Naked Science Forum
General Science => General Science => Topic started by: Mike Berry on 06/02/2011 18:30:03
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Mike Berry asked the Naked Scientists:
Hi
I want to make a small model which levitates using permanent magnets. I have failed dismally.
Is there any static arrangement of that will work. It seems so easy in the mind but seems impossible in practice. The thing always flips over or something.
Does this defy some basic laws of physics or am I missing something.
I see some models doing this but they all use an electromagnet with some sort of sensor providing feedback. Other have the model spinning. I just want it "still".
Mike
What do you think?
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You can't do it statically; it's impossible. You can with an oscillating field or use of moving parts.
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You can't do it statically; it's impossible.
Maybe only slightly impossible.
http://sci-toys.com/scitoys/scitoys/magnets/pyrolytic_graphite.html
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Define "static".
If the electrons in the graphite were not moving it wouldn't work.
Without using paramagnetic materials it's impossible.
I'm told this proves it
http://en.wikipedia.org/wiki/Earnshaw's_theorem
but, I don't understand a word of it.
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Define "static".
If the electrons in the graphite were not moving it wouldn't work.
Without using paramagnetic materials it's impossible.
I'm told this proves it
http://en.wikipedia.org/wiki/Earnshaw's_theorem
but, I don't understand a word of it.
On that basis, nothing is really static.
We'll have to wait for the OP to advise what was meant by "static", but usually it means "without moving parts" (above the atomic level).
I didn't think the question prohibited the use of paramagnetic materials either (which, apparently, have never heard of Earnshaw) although I may have missed that bit.
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There are a number of ways to do it, but the obvious ways that would look like they 'ought' to work, don't due to Earnshaw's theorem.
Check out:
http://en.wikipedia.org/wiki/Magnetic_levitation
The easiest way is probably to put Pyrolytic carbon on neodymium magnets ("diamagnetism") and it will float, but the carbon has to be pure with absolutely no iron contaminants.
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How strong of a magnet do you have....
A researcher has levitated a frog... with a magnet
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fi.ytimg.com%2Fvi%2FA1vyB-O5i6E%2F0.jpg&hash=5cd2e385d17359602aa0bab79acdc719)
But, it requires a very powerful electro-magnet.
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There are a number of ways to do it, but the obvious ways that would look like they 'ought' to work, don't due to Earnshaw's theorem.
Check out:
http://en.wikipedia.org/wiki/Magnetic_levitation
The easiest way is probably to put Pyrolytic carbon on neodymium magnets ("diamagnetism") and it will float, but the carbon has to be pure with absolutely no iron contaminants.
You'll find a nice video in this 'ere post too.
http://www.thenakedscientists.com/forum/index.php?topic=37087.msg343118#msg343118
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The frog thing needs a few megawatts and a huge water cooled electromagnet, it relies on having a stronger field under the frog than above, and the fact that water is slightly diamagnet; it is repelled by water.
You can also levitate using other diamagnetic materials like bismuth; you can use that to stabilise permanent magnets.
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Is there any particular reason why something like this wouldn't work?
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fi198.photobucket.com%2Falbums%2Faa176%2FKryptid%2FMagnets.jpg&hash=6d8c14738152c66f9ca805ab317d0b34)
Take note that this is only a 2D representation. Instead of a circle in a semi-circle, instead imagine that the bar magnets and braces form a sphere instead or a regular polyhedron instead of a circle and that the semi-circle is instead a bowl shape.
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Yes, a hemisphere creates a linear magnetic field, which means it's neither a north, nor south pole (north poles are where the field diverges, south poles are where the field converges- or vice versa I forget), so there's no levitation force, and it's completely unstable.
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Perhaps Wolfe can also comment on the sphere? Am I right in thinking that the magnetic field lines would still have to connect and you in fact end up with a rather dull ball with a small overall dipole caused by the fact that you cannot "balance" the magnetic forces evenly. I think it is close to impossible to create a macro version of the magnetic monopole (which is what that ball would be)
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Yes, that's right.
Uniform spheres have no overall magnetic moment at all, all the north and south poles cancel.
The discrete sphere like shown above will have fields at close ranges, but the spheres will rotate until the north poles and the 'holes' (they're actually really south poles because the field lines converge there) in between line up and then they will suck towards each other until they touch. If the pitch of them is such that they can't all line up simultaneously, then the attraction and repulsion will cancel, and there will be no overall attraction, and they will just fall under gravity.
The maths behind Earnshaw's theorem is only little more than simple calculus. It's actually harder to see why it won't work in particular situations, than why it won't work in the general one.
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To levitate you need to constrain movement in 2 directions, leaving the model free to move only in the z direction. Easiest is by using a glass rod and allowing the magnets to slide on it ( it needs a hole in the model that prevents it from flipping over or a magnet with a hole in it). Easiest to demonstrate using 2 cheap ferrite speaker magnets, on a wooden dowel. They will repel each other when placed correctly (and move together and shatter the other way round). Another method is to use a tensioned wire or string as a substitute for the rod, same caveats apply.
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A diamagnetic material will be repelled by all magnetic fields, whether it is N or S. So, no worry about it flipping. However, they are most effective at very cold temperatures. For example, frozen mercury is supposed to be diamagnetic.
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To levitate you need to constrain movement in 2 directions, leaving the model free to move only in the z direction. Easiest is by using a glass rod and allowing the magnets to slide on it ( it needs a hole in the model that prevents it from flipping over or a magnet with a hole in it). Easiest to demonstrate using 2 cheap ferrite speaker magnets, on a wooden dowel. They will repel each other when placed correctly (and move together and shatter the other way round). Another method is to use a tensioned wire or string as a substitute for the rod, same caveats apply.
If you're using a mechanical constraint for the third dimension, that's 'pseudo levitation'.
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Of course, you could always paint a pingpong ball to look like a monopole and levitate it over an array of magnets with a jet of compressed air shooting up from the centre of the array.
(Pay no attention to the man behind the vacuum cleaner.) [;D]
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Do you have access to liquid nitrogen ? ...
http://en.wikipedia.org/wiki/Meissner_effect
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Bismuth might be easier to get than pyrolytic graphite or liquid nitrogen.
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Bismuth might be easier to get than pyrolytic graphite or liquid nitrogen.
Does graphite become pyrolytic if it has too much to drink?
Kindly mind your own bismuth! [;D]
OK! OK! I was about to leave anyway. There's no need to shove!
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Nice model railway at 0:53 on the video ...
http://www.thenakedscientists.com/HTML/content/kitchenscience/exp/superconducting-levitation/