Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Atomic-S on 26/10/2012 04:34:26
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Consider a dewar flask containing liquid helium resting within the vicinity of a superconducting magnet at a very low temperature. Consider another like dewar flask located in the vicinity of a conventional magnet at room temperature that is producing what, acording to conventional measurements, is a like magnetic field. Will the flasks lose contents at the same rate?
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Helium is diamagnetic, and is only weakly affected by magnetic fields. The magnetic field will not affect its rate of escape from an insulating dewar flask significantly. It does not matter if the magnetic field is produced by a superconductive magnet, a room-temperature electromagnet or a room-temperature permanent magnet.
However, the type of Helium does make a difference:
- The Helium 4 nucleus is a boson, and fairly readily forms a superfluid state, in which it can climb out of open vessels.
- Helium 3 is about a million times rarer, and the nucleus is a fermion; this requires much lower temperatures to form a superfluid. This superfluid is affected by a magnetic field.
However, in all cases, the effect will be the same regardless of the type of magnet producing the magnetic field.
http://en.wikipedia.org/wiki/Helium-3#Cryogenics
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Heat is radiation, magnetism is a 'field'. If I assume a static field by a permanent magnet, no relative motion involved in whatever system, with some receiver of that magnetic field, at absolute rest relative that permanent magnet, what will conduct heat/radiation? If the field involves moving, then it's not static. You have also Heisenberg's uncertainty principle to consider there, and if one takes that one seriously, at what scale do we find a static field?
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Are you perhaps thinking of nuclear fusion experiments using a tokamak torus, where an incredibly hot plasma is held in by a magnetic field? http://en.wikipedia.org/wiki/Tokamak#Toroidal_design
Depending on the fuel mix, this plasma may contain various isotopes of hydrogen and helium.
The plasma cools rapidly on contact with the walls of the torus (and also rapidly erodes the walls of the torus), so the goal is to heat the plasma and also to keep the plasma away from the walls. This is done with the aid of a magnetic field which compresses the plasma.
The magnetic field has almost no impact on neutral Hydrogen and Helium, but once the atoms are ripped apart into positive nuclei and negative electrons, they are restricted to spiral around the magnetic lines of force, which keeps them away from the walls of the torus.
In this application, there are a few differences between superconducting magnets and room-temperature magnets:
- Superconducting magnets are likely to suddenly lose their superconductivity if the magnetic field (and supercurrent) change too rapidly. So superconductive electromagnets are better at producing an intense static magnetic field. They don't have any resistive losses, but the cryogenic refrigeration consumes a lot of power, and the thermal insulation is quite bulky.
- Permanent magnets can only produce a static magnetic field, but usually a lot weaker than a superconductive magnet. On the positive side, they don't have any running expenses.
- Electromagnets are good at producing variable magnetic fields, but lose more power to electrical resistance than superconductive electromagnets
In theory, the results would be identical regardless of how you produced the magnetic field, provided it was of equivalent strength.
However, in practice a variety of economic and engineering tradeoffs suggests a mix of magnet technologies.
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You have also Heisenberg's uncertainty principle to consider there, and if one takes that one seriously, at what scale do we find a static field?
Yes, that may be involved, but the question here would be, is it a function of the temperature of the magnet itself? And even if it is, is the effect on the helium evaporation observable?
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Compromises are possible cooling a "room temperature" magnet to liquid Hydrogen temperatures can reduce the overall power consumption.
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You know Atomic, considering a indivisible universe,same for us all, and also considering entropy. Then considering how a magnetic field is observer dependent according to relativity. What effect would it have if we found that heat indeed was transported according to one observer, but not according to the other as he found no magnetic field to exist?
How would it be explained?
http://en.wikipedia.org/wiki/Electromagnetic_radiation
And
http://physics.stackexchange.com/questions/10745/can-heat-be-transfered-via-magnetic-field-in-a-vacuum
Seems a tricky question you made there.
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Yor_on, a field will change between electric and magnetic components as you change reference frame, so I suspect that if heat is transferred via magnetic field in one reference frame, it would be transferred by electromagnetic fields in another. In addition, for heat to be transferred, you'd almost certainly need to have a field that varied over time, and a time-varying magnetic field automatically creates an electric field component. I doubt there's an issue, but you'd probably have to work out the details to convince yourself.
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In view of the analysis of the problem in terms of agitated charges in the material, one can say that the field cannot be strictly static, and one can also say that the material radiates energy by reason of its temperature. It is not clear to me that these two statements are actually different. So I guess the question that would have to be asked would be, if the flask is placed next to a magnet of a certain temperature, and another like flask is placed next to an unmagnetized body at the same temperature, is there any difference in the heat leakage? If not, the distinction between whether the heat enters via a fluctuating field or via radiation becomes simply semantic.
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Yes JP, it should, shouldn't it?
But it would be very interesting to find some simple observer connected effect that expressed itself without. And I'm looking for it :)
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You can of course have a "magnetic fridge", which is based on a material which absorbs or expels heat when it is placed in or removed from a magnetic field...
http://en.wikipedia.org/wiki/Magnetic_refrigeration
and
http://www.guardian.co.uk/technology/2006/dec/14/energy.insideit
Oh yes, the CamFridge!