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Magnetic Hysteresis

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Dr. Slack

Thu Nov 19 2009, 06:55AM

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

A moving magnetic field will drag any conductor along with it by induction. It works really well with copper and alli armatures because they are good conductors. With water, it will work but with many orders of magnitude less effect. Saline will work better than pure water, conductivity is all.

I would expect that if you set a round bowl of water above a turntable of Neo magnets and spun it fast, and perhaps floated some indicator like squares of tissue paper or lycopdium powder on the water, you may see an effect.

MRI machines are already creating changing fields at which nerve stimulation effects in the brain are caused, with still no mechanical sensation. If you turn the wick up on that, both effects will get stronger.

Proud Mary

Thu Nov 19 2009, 08:03AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

I can see that my opinion that pure water was not affected by magnetic fields was theoretically wrong, and I should have said that water was not significantly affected.

Also, I find it hard to imagine eddy currents in a substance with a dielectric constant of 80, but here again I suppose the shadow falls between the practical and measurable (my usual bias) and more theoretical insights.

It seems a very interesting field of study, and one which I hadn't really thought about till now.

Bored Chemist

Thu Nov 19 2009, 06:26PM

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

Here is a video clip of, essentially, some impure water being affected by a magnetic field.

Since the water concerned is a levitated live frog it's more interesting than you might have expected.

With absolutely stupidly strong (and steeply graded) magnetic fields the effects can be quite significant.

However unless you have access to this sort of field (and I doubt you have) you could only move the water about much by using the eddy current effects.
To do that you need to induce a large changing current in the workpiece.
It's not a good idea to induce large currents in living things.

cduma

Sun Nov 22 2009, 04:18PM

Registered Member #1822 Joined: Fri Nov 21 2008, 08:04PM
Location:
Posts: 300

I see. It seems that no one is sure about this at all.

klugesmith

Sun Nov 22 2009, 06:37PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

cduma wrote ...
I see. It seems that no one is sure about this at all.

Looks like a consensus when -I- read the thread. To deflect water in an easily detectable way by diamagnetic repulsion: trivial. To move water by eddy currents: probably do-able if you work at it. To induce a force you can -feel-, or even a motion that will interest your non-geek friends: not likely.

Re. the question in your OP: magnetic field strength and rotational velocity are equally important. I see two ways to approach it: mechanically spin permanent magnets, or make a rotating field with phased electromagnets (as in induction motors or "brushless DC" motors). The principles applied to compute torque of an induction motor would also apply to water.

I just got a negative result from a little experiment to see if it was real easy.
Dusted off can-spinner #1, which briskly rotates soda cans in the field of a shaded-pole fan motor.

In place of the can I put a plastic cup with a saturated solution of table salt and some stuff floating in it. No apparent rotation of the water after 5 minutes at 20% voltage (normal for can spinning). No motion after 1 minute at 60% voltage (9x stronger force from electrical eddy currents). Hope this helps!

Renesis

Sun Nov 22 2009, 07:14PM

Registered Member #2028 Joined: Mon Mar 16 2009, 08:13PM
Location: Norway
Posts: 319

Klugesmith wrote ...

I just got a negative result from a little experiment to see if it was real easy.
Dusted off can-spinner #1, which briskly rotates soda cans in the field of a shaded-pole fan motor.

Further increasing the voltage would probably fry your windings, but what if you increased the frequency? I presume that you ran these motor guts at 50/60Hz, if it was hooked up to a VFD and driven at lets say 200hz, would that increase the eddycurrents and thus the torque? I would love to see this happen somehow.

klugesmith

Sun Nov 22 2009, 07:28PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Renesis wrote ...
Further increasing the voltage would probably fry your windings, but what if you increased the frequency? I presume that you ran these motor guts at 50/60Hz, if it was hooked up to a VFD and driven at lets say 200hz, would that increase the eddycurrents and thus the torque? I would love to see this happen somehow.

Sorry, got no VFD here. Maybe someone else is set up for that -- don't need to find a motor that matches the diameter of soda cans.

Shaded poles might be optimized for 50/60 Hz, but a capacitor-run motor could have C value changed according to frequency. Of course a 3-phase motor, as used with proper VFD's, would be ideal. Or "brushless DC" motor.

Here's a possibly better way to get large, rapidly changing magnetic fields in water. Take a "disk launcher" based on HV capacitor discharge into a pancake coil. Replace the conducting disk/projectile with a dish of salt water (not actually touching the coil) and see what happens. This would be relatively easy to analyze on paper -- flux coupling could be very good, but induced current so low (because of high resistivity) that primary RLC circuit is not much perturbed by presence of secondary.
Who is reading this and has a disk launcher?

-Rich

cduma

Tue Nov 24 2009, 09:05PM

Registered Member #1822 Joined: Fri Nov 21 2008, 08:04PM
Location:
Posts: 300

I have a 35KV 45uF capacitor bank that I use for shrinking coins. During a 22KV run it was able to knock over a bottle of fresh water. I could easily convert it to a disk launcher and already have the pancake coil.

teravolt

Wed Nov 25 2009, 03:21AM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

water is diamagnetic I thought. do you have a video of this and or was it pushed back by the explosion.

cduma

Tue Dec 01 2009, 02:23AM

Registered Member #1822 Joined: Fri Nov 21 2008, 08:04PM
Location:
Posts: 300

I have no idea. It was on a wooden box that surrounds the +1/16" blast chamber and I doubt the lid was forced up to hit the box and knock over the bottle.

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