Magnets!

[mrbiggs]

Originally Posted by ThatReallyHurt

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...

The book is even better.

Also, Brian Greene has another book about spacetime that is quite good, although not quite as good as The Elegant Universe.

[Soldier]

Originally Posted by HillBilly

no such thing as a wasted post

Now a wasted poster...that's where it gets fun.

[Soldier]

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.

Woot! Mechanical Engineer to the rescue!

The magnets aren't actually holding themselves up; they're being held up by friction. You've got gravity pulling down on your magnet, the magnetic force of the magnet pulling itself against the 'fridge, and friction (a reaction) exerting a force upward, against gravity - equal to it, actually. Please see Exhibit A, and note your coffee cup, held by your invisible hand (since I suck at drawing). Please also note the "aroma" lines coming off the top.

The force of friction is some number (the coefficient of friction, variable muu - basically a "slipperiness" value) times the force applied...this coefficient is always between zero and one. So if your magnet is pulling itself to the 'fridge with a force of, say, 1N, and the coefficient of friction is .64, then the force of friction is .64N upward. Now, gravity is mass * acceleration due to gravity, which happens to be about 9.81 m/s^2 at sea level (about 9.79 around here). So if your magnet has a mass of, say, six grams (.006 kg), then it weighs .006*9.81 Newtons, which is about .059N. This means that since the force of friction upward is greater than the force of gravity (weight) downward, your magnet will stay stuck to the 'fridge in that place.

Unless, of course, your refrigerator is old and rusty and unable to withstand the pressure of 1N across the surface area of the magnet, in which case your magnet would start bending the refrigerator and pulling itself in...but that's another discussion for another time.


The issue of magnets, now, is another entirely. Magnetic and electric fields are inexorably entwined...each creates the other (and in inductors and transformers, they go back and forth many times). Electricity, we pretty much understand - it's the flow of electrons through a material. We understand how it conducts, how it can be held back from conducting, et cetera. Magnetism, not so much. Yes, it's a force field...but it's what's referred to as a "force at a distance". There's nothing actually to a magnetic field. So when a magnet pulls on something far away from it...they're interacting without actually touching. Even when electricity arcs, we know that it's electrons jumping through the air. Nobody understand quite how magnetism works at a distance. Also, if you break a magnet in half, you don't end up with a North end and a South end. You end up with two smaller magnets, each with a North and South end. What's up with that?

Going back to one of your first questions...
Energy isn't mass. The two are related, though (I think we're all familiar with E=MC^2). What this means is that a given mass has a certain amount of energy that could be gained by breaking it down...and, theoretically, that enough energy could be built back into a certain amount of mass. Energy, however, doesn't have mass.

Of course, electricity is classified as energy. But then we're told that electricity is the flow of electrons. And electrons are particles, that should have mass. What's up with that?


Wow...I could have sworn I forgot all that after finals last semester. Guess not...

[Soldier]

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.

Woot! Mechanical Engineer to the rescue!

The magnets aren't actually holding themselves up; they're being held up by friction. You've got gravity pulling down on your magnet, the magnetic force of the magnet pulling itself against the 'fridge, and friction (a reaction) exerting a force upward, against gravity - actualy equal to it. Please see Exhibit A, and note your coffee cup, held by your invisible hand (since I suck at drawing). Please also note the "aroma" lines coming off the top.

The force of friction is some number (the coefficient of friction, variable muu - basically a "slipperiness" value) times the force applied...this coefficient is always between zero and one. So if your magnet is pulling itself to the 'fridge with a force of, say, 1N, and the coefficient of friction is .64, then the force of friction is .64N upward. Now, gravity is mass * acceleration due to gravity, which happens to be about 9.81 m/s^2 at sea level (about 9.79 around here). So if your magnet has a mass of, say, six grams (.006 kg), then it weighs .006*9.81 Newtons, which is about .059N. This means that since the force of friction upward is greater than the force of gravity (weight) downward, your magnet will stay stuck to the 'fridge in that place.

Unless, of course, your refrigerator is old and rusty and unable to withstand the pressure of 1N across the surface area of the magnet, in which case your magnet would start bending the refrigerator and pulling itself in...but that's another discussion for another time.


