Quiz - can mag force cause N side of one PM stick to N side of another (without any help)?

[electricpete]

No funny business of introducing extra metal, extra fields, extra mechanical restraints….

Just two magnets, each with a clearly-defined N and S side....

Are there situations where magnetic force makes the N side of 1 stick to the N side of the other?

Before you shout "No", I'll tell you the answer:

"Yes". It can occur for example when the two magnets are roughly pancaked shape and one is much larger diameter than the other.

http://www.kjmagnetics.com/blog.asp?p=magnets-are-weird

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(2B)+(2B)' ?

 

[electricpete]

Before someone objects: "without help" may have been misleading. It would take some external force to move the magnets into position where the N will attract the N. But after the external force is removed the N remains attracted to the N.

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(2B)+(2B)' ?

 

[MagBen]

Thanks for sharing. Very interesting, although the explanation is not that professional. It is true that the field strength at the center of the big disc magnet is weaker than on the edges, but it doesnot explain where the attraction comes from, and why the attraction is bigger than repulsion.

 

[electricpete]

I thought the flux lines sort of told a story.

The flux density toward the outer edges of the large magnet are smaller than toward the center even before we add the small magnet (because flux from outer edge has shorter distance to travel to reach the opposite face).

When we insert the small magnet in the center, we block the flux in the center even more producing an even lower flux density area in that center. Now that high flux density from the outer edges tends to bulge toward the center. In a fuzzy way, we can think that flux lines want to push things to straighten themselves out, at some point the easiest direction to push the magnet to straighten the pattern reverses.

Perhaps slightly less fuzzy (more rigorous) we can look at energy density. The magnet will be attracted in a direction that minimizes the integral of energy density. Energy density is proportional to flux density squared to we generally focus on highest flux density areas.

Two competing effects as we move the smaller magnet toward the larger magnet:
(we will use orientations shown in the animation):
1 - the flux lines are becoming more crowded immediately above the larger magnet on each side of the small magnet. That is an increase in energy and accounts for repulsion
2 - the flux lines are becoming slightly less crowded in a large volume farther up above the magnets. This is a smaller change in energy density but seems to affect a larger volume. This would be a decrease in energy that could account for attraction.

Which effect is larger determines which direction the magnetic force acts.

The 2nd effect is not relevant until the magnet gets close enough so the flux lines start bulging around behind it. So the 2nd effect doesn't win until the magnet gets very close.

Also geometry of the problem was chosen to enhance the 2nd effect (large diameter disk and big difference in magnet diameters) and to minimize the 1st effect (small thickness of disk).

That's my thinking anyway. I'll be glad to hear if anyone wants to offer a different or revised explanation.


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(2B)+(2B)' ?

 

[electricpete]

The flux density toward the outer edges of the large magnet are smaller is larger than toward the center even before we add the small magnet (because flux from outer edge has shorter distance to travel to reach the opposite face)

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(2B)+(2B)' ?

 

[MagBen]

I see your point but still think there might exist a clearer explanation.
Still, you are smarter than I am 'cause I couldnot come out a better explanation.

 

[MagBen]

I tried to expain this way:
when the small magnet approaches the middle of big disc magnet, the field strenghes of N (top) and S (bottowm) both are decreased, however, the decrease at N is bigger than at S, therefore, the field is larger at S than at N. At some point/distance, the repelling force at N is smaller than the attraction force from the S (bottom), and the small magnet is attracted.
The key is the thickness of the bigger magnet is such small (disc) that attraction from the bottom is larger than the repellation from the top regardless of distance difference.

attration force F1 = kB1^2(1/x1^2+...)
repelling force F2 = kB2^2(1/x2^2+...) (B-flux density, X-distance between two magnets)
since x1-x2 = thicknes of disc magnet is small, a small difference of B1-B2 leads to F1>F2.

 

[electricpete]

I agree my explanation relies too much on the field lines presented and doesn't really answer "why".

I'm stumped at the moment.

I'll think about what you're trying to say.

There are two alternate representations of magnets I'm familiar with.

1-"magnetic charge"on each face of the magnet.
2-current flowing around circumference of the magnet.

The second approach seems to lend better to simple analysis of this participate problem, but fails to prove the result.

Consider force generated by interaction of field from large magnet (large loop) with current from small magnet (small loop) with both loops centered on same axis. Radial component sums to zero by symmetry. Axial component comes from the field from large coil spreading apart as we move up from plane of the coil. To get a downward (attracting) force on the small loop, the field from large loop (alone) would have to compress inward as we move up from plane of large coil. i.e. field lines from large coil alone would have to bulge inward. But that is not how field from coils act.

=====================================
(2B)+(2B)' ?

 

[cloa]

Isn't this a simple explanation- the big magnet wide magnet is strongly attracted to itself and the little magnet can't move perpendicularly away because of this powerful attraction simply overrules it. The magnetic flux for the large magnet have a converging-diverging bubble area that simply makes it potential wall for the small magnet. Not a mutual attraction rather its the dominate effect of the big magnet.

http://www.eng-tips.com/threadminder.cfm?pid=1529

Use translation assistance for Engineers forum

Note the rules include No Student posting

 

[MagBen]

This is a genius description of the phenomenon, but seems still not give the physical reasons.

 

[cloa]

There really aren't physically reasons- we don't know the actual reason for the EM force- only one level down.

http://www.eng-tips.com/threadminder.cfm?pid=1529

Use translation assistance for Engineers forum

Note the rules include No Student posting

 

[FiatfLux]

ElectricPete,

We're assuming equal magnetic properties, correct?

Are you answering your pop quiz due to directly observed phenomenon? Just curious.

I think you were close to the mark with the flux line of thought, and registered today to add a little more.

Assuming equal mag properties, the larger disc will have the higher internal demag field. The smaller magnet having less work to push through all that air, from pole to pole, has a much higher flux density, and lower internal Hd.

Once you force contact between little N [n1] and big N [n2] you are also halfway to a closed circuit. n1's Hd is reduced by being afforded the easier flux path through n2, while n2 essentially suffers just a little more flux leakage. Pending thickness, s2 may even approach a pole flip on the normal curve [before the intrinsic curve anyhow], given the rest of s2 pole surface a much closer neighbor for flux 'travel'.

 

[MagBen]

FiatfLux,
I like the way you explain. Lot of points make sense to me. One point I would like to clarify: Does the pole flip between normal and intrinsic curve mean that the applied field is between Hc and Hci, such that it recoils back when seperating the magnets? How to expain why these two magents still expel at the edge surface of the big magnet? note demag field Hd (=-N.M) is opposite to the direction of its B.

 

[FiatfLux]

MagBen,

If the polarity were to flip local to n1 surface as it contacts n2? I would expect it to be less than Hci, leading to no permanent change in sign, but their may be permanent reduction in net mag moment. Again, assuming magnet 1 and magnet 2 are identical in material properties.

That recoil may be stronger from other portions of n2. Though all of this imagining can get out of hand.


Could you re-phrase your last question, or did I come close to it above?

 

[MagBen]

If pole flipping occurs, why the N-N didnot stick at the edge of big disc, but only in the center

 

[FiatfLux]

Ah,flux has its shortest path [so, higher density] at the edge, perhaps more likely a distribution of force requirement, as in, easier to symmetrically distribute from the middle, than off center.

Not entirely sure though.