capacity of magnet to stick to hot steel

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Offline bprashant05

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capacity of magnet to stick to hot steel
« on: 01/08/2006 13:33:38 »
Hi everybody,

 please let me know if the temperaure of the steel metal is 100 degree centigrate will the magnet be able stick to the steel as it sticks to it in normal temperature .

Thanks [?]


Offline eric l

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Re: capacity of magnet to stick to hot steel
« Reply #1 on: 01/08/2006 19:25:59 »
I never actually measured it, but I have some experience with magnets applied on heated surfaces, and I noticed that if applied to a hot surface (in the order of 120C) the magnet still sticks, at least for hours.  But the magnets seem to loose at least part of their power over longer periods of time at high temperatures.  For example, if stuck to the outer surface of an oven, the magnet may drop after a week or so.
"Wonder is no wonder" (Simon Stevin 1548-1620)



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Re: capacity of magnet to stick to hot steel
« Reply #2 on: 01/08/2006 20:01:22 »
As far as the magnetism of a permanent magnet is concerned, at higher temperature, the magnet itself will increasingly lose magnetism lose

The Curie point is a term in physics and materials science, named after Pierre Curie (1859-1906), and refers to a characteristic property of a ferromagnetic material.
The Curie point, or Curie temperature, Tc, of a ferromagnetic material, is the temperature above which it loses its characteristic ferromagnetic ability: the ability to possess a net (spontaneous) magnetization in the absence of an external magnetic field.
At temperatures below the Curie point the magnetic moments are partially aligned within magnetic domains in ferromagnetic materials. As the temperature is increased from below the Curie point, thermal fluctuations increasingly destroy this alignment, until the net magnetization becomes zero at and above the Curie point. Above the Curie point, the material is purely paramagnetic.
At temperatures below the Curie point, an applied magnetic field has a paramagnetic effect on the magnetization, but the combination of paramagnetism with ferromagnetism leads to the magnetization following a hysteresis curve with the applied field strength. The destruction of magnetization at the Curie temperature is a second-order phase transition and a critical point where the magnetic susceptibility is theoretically infinite.

Ofcourse, this does not directly answer the question as to whether hot steel will be attracted to a cold magnet, or whether a hot steel plate can be attracted to an electromagnet.

In both cases, my own feeling is that as long as the steel remains solid, there should be no problems with attracting it to a magnet you merely have to be sure that the magnet itself remains cool, or that you use electromagnets rather than permanent magnets.



Offline daveshorts

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Re: capacity of magnet to stick to hot steel
« Reply #3 on: 01/08/2006 20:36:57 »
Hot steel will be attracted to a magnet, definitely at 100°C.

Magnetic fields are produced by electric currents. In some atoms the electrons orbit on average more in one direction than in the other hence producing little circulating electric currents. Normally these are arraged randomly in the material, but in a magnetic field they will tend to align so their S poles point to the N pole of the applied field and therefore are attracted to a magnet, this is called paramagnetism but is quite weak.

In Iron, Nickle, and Cobalt (and various alloys) a much stronger effect is present called ferromagnetism the little atomic magnets will spontaineously align with one another (due to some specific quantum effects to that material). So a lump of iron will have areas where all the atomic magnets are pointing in the same direction which are called domains. These are normally aligned at random but in a field there will be more of them pointing with the field than away from it, and because moving these domains about costs energy the domains will stay aligned even if you remove the field.

If you heat a ferromagnet above it's curie point the tendancy for the atomic magnets to align is overwhelmed by thermal vibrations, and it will revert to being a paramagnet. So steel will allways be attracted to a magnet even when molten, but above it's curie temperature (several hundred °C) not very strongly.

The magnet loosing it's magnetisation in an oven is probably that the thermal energy allows the domains to move around and point in random directions (which is a lower energy state than them all aligned).