The issue of magnets, now, is another entirely. Magnetic and electric fields are inexorably entwined...each creates the other (and in inductors and transformers, they go back and forth many times). Electricity, we pretty much understand - it's the flow of electrons through a material. We understand how it conducts, how it can be held back from conducting, et cetera. Magnetism, not so much. Yes, it's a force field...but it's what's referred to as a "force at a distance". There's nothing actually to a magnetic field. So when a magnet pulls on something far away from it...they're interacting without actually touching. Even when electricity arcs, we know that it's electrons jumping through the air. Nobody understand quite how magnetism works at a distance. Also, if you break a magnet in half, you don't end up with a North end and a South end. You end up with two smaller magnets, each with a North and South end. What's up with that?

Going back to one of your first questions...
Energy isn't mass. The two are related, though (I think we're all familiar with E=MC^2). What this means is that a given mass has a certain amount of energy that could be gained by breaking it down...and, theoretically, that enough energy could be built back into a certain amount of mass. Energy, however, doesn't have mass.

Of course, electricity is classified as energy. But then we're told that electricity is the flow of electrons. And electrons are particles, that should have mass. What's up with that?


Wow...I could have sworn I forgot all that after finals last semester. Guess not...

[PeterGustafsson]

Hi!


I have a set of Neodymium-Iron-Boron magnets (Nd-F-B) at home. Those things are so strong so that they can stir water around with their magnetic force. Looks strange. They can also cause thin layers of carbon to levitate. Together with a set of Bismuth blocks, you can get a fairly large one of those magnets to levitate!


Have a nice time!

Peter Gustafsson

[Elemental]

Originally Posted by PeterGustafsson

I have a set of Neodymium-Iron-Boron magnets (Nd-F-B) at home. Those things are so strong so that they can stir water around with their magnetic force.

And to think this whole time I've been stirring my water by hand like a loser

[lochinvar]

Originally Posted by Soldier

Please also note the "aroma" lines coming off the top.

Those are "Consumate Lines of Majesty", like Trogdor the Burninator...

[Have At You]

Originally Posted by PeterGustafsson

Hi!


I have a set of Neodymium-Iron-Boron magnets (Nd-F-B) at home. Those things are so strong so that they can stir water around with their magnetic force. Looks strange. They can also cause thin layers of carbon to levitate. Together with a set of Bismuth blocks, you can get a fairly large one of those magnets to levitate!


Have a nice time!

Peter Gustafsson

Me want! Me want!!

[mrbiggs]

Originally Posted by lochinvar

Those are "Consumate Lines of Majesty", like Trogdor the Burninator...

Actually, those are magnets hovering over the magnetic coffee, which is being developed by the air force.

(By the way, you draw the friction as pulling the magnet up. Yet I'd say that it's just making the magnet stay where it is. In other words, if the gravity were removed, the magnet wouldn't travel up. What am I missing?) ('Cuz I know I'm missing something.)

[Cipher]

Originally Posted by mrbiggs

(By the way, you draw the friction as pulling the magnet up. Yet I'd say that it's just making the magnet stay where it is. In other words, if the gravity were removed, the magnet wouldn't travel up. What am I missing?) ('Cuz I know I'm missing something.)

He drew the friction force up not because it forces anything up, but by the nature of friction it exerts force in an opposite direction (parallel to the movement surface) to the external force (in this case gravity) that is trying to move the object.
You'll learn that in Physics 1 if you take it in college. (or in high school if you're lucky)

[lochinvar]

Originally Posted by Cipher

He drew the friction force up not because it forces anything up, but by the nature of friction it exerts force in an opposite direction (parallel to the movement surface) to the external force (in this case gravity) that is trying to move the object.
You'll learn that in Physics 1 if you take it in college. (or in high school if you're lucky)

"For any action there is an equal, but opposite, reaction." ~Newton

Friction is the reactive force to gravity's active force.
If the active force of gravity is removed, so is the reactive force of the friction.

[mrbiggs]

Originally Posted by Cipher

He drew the friction force up not because it forces anything up, but by the nature of friction it exerts force in an opposite direction (parallel to the movement surface) to the external force (in this case gravity) that is trying to move the object.
You'll learn that in Physics 1 if you take it in college. (or in high school if you're lucky)

I'ma take it in high school, but that's next year.

Thanks for the clarification.

[Cipher]

Originally Posted by scrapinpeg

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?

1. The energy isn't actually associated with the actual magnet. The energy is derived from and exists within the field that is produced by the magnet. This is one of the principle concepts in field theory that must exist in order to preserve a number of other physics concepts, including relativity.

