How do permanent magnets generate a magnetic field? It doesn't feel like there's any electricity flowing, my hair doesn't stand up like there's any static, and I don't get any shock from handling the magnet. What's happening?

Prompted by something I found on the internet, I put the polar end of a magnet up against an old crt monitor screen to observe the field. It produced a nice array of small circles, getting larger further away from the magnet. When I rotated the magnet, I expected to see the spots rotate by a similar amount, but they didn't move.

So I wondered if the field lines from a permanent magnet are held in place by a stronger field somehow?

The magnetism comes from the atoms themselves. This was one of the surprises of quantum mechanics, you can have spin, and magnetism, from charges that are not part of any electric currents. In a sense the kinetic energy inside the atom is associated with currents, but they are not classical in nature. Magnetic field lines are not real entities, they are simply the way we imagine drawing lines along the magnetic field direction. The field of a dipole magnet is rotationally symmetric, so you don't notice anything when you rotate the magnet.

Ken G

^Magnetic field lines are not real entities, they are simply the way we imagine drawing lines along the magnetic field direction. The field of a dipole magnet is rotationally symmetric, so you don't notice anything when you rotate the magnet.^

Not real and not even a good visual aid.
Rotationally symmetric yes.The field is better thought of as planes than lines.
On a bar magnet they would be donuts ,enclosing the magnet ,each enclosing the next one nearer to the magnet.No more real than lines but can explain extra things.

So how come if I spread iron filing on a piece of paper above the magnet, they form lines with spaces between. Isn't this indicative of real lines of force or the interference of polarity orientated waves?

So what explains the pattern of circular blobs on the crt?

Try it yourself, but use an old fashioned monitor with a degaussing button!

I tried it with one end of the magnet against the screen, so the 'cross section of the doughnuts' would be passing the plane of the screen.

Each little piece of iron is magnetized and aligned by its interaction with the field produced by the magnet.
The filings form lines because the induced magnetic moment of each piece interacts with its neighbors, forming a sort-of chain where the north pole of one piece is attached to the south pole of the neighboring piece.
The spaces between these chain come from the repulsion of magnetic dipoles which are aligned parallel to each other.

You can work this out by looking up the the field produced by a magnetic dipole.


The Lorentz force on the electrons in the CRT due to the magnet.

You can work out the amount of deflecton from the magnetic field of your magnet, and from the velocity of the electrons (related to the high voltage used in the CRT).

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papageno


"Why waste time learning, when ignorance is instantaneous?" - Hobbes (Calvin and Hobbes)

"It's all about context!" - Vince Noir (The Mighty Boosh)

"I've never heard of such a brutal and shocking injustice that I cared so little about!" - Zapp Brannigan (Futurama)

As papageno pointed out, the answer has more to do with iron filings than it does with "lines of force". The latter is a very useful construction we make in our minds, but the lines you see with the filings are an example of "spontaneous symmetry breaking". You might test how much of it has to do with random characteristics of each trial by marking where the lines are and repeating the experiment-- do they always fall in the same place? I haven't tried it, but I suspect they won't, especially if you hold the magnet to conform to a symmetry (like the way you put it against the CRT screen).

Thanks for the replies, I'll do some more experiments and report back.

Shouldn't this be in Q&A

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'The eye can only see what the mind is prepared to accept'

No, it's a general science question, not an astronomical question.

Fred

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"For shame, gentlemen, pack your evidence a little better against another time."
-- John Dryden, "The Vindication of The Duke of Guise" 1684

I looked up Lorentz force on Wikipedia and was intrigued by the last part of this passage:
Is this implying that it is easier to work out the magnetic forces if they are concieved of as operating within an envoronment which has forces acting at a distance - deus ex machina?

What I have never been able to grasp is why the quantum spin does not get used up as passing electrons have an acceleration (deflection) imparted on them.

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(By the way, I hate it that so many papers in the areas of planetary science and geology are not easily avaiable to the dreaded "non-subscribers". It is like they are screaming at me: "YOU CAN'T HANDLE THE TRUTH". Good, I feel better now.)

I know you are a person who takes his physics seriously, but isn't it said that most great discoveries aren't discovered with "Eureka!" but with, "Hmmm, that's funny." Big Don

You mean, the quantum getting used up a small bit at a time?Now that I think about it, I always thought they were screaming: "GET A JOB"

Move a straightened out paper clip near a magnet.If lines of magnetic force were real then the clip (paramagnetic) would move in jerks as it jumped from one line to the next.

So if there is no differentiation in the field other than a gradual falloff in strength with distance, what causes the bright circular areas of excited electrons in the crt interspersed with non-excited areas?

In your example, once the paperclip overcame the friction of the surface it's sliding across, would it's rapidly increasing momentum not overcome the apparent effect that differentiates lines of iron filings so that you wouldn't be able to spot the variation in the flux strength if there were any?

Whoever wrote that part, he does not know what he is talking about.


He does not understand how systems with many particles are dealt with in physics, and throws around some buzzwords to hide it.


Why should it be used up? It is an intrinsic property of the particle like the mass and the electric charge.

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papageno


"Why waste time learning, when ignorance is instantaneous?" - Hobbes (Calvin and Hobbes)

"It's all about context!" - Vince Noir (The Mighty Boosh)

"I've never heard of such a brutal and shocking injustice that I cared so little about!" - Zapp Brannigan (Futurama)

Magnetic field are vector fields. For a simple bar magnet (dipole) with moment mthe equation for the field is:

B = (m•n - m) / r³

where n is a unit vector from the dipole to the location r and r is the length of r. Now, you can find the field strength and direction, both contained in B. Now we can set certain levels of B, (which is the magnitude of B) and then plot lines of equal magnitude. These turn out to look like what you see when you sprinkle iron filings over a bar magnet (but only a little). Papageno already told that the small filing align themselves with the direction of the field. Now, this field is continuous, and if you put enough iron filings on the plate on top of the bar magnet you will find that the whole thing will be grey, but because of the finite size of the filings you will still see structure.

Now, field lines we draw to get an indication of how the magnetic field is directed and how strong it is. we have to make a choice about at which levels we draw these lines, e.g. at the middle of the bar magnet, where the field is parallel to the bar, we draw lines at every nanoTesla field strength. Then we follow these lines (along the equation given above, do the math and you will see what the exact equation is for a field line of a dipole, R sin(θ) = constant, where θ is the angle with the direction of the dipole) and then we see that the field lines come closer together near the pole of the magnet, indicating that the field is stronger there.

Now, like I say the magnetic field is continuous, it does not mean that there are steps, which are described by the field lines. Therefore there is no need for a paperclip to jump from field line to field line.

In all, field lines just tell us what the direction and what the strength of the field is, which is very useful, in describing e.g. the field of the Earth and what happens when a solar flare happens or when a CME hits the magnetosphere, etc. etc. etc.

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Optimism does not change the laws of physics. (T'Pol)
A good scientist has freed himself of concepts and keeps his mind open to what is. (Dao De Jing 27)
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Martin ( http://www.geocities.com/DrMartinV )

I'm sorry, I think I am being misunderstood. An electron trajectory experiences an acceleration when it passes a dipole closely enough. F=M*A, where does the energy for the F come from is what I do not understand.

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(By the way, I hate it that so many papers in the areas of planetary science and geology are not easily avaiable to the dreaded "non-subscribers". It is like they are screaming at me: "YOU CAN'T HANDLE THE TRUTH". Good, I feel better now.)

I know you are a person who takes his physics seriously, but isn't it said that most great discoveries aren't discovered with "Eureka!" but with, "Hmmm, that's funny." Big Don