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Levitating a coil

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Marko

Sun May 14 2006, 09:39PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

It is eddy current repulsion from AC being feed to the coil, care to give more infomation on this TDU? frequency? power?

100VAC, at probably 50Hz and 6,5A (650W).

...

Sun May 14 2006, 11:05PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

Alessandro wrote ...

It not diamagnetism as the most diamagnetic substance on Earth (pyrolytic graphite) can only float mere millimeters above grade N50 neodymium super magnets and barely lifts its own weight.

It is eddy current repulsion from AC being feed to the coil, care to give more infomation on this TDU? frequency? power?

Actually the most diamagnet substance is any of the superconductors... They exhibit enough superdiamagnetism to levatate the magnet above the superconductor

In any case, if we can levate a frog from the diamagnetism of the water? in it...

Maz

Sun May 14 2006, 11:51PM

Registered Member #111 Joined: Fri Feb 10 2006, 01:04AM
Location: Menasha,Wisconsin
Posts: 65

now how plausible would it be to make something that wasnt tethered?

the power supply and a source of batteries on the coil.
this wouldnt do it, but i wonder what could????

G^3

Sun May 14 2006, 11:53PM

Registered Member #97 Joined: Thu Feb 09 2006, 05:40PM
Location:
Posts: 61

Next time you get some liquid nitrogen cool the copper way down. In my physics class there was a demo with a supercold copperplate and a magnet and you could drop the magnet and it would bounce, with out touching the plate, and then slowly settle down. I bet if the copper plate was very cold the coil could lift way over its own weight.

Tesladownunder

Mon May 15 2006, 12:50AM

Registered Member #10 Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424

G^3 wrote ...

.. In my physics class there was a demo with a supercold copperplate and a magnet and you could drop the magnet and it would bounce, with out touching the plate, and then slowly settle down. ...

What you saw in your physics class was a magnet dropped onto a superconductor, not copper. A superconductor repels a magnetic field (perfect conductor = perfect eddy currents to repel). A stationary magnet will not levitate above a stationary plate since no eddy currents are generated.

I found this copper plate in a scrap metal yard and I think it cost some AUD$20 for the copper weight but I knew it would be great for magnetic repulsion stuff.
It is really interesting to watch a NIB magnet slide down this in slow motion when it is held at a 70degree slope just by the action of eddy currents. It takes perhaps 10 seconds to reach the bottom. Similarly if you try to slide the magnet over the copper it gives a lot of rate dependent resistance. This is a real force measured in kg and way above any para/diamagnetic force measured in mg. The simple test is to see if it works with DC (it doesn't).
Although it can lift its own weight there is a huge power requirement which is one of the reasons that we still use wheels on our SUV's. To get 650W out of a battery plus an inverter to give AC would be a challenge if limited to 200g.

I have two of these coils which will fit on the plate. I might try them together which should make them stable in 2 dimensions. I figure only 2 support strings will be needed then which can be the input wires.

A question: My setup was on my table with an iron frame. Would this make a difference through the copper plate in this setup?

Peter

Simon

Mon May 15 2006, 01:14AM

Registered Member #32 Joined: Sat Feb 04 2006, 08:58AM
Location: Australia
Posts: 549

Anyone who's still confused about the physics of this should look up Lenz's Law. It's the simplest way to qualitatively understand this sort of stuff.

EM isn't my best field yet, but my guess to your question, TDU, is that the iron bench won't have a significant effect.

Dr. Slack

Mon May 15 2006, 07:14AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Nice demonstration Peter

The discussion about diamagnetism - with AC, the copper is effectively diamagnetic, the eddy currents induced in it reduce the field in the material. This only works at DC in superconductors.

Perhaps somebody could look up the skin depth for copper at 50Hz, this is the distance through the copper at which the field has reduced to 1/e (my gut feel thinks it's a few mm, how good is my guess?) A sheet of copper several skin depths thick would prevent any significant field penetrating to the other side (this answers the iron bench question)

That 100v x 6.5A is not 650W of course, but 650VA. The actual power dissipated in the coil will be given through the DC resistance of the coil (at operating temperature) by I[sup]2[/sup]R. A measurement of true electrical power consumed at the terminals would give the heating in the copper plate as well as the heating in the coil. Not having a watt meter, I usually make that measurement by substituting various power factor correction capacitors across the input terminals to resonate out the reactive current, until I get a minimum recorded current, which is the resistive current.

How long can you run it before that interesting smell prompts you to turn it off?

Bored Chemist

Mon May 15 2006, 08:03AM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

Just a thought; one way of looking at diamagnetism is that it's the effect of eddy currents on the atomic level. Because (classicly) the electrons are orbiting already the diamagnetic effect still works with DC.
I'd still be interested in knowing the effect of cooling the coil and plate in liquid nitrogen. Dropping the resistance roughly 4-fold should make some difference

ragnar

Mon May 15 2006, 08:27AM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425

Skin depth in copper at 50Hz-60Hz is 8mm-9mm, depending on where you live...

C'mon dude, square-root of all rho on pi times F times mu. =)

stop4stuff

Mon May 15 2006, 10:43AM

Registered Member #64 Joined: Thu Feb 09 2006, 06:25AM
Location: Southampton, UK
Posts: 68

A little bit of research later...

Also works with aluminium

A maglev train uses the same technology - 3 phase electromagnets on the train induces repelling magnetic fields in aluminium tracks, the 3 phase provides a 'magnetic ripple' propelling the train along.

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