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If a permanent magnet is dropped into a deep body of fresh water such as Lake Superior, as it descends down, the rising water pressure should continually reduce the size of the magnetic field surrounding the magnet. This is because fresh water is diamagnetic and repels an external magnetic field.

So, I believe that the deeper the magnet descends, the more its magnetic field should be repelled by the ever-increasing pressure of the diamagnetic water molecules pushing against it. Another way of looking at this is that the magnet's magnetic field lines should become more and more compacted together as the magnetic field is compressed by the surrounding water molecules.

Does water pressure have any effect on the size of a permanent magnet's magnetic field?

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  • $\begingroup$ see van.physics.illinois.edu/qa/listing.php?id=368 $\endgroup$
    – user207455
    Commented Jun 17, 2019 at 16:22
  • $\begingroup$ @Solar Mike, thanks for that link. The reason I asked this question was from watching water-magnet interaction videos like this one: youtube.com/watch?v=lTmFjQCPfCg , it made me wonder if a magnetic field would have a hard time penetrating fresh water molecules under high pressure. $\endgroup$
    – user217618
    Commented Jun 17, 2019 at 17:19
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    $\begingroup$ Shouldn't water density and not pressure be the relevant parameter? $\endgroup$
    – Javier
    Commented Jun 17, 2019 at 19:49
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    $\begingroup$ @HRIATEXP The exception being truly deep water like the Marianas Trench, where the very high pressure causes iron to behave as a fluid, which would cause it to lose it's magnetic field. As I remember it, this applies to the deepest 5% of the oceans, and has made exploring them very difficult. $\endgroup$
    – CriglCragl
    Commented Jun 18, 2019 at 0:36
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    $\begingroup$ @CriglCragl - Pretty sure that iron does not lose its room-T ferromagnetism under just 1 kbar of pressure (the approximate pressure at the bottom of the Marianas Trench) and I know that it doesn't behave like a fluid under 1 kbar of pressure at room-T. A pressure of 1 kbar is virtually nothing to most solid metals. The unit cell volume of iron decreases by less than 1% under 1 kbar of pressure. The strength and elastic properties of iron are virtually unchanged under such a pressure. $\endgroup$
    – user93237
    Commented Jun 18, 2019 at 3:52

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Water density at the bottom of the Marianas Trench is 2.7% higher water at RTP. That increased density will have an impact. The presence of any bubbles or voids containing gases, could cause fracturing. The increase in pressure is not enough to alter the grain-structure the ferromagnetism depends on.

In short, water pressure has no direct effect. There are density and some structural considerations.

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