A question about magnets

[Anders W. Bonde]

After an unsuccesful (although probably not exhaustive) Google search, I'm left with the following question to display my ignorance and forgetfulness:

If you stack alternatingly oriented permanent magnets (e.g. NS-SN-NS-SN-NS-SN) by clamping them together with a strong clamping device (they would repel each other if not clamped), will they eventually become aligned or "de-gaussed" altogether, and if so, would this depend on whether they were ferrous or rare earth magnets or whatever?

Would an NS-SN-NS stack (i.e. an asymmetrical stack) eventually become a single NS dipole, as it were, and a symmetrical NS-SN-NS-SN eventually simply loose its magnetism altogether?

 

[Just thinking]

Over time, I blieve they would de-gauss all together --- but you can speed the process up by heating them quite a bit.

 

[Soapy Sam]

Dunno- but I bet you have fun finding out.

(Watch the rare earth ones- potent, but brittle, they can shatter if they hit something hard. Wear safety glasses when experimenting).

 

[Ziggurat]

If you stack alternatingly oriented permanent magnets (e.g. NS-SN-NS-SN-NS-SN) by clamping them together with a strong clamping device (they would repel each other if not clamped), will they eventually become aligned or "de-gaussed" altogether, and if so, would this depend on whether they were ferrous or rare earth magnets or whatever?

It would depend on a property of the magnet known as the coercivity, as well as the strength of the field each magnet creates. Ferrous or not, a lot of "hard" magnets can only really be switched (or demagnetized) by applying a magnetic field much larger than the one they produce. In which case, this arrangement would not work to demagnetize them, or at least not within any relevant time scale.

Would an NS-SN-NS stack (i.e. an asymmetrical stack) eventually become a single NS dipole, as it were, and a symmetrical NS-SN-NS-SN eventually simply loose its magnetism altogether?

Assuming that the coercivity is low enough, something like that may indeed happen. If the coercivity is REALLY low, then your arrangement could also just end up magnetizing along the background field of the earth.

 

[Yllanes]

But if you apply a field stronger than the coercitive field of the material, you would magnetise it in the opposite direction. To reliably demagnetise a ferromagnetic material you need to expose it to succesive hysteresis^WP cycles of decreasing amplitude.

 

[Ziggurat]

But if you apply a field stronger than the coercitive field of the material, you would magnetise it in the opposite direction. To reliably demagnetise a ferromagnetic material you need to expose it to succesive hysteresis^WP cycles of decreasing amplitude.

To do it quickly, sure. But that wasn't really the question. My point wasn't really about how to demagnitize it reliably, but more just that you can't really do much at all to a fully magnetized hard ferromagnet (in terms of reversing the polarization OR demagnatizing it) if the field you're using is well below the coercive (not coercitive) field. In such a case, the magnetization will be stable even over very long time periods.

 

[Yllanes]

To do it quickly, sure. But that wasn't really the question. My point wasn't really about how to demagnitize it reliably, but more just that you can't really do much at all to a fully magnetized hard ferromagnet (in terms of reversing the polarization OR demagnatizing it) if the field you're using is well below the coercive (not coercitive) field. In such a case, the magnetization will be stable even over very long time periods.

I agree with that. Actually my post was an excuse to post the link about hysteresis, so that people can see what the coercivity or the remanence mean.

the coercive (not coercitive) field

Thanks for the correction. That was Spanish creeping in. Although I thought 'coercitive', while not the common choice in physics, was also valid.

 

[c186282]

There are many examples of NSNSNSNSNS stacks around that stay
magnetized for long periods. Have you ever noticed that those thin
refrigerator magnets always stick together and never repel each other?
They are actually NSNSNSNS stacks. If you take two of these magnets and
stick them together then slide them apart they may slide smoothly or
snap from spot to spot. If they slid smoothly rotate one of them about
an axis perpendicular to the plane of the magnets and try again. Once
you get them to snap, if you are skilled you can hold them just right
to make them repel. They are made this way because the force that a
magnet will attache to a non-magnetized piece of metal is related to
the gradient in the magnetic field not its absolute strength. A
NSNSNSNS stack will produce a very large magnetic field gradient
without a large field. However, one is more likely to loose these
magnets before they de-magnetize.

An example of a NSNSNSNS stack that has stayed magnetized a long time
is the ocean floor along the Mid-Atlantic Ridge.

see: istp.gsfc.nasa.gov/earthmag/reversal.htm
Sorry I can not post links yet

 

[h.g.Whiz]

would it be hazardous to make an extension cord with a magnetic skin (by the way if anyone patents this. if you dont feel like hooking me up with any of the money that you make from it . can you at least mail me free one)

 

[The Man]

No, I do not believe it would be “hazardous”; however, “pointless” would probably be a more descriptive term for the application you’re considering. What do you think would be the advantage of an extension cord with a magnetic skin, other then possibly enabling it to stick to your refrigerator? For that application the strength of the alternating magnetic fields of the skin combined with the surface area of engagement with the metal surface and the resulting coefficient of friction verses the force on the cord (due to it’s weight) parallel to that metal surface, would determine if the cord remained in place, slid down the surface or fell off. The real hazard would be having the extension cord up against a metal surface, any failure in the insulation and you have a ready made short.

 

[h.g.Whiz]

Magnetic stripping has a tendency to self coil and and is knot resistant

 

[h.g.Whiz]

Ideal properties for an extension cord

 

[The Man]

Magnetic stripping has a tendency to self coil and and is knot resistant

Ok, interesting, but is this due to the magnetic properties of the strip or just the general (non magnetic) material properties of the strip. Personally I prefer my extension cords not to self coil (especially when I’m using them).