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I am thinking about induction. I understand it just
barely,
which makes it fascinating. I am thinking of the
experiment where a magnet falls slowly down the center
of a conductive pipe, slowed by the induced current and
cosequent opposing magnetism.
Everyone knows metals conduct electricity.
But what
about plasma? I think they conduct electricity too. The
handiest plasma is that inside a long fluorescent bulb.
I propose that a plugged in fluorescent bulb could serve
as
the pipe. On placing a ring magnet circumferentially
around one end of a vertical tube bulb and letting it fall,
I
predict that the magnet will fall slower than would be
the
case if the bulb were unplugged.
Maybe the phosphors in the bulb will glow extra as the
magnet passes by?
Related: lamp that works this way
https://en.wikipedi.../Electrodeless_lamp
[notexactly, May 29 2015]
B-H curve
http://www.electron...tic-hysteresis.html
[Ling, Jun 01 2015]
[link]
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Or a glass pipe with hollow walls that the plasma can propagate excitedly and the magnets can drop though the centre of the pipe. I would definitely want to see the visual effect of this. |
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This is a brilliant idea! [+] |
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I suspect that there will be an effect, but the gas can move
with the magnet whereas usually the aluminium is fixed. |
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Would make for more interesting streetlights, and very pictureskew scene with lots of council workers singing shanties at the capstan, while hauling the magnets back up to the top of the 200 ft tube. |
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There's a huge and very important difference between an ionized gas (as found in gas-discharge tubes) and a true plasma ... |
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/gas can move/
So induced eddy currents pull themselves apart? |
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Not sure...I guess the force on the magnet is balanced by the
force on the conductor (gas). |
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.I*I.
.I*I.
.I*I.
(I+I)
.I I.
.I I. |
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I predict that there will be interesting effects on the
tube, much as [bugston] suggests, but the magnet
will drop like a rock. Might be best to try with a dc
HV supply instead of the standard ac mains. |
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A multi-megohm hv resistor could limit the current. Or Use the HB's
favourite hv generator, a van de Graaff. |
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I found a video where a man turns a lit fluorescent
bulb on and off by bringing a magnet close to it. I
suspect, though, that the magnet is interfering with
the action of the magnetic ballast. |
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If the plasma can slow the magnet, then perhaps you've come up with a method of increasing drag when entering an atmosphere. Generate a strong, static magnetic field along the axis of a spacecraft and pull a bunch of plasma along for extra drag. |
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Usually when entering an atmosphere you want to get
rid of the plasma as quickly as possible, so its heat can't
get through the shockwave to your vehicle's surface. But
if you drag on it as it flows beside the vehicle, it might
be okay. |
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Thinking about drag - that is already how a vehicle
slows on re-entry: conversion of kinetic energy to
heat via friction. If one used magnets to increase
the effective profile of the ship by recruiting
plasma, would that decrease heating of the hull? |
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Here is a very easy physics question. A magnet
swings past an iron sheet and slows as it passes.
Does the kinetic energy lost by the magnet turn into
heat of the magnet / iron sheet? |
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[bungston] I saw this really neat braking system on a salt-flats racer that used what you're describing in your question. |
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A set of, I think it was, 4 magnets were attached to a rotor, that was attached to a wheel. If this was a motor it would be considered an axial-flux arrangement. |
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An aluminum plate was brought either in towards the spinning rotor, or away. As the driver brought the plate closer, the magnets would induce current in the plate, creating a large drag on the rotor and heating the plate. This was the racer could brake without any surfaces touching, with a simple and reliable system. |
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I've considered building a mini version of this to prevent kid#1 from riding his trike faster than about 10km/h, but below that speed not feeling much drag. |
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With regards to your question: A magnet passing over a flat plate conductor will induce current and cause heating (I^2R losses). I'm not sure if the permanent magnet will heat as a result of interactions with the eddy currents. From what I've seen, not substantially. An iron conductor will probably behave differently to something non-ferrous. |
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I have heard of induction brakes, I think. I very
much like the lack of friction: nothing to wear out. |
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But the magnet! I know current causes heat which
subs in for friction in the service of entropy. But
what about a plate which is magnetic but
nonconductive? No eddy currents. Does plate or
magnet heat up thru magnetic attraction alone? |
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I think that the heating is caused by induced electrical
currents in the plate, because the parts of the plate
experience a change in magnetic field as they pass by the
magnet. The magnet, however, sees the same magnetic
field all the time. |
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If the plate is made to be magnetic, but not conductive,
then its parts will also experience a change in magnetic
field as they pass the fixed magnet. The point to consider
here is: does the magnetisation and demagnetisation
generate heating effects in the plate? The area in the B-H
curve is a possible measure of the energy involved. |
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// But what about a plate which is magnetic but
nonconductive? No eddy currents. Does plate or magnet
heat up thru magnetic attraction alone? // |
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Like ferrite? Hmm, as [Ling] said, I think. If permanent
magnets: it would certainly cog quite badly. |
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/does the magnetisation and demagnetisation
generate heating effects/ |
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I cannot recall having read about magnets heating
up for any reason other than induction. But if a
swinging magnet or anything else can slow purely
because of magnetic effects I cannot imagine
where else that energy would go. |
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Whenever I start thinking about magnets and their
spooky ways I worry I am on the slippery slope to a
perpetual motion machine. |
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Ling it usually takes me a few posts to catch up
with you. Magnetic hysteresis is the effect
whereby change in magnetic field is turned into
heat. I wonder if hysteresis could be used to heat
a plate, for example to make tea. |
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