe, it travels across space till it gets to point B at the end of the universe, and from the perspective of the photon the whole thing happened all at the same time.

Sorry, but I have no idea what the answer to the magnet question is. This is sort of a wasted post, I guess.

[HillBilly]

Originally Posted by Have At You

Wow, that totally... makes sense.
So it must be wrong. First rule of quantum dynamics.

Photons are great. Emit one at point A at the beginning of the universe, it travels across space till it gets to point B at the end of the universe, and from the perspective of the photon the whole thing happened all at the same time.

Sorry, but I have no idea what the answer to the magnet question is. This is sort of a wasted post, I guess.

no such thing as a wasted post

[Capt. Slo-mo]

I'm no longer a current member of the Magnetology Club, but I'm guessing the magnetic bond between the magnet and the iron in the refrigerator is exerting a stronger pull than gravity on the magnet itself. They've established a dialogue something like: "hey dude...you cling that hard and you can stay...just don't try to hold up THREE pictures of Aunt Edna, or it's floor city for all of you!"

Kinda like the arrangement between gravity and the centripetal force on a Direct TV satellite.

A better question--and more direct to Scrapinpeg's question--is whether entropy affects a magnet...as in, does the intensity of the magnetic field tend to diminish over time, thus rendering victory to the patient fridge?

[lochinvar]

Entropy affects all things, including magnets. It just happens very, very slowly...

Magnetic lines of force are set up when the "domains" within magnetized material "line up" in the same orientation. Think of it as little magnets inside the magnetic material that either point randomly or point in the same direction; the strength of the magnetic field is directly dependent on how many of the "domains" line up facing in the same direction. All or most, and the material is magnetized. Few or none, and the material is non-magnetized.

Magnetic domains are, however, subject to misalignment due to physical influence--anyone who's ever dropped a magnet and has seen the subsequent loss of magnetism can attest. Magnetism is vulnerable to impact, momentum, and a host of other physical phenomenon.

Every time you open and close your refrigerator door, every time you take the magnet off and put it back on again--every time you handle it, or handle what it's mounted to, you impart some physical shock or momentum to it. This in turn affects the domains, knocking some small number of them out of alignment; eventually, the magnet will no longer stick, since not enough domains are aligned to make the magnetic field strong enough.

So, in short, magnets do wear out eventually. Some just take longer than others.

[BrianH]

Why don't magnets stick to a stainless steel fridge door?

[Have At You]

Originally Posted by BrianH

Why don't magnets stick to a stainless steel fridge door?

They do, unless it's a particular kind of stainless steel, called "austenitic." All stainless steels, with the exception of the austenitic group, are strongly attracted to a magnet.

Some stainless steels (like those used in solenoid shafts) have the quality of being "magnetically soft" -- their magnetic fields collapse quickly, so the material can reset back to the original position. These tend to have molybdenum in the mix.

The austenitic stainless steels have little magnetic attraction, but only when they're annealed. If they've been cold-worked instead (extruded, ground, rolled, pounded, etc.) they'll still be strongly magnetic.

The reason why the annealed austenitic stainless steels don't attract magnets is because they have a high nickel-to-chromium ratio, which makes a more stable lattice, which is less responsive to magnets.

[Larrison]

Originally Posted by scrapinpeg

So I was standing in my kitchen this morning, in my pre-coffee zombie zone, and found myself wondering what keeps the magnets stuck to the refrigerator door.

Gravity wants them to fall down. But they're holding themselves up.

If that takes energy to do, and energy is mass, then are my magnets using themselves up?

If it doesn't take energy for the little buggers to overcome the force of gravity, then what the heck is going on?

Okay, gonna drink my coffee now. Mmmmm.

Dude! good question -- while I have to confess that when I got up, my first thought was "#$$!! daylight savings time...."

I think the answer is that the magnet is in a local energy minimum. You put energy into the magnet picking it up off the floor and putting it against the fridge door. The magnetic field lines of the magnet meshed with those of the fridge door so that there is a force pulling the magnet towards the fridge. (And there is force on the fridge pulling it towards the magnet too, just that its so much bigger it ain't gonna move...much). The act of bringing the magnet to the door, expended energy -- kinetic and potential energy of course, but also some electromagnet energy as the field lines are transferred into the door. You won't notice these, but there is not a small magnetic field which has spread through the door.

You could feel yourself putting energy into the magnet/ door combination, as you held it up, and moved it over to the door (including the energy you put in, not allowing the magnet to "snap" against the door, marring the paint...) And if you just move the magnet back and forth without touchign the door, since you are manipulating the magnetic moments of the iron atoms in the door, you are actually putting energy into them. (If you do this enough, you can actually heat up the atoms in the door, to the point they get hot -- this is something called hysteresis, and is used in at least one brand of high end cook tops -- to continue the kitchen analogy.)

So, you have put energy in the magnet, ending up with a configuration that has force (down) on the magnet from gravity, force (into the door) from the meshing of magnetic field lines. Through friction, the force into the door, doesn't allow the gravity force to overcome, and pull the magnet down, and you end up with a stable configuration.

[latenight]

Well, I did find my keys................lol

[scrapinpeg]

Excellent! That answers a lot of questions for me.

But I still have one question remaining:

The force from the magnetic field lines... that force is doing work, by pulling the magnet to the metal. So where is the energy coming from, with which to do this work? And why doesn't it run out? (okay, 2 questions)

[epee-girl]

*grabs more Oreos and scotch in hopes that this though very interesting will make sense the drunker I get!*

[mrbiggs]

The magnet thing reminds me of a question I have that I know the answer to, but that I don't care about.

Have you ever gone outside during a rainstrom, and thought "where is all that water coming from"? It's just falling...out of the sky. Gallons and gallons of it, out of the air we're breathing right now. I find that incredibly weird.

(And yes, I know it's caused by the condensation of water around tiny pieces of dust...but still...)

[lochinvar]

Originally Posted by scrapinpeg

Excellent! That answers a lot of questions for me.

But I still have one question remaining:

The force from the magnetic field lines... that force is doing work, by pulling the magnet to the metal. So where is the energy coming from, with which to do this work? And why doesn't it run out? (okay, 2 questions)

Not quite. Work is force applied over distance (W=fs is the formula if you remember your high school physics). Something has to move in order for work to happen.

The magnet is stationary, so no work is being done. You are working if you move the magnet, but the magnet isn't working. The magnetic field holding the magnet to the door is stable potential energy--it neither increases nor decreases unless acted on from the outside.

The relationship--Unified Field Theory--between electicity, magnetism, and gravity has been studied for decades, with no one much closer to explaining it. I'm hoping quantum mechanics will bring us closer.

[ThatReallyHurt]

Originally Posted by lochinvar

Not quite. Work is force applied over distance (W=fs is the formula if you remember your high school physics). Something has to move in order for work to happen.

The magnet is stationary, so no work is being done. You are working if you move the magnet, but the magnet isn't working. The magnetic field holding the magnet to the door is stable potential energy--it neither increases nor decreases unless acted on from the outside.

The relationship--Unified Field Theory--between electicity, magnetism, and gravity has been studied for decades, with no one much closer to explaining it. I'm hoping quantum mechanics will bring us closer.

According to some physicists, quantum theory isn't going to cut it, either - they're banking on string theory.

I don't watch Nova very much at all, but I turned on the TV a little while ago and happened across Nova's presentation of "The Elegant Universe". Very well done, and makes a lot of sense...