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Q & A: water diamagnetism

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Most recent answer: 09/29/2014
Water is attracted to electrostatic charges due to its molecular polarity. Then why does water create a magnetic field to repel an external magnetic field (diamagnetism), when magnetism can be induced by movements of charges? How can water be attracted in one instance, and then repel the other when both are essentially made up of similar stuff? Am I misunderstanding something?
- Harold (age 17)

That's an nice interesting question.

There's no real contradiction between the tendency of water to be attracted by electric fields and (weakly) repelled by magnetic fields. Think of the two physical situations. The electric field may come from some unbalanced charge in some region. The magnetic field may come from a completely neutral region in which the positive and negative charges are moving opposite ways. There's no logical reason why these different physical situations should have to produce the same effect on the water.

Still, even viewed by itself, the diamagnetism is interesting. If you were to picture water as made up of little magnetic dipoles (sort of like how you can picture water as made up of little molecular electric dipoles) then you'd expect it to be attracted to magnetic fields just by analogy to how it's attracted to electric fields. That analogy would be good even if electricity and magnetism had nothing to do with each other.  So that tells us that a classical picture of water as made up of fixed magnetic dipoles has problems. In fact, the diamagnetism is a purely quantum mechanical effect, not describable in these classical terms.

Mike W.

(published on 09/29/2014)

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