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Magnetic Hysteresis

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cduma

Wed Nov 18 2009, 03:47PM

Registered Member #1822 Joined: Fri Nov 21 2008, 08:04PM
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I am trying to design a setup with powerful rotating magnets that can cause water to rotate along with it. My eventual goal is to make it powerful enough that the effect can be felt when your hand is placed near it. What would be more important/easily achieved, rotational velocity or field strength?

This would be a great platform for future experiments involving rotating magnetic fields!

Proud Mary

Wed Nov 18 2009, 06:41PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
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Water has some magnetic properties, but is not itself magnetic.

No matter how powerful the magnet, it will not pick up or attract pure water.

But it can alter the rate of spin of protons in the water, forming a very sensitive magnetometer, and many other important uses besides in physics and diagnostic medicine.

Renesis

Wed Nov 18 2009, 07:08PM

Registered Member #2028 Joined: Mon Mar 16 2009, 08:13PM
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Is it not possible that the water may form weak eddycurrents, and make an opposing magnetic field just like in induction motors? I know for a fact that induction motors and Columbus' eggs will work with aluminium rotors/eggs, with is not magnetic, just highly conductive.

Proud Mary

Wed Nov 18 2009, 07:24PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
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I don't know Ren, but I feel certain that if I take a very strong neodymium magentic from the shelf, it will not be able to attract or repel tap water in any way I can detect - beyond the proton spin if I have lots of equipment.

cduma

Wed Nov 18 2009, 07:32PM

Registered Member #1822 Joined: Fri Nov 21 2008, 08:04PM
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When you have a powerful magnetic field IN MOTION near a conductive object it will induce an electric charge which will in turn create an electromagnetic field which will interact with the magnetic field that created it.

IamSmooth

Wed Nov 18 2009, 07:39PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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A magnet can induce and electric current which will itself create a magnetic field. That is correct. See this video:

However, I agree that a powerful magnet will not induce a magnetic field in water. Think about MRI on humans. This is a very powerful field; more powerful than you will be able to produce. If this induced currents in a human, which is +70% water, what do you think would happen to this individual?

ConKbot of Doom

Wed Nov 18 2009, 07:41PM

Registered Member #509 Joined: Sat Feb 10 2007, 07:02AM
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I'm sure if magnets could significantly affect water, we'd have seen 'caterpillar drive' a long time ago :-/

edit:

sooooo it has been built just not very successful.. They seem to indicate that there is current being run though the water, not just the intense field itself... What would be a good electrode for high currents though fresh or salt water? Could you use an AC-excitation, along with an in-phase magnetic field to prevent or at least reduce electrolysys effects?

Proud Mary

Wed Nov 18 2009, 08:10PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
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ConKbot of Doom wrote ...

I'm sure if magnets could significantly affect water, we'd have seen 'caterpillar drive' a long time ago :-/

edit:

Pure water has both a very high dielectric breakdown and dielectric constant.

I'll leave it to someone else to comment on the effect of magnetic fields on saline electrolysis, because I have no clear idea.

Bjørn

Wed Nov 18 2009, 08:26PM

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If the field is strong enough and changing rapidly enough it will affect nerve cells which can be a bad idea.

Water is dimagnetic so it will generate a weak magnetic field of opposite polarity. It is possible to see this if you fill a saucer with water and place a very strong magnet under it. The water will bulge slightly and distort the reflections. If the field is strong enough you should be able to feel it.

Copper coils that are used for dimagtnetism research require several MW of power so it is pretty hard to reach the required strength for the effect to be really interesting.

klugesmith

Wed Nov 18 2009, 10:29PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
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BjÃ¸rn wrote ...
Water is dimagnetic so it will generate a weak magnetic field of opposite polarity. It is possible to see this if you fill a saucer with water and place a very strong magnet under it. The water will bulge slightly and distort the reflections. If the field is strong enough you should be able to feel it.

Yup, here's a picture showing distorted reflection of an overhead light. Ferrite magnets from loudspeakers or magnetrons are strong enough to get a visible effect. For best sensitivity the magnet should be in (not under) the dish, with just enough water that you can get it to cover the magnet.

Ferric chloride solution gives similar effect with opposite sign (it's paramagnetic) and magnitude up to 100x stronger depending on concentration. So the surface is convex instead of concave upwards in places of strong magnetic field gradient. Once I experimented to determine the dilution that would be magnetically neutral.

And aside from diamagnetic/paramagnetic effects, water (like "nonmagnetic" metals) will be dragged along by moving magnetic fields because of eddy currents. The effect is many orders of magnitude weaker than metals, because of higher resistivity. But should be easily detectable (like the diamagnetic repulsion) if water conductivity is enhanced.

Here is a homopolar motor whose rotor is salty water.

[edit] By the way, the thread title is misleading. Nothing discussed so far depends on hysteresis.

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