2. What is going on is called ferromagnetism. It is a property of certain substances to produce a magnetic field due to the orbit of electrons around the nuclei of the matter and the "spin" property of those electrons. It is very similar to paramagnetism which arises for the same reasons, but ferromagnetism is generally the stronger type of magnetism. That is because in ferromagnetic structures, the crystalline structure causes the adjacent magnetic moments to align due to coupling interactions, whereas the magnetic moments in paramagnetism are fairly randomly distributed. The magnetic moments are aligned and organized into domains, not all of which will point the same direction, causing many ferromagnetic substances to be normally non-magnetic. When they are exposed to an external magnetic field, they align and then also produce a magnetic field and are attacted to the source of the external field (i.e. nails are not magnetic, but when they are exposed to the field of a permanent magnet, they become magnetic and are thus attracted to the permanent magnet). If those items remain in the field long enough, then the domains will permanently align and the items will become permanent magnets themselves.

To understand why the nails in the example are not attracted to the magnet until they themselves produce a magnetic field, you must understand the idea of recopricity. A substance will not be subject to the forces of a field unless that substance also produces that same kind of field. That is true regardless of the field in question, magnetic, gravitational, electric, etc...

Wow, that was a lot of rambling... I love physics. Hoped that kinda answered the question.

[WhipLash]

Originally Posted by Have at You

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.

beginning? end? i thought that the universe is spherical? and scientists generally agree that the universe is expanding, so for your photon to reach the other side the rate of expansion has to be less than the speed of light right? anyone knows the current estimate on the expansion rate? know u can do the calculations from the red/blue spectral shift of stars, but can't remember much other than that.

oh, and can someone elaborate on the theory that magnetism is caused by the convection currents within the earth's core? does that mean that gas giants don't have magnetic fields?

[Have At You]

Originally Posted by WhipLash

beginning? end? i thought that the universe is spherical? and scientists generally agree that the universe is expanding, so for your photon to reach the other side the rate of expansion has to be less than the speed of light right? anyone knows the current estimate on the expansion rate? know u can do the calculations from the red/blue spectral shift of stars, but can't remember much other than that.

oh, and can someone elaborate on the theory that magnetism is caused by the convection currents within the earth's core? does that mean that gas giants don't have magnetic fields?

For a closed universe, picture for yourself a graph where the Y and Z axes represent length and width (leaving out the third dimension of depth for the sake of clarity, and the left-to-right X axis represents time.

Now picture a sphere graphed on these axes. This is the graph of everything in the entire universe, for its entire existence, that ever was or will be.

The sphere you refer to (or football, or whatever closed shape you prefer) has an extreme left boundary and an extreme right boundary, where the YZ area approaches zero.

These points are the beginning and the end, since they are the extreme boundaries of time.

Hope that clears up at least that part of your question.

[Have At You]

The red shift part of your question arises from a common confusion about what causes it. Popular conception is that it is a Doppler effect, that the stars are moving away and therefore the light appears to have a longer wavelength, like a passing siren gets lower in tone as it recedes.

In fact, there is no Doppler effect to light waves, because they are the ultimate relative phenomenon. That's a big part of relativity.

Which means that no matter how fast or slow the universe expands, light travels from point A to point B at the same relative rate.

[WhipLash]

well it is commonly thought as the doppler effect and is widely published as such in some physics textbook as well as famous books, like steven hawkings. but now i'm confused... i know that light travels the same speed, so what is causing the red/blue shift?

[Have At You]

To answer the last bit of your question, yes the gas giants do have magnetic fields that have been measured by probes.

Barring a rocky core that could produce them, there are several other ways the magnetic fields could be generated.

The most likely way is pretty simple -- at certain pressures, the methanes and other gases become ices which are very good conductors of electricity. Slipping and sliding between layers of these ices, cause by rotation and other movements, would create a powerful dynamo. Which would, in turn, generate a magnetic field.

Intriguingly, the magnetic fields of Uranus and Neptune are off on angles about 60 degrees away from the poles of rotation. Anyone have an idea why that should be? Cause I don't.

[Have At You]

Originally Posted by WhipLash

well it is commonly thought as the doppler effect and is widely published as such in some physics textbook as well as famous books, like steven hawkings. but now i'm confused... i know that light travels the same speed, so what is causing the red/blue shift?

Well, I'm going to bed, but here's a decent web site that explains it somewhat:

http://www.astronomycafe.net/anthol/expan.html