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Q & A: diamagnetism

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Most recent answer: 05/30/2013
Q:
i have heard of the moses effect where a really powerful electromagnet was placed in a tube with water. when the magnet was turned on, all the water moved away from the magnet. i have 2 questions about this 1: if the polarity was reversed, would the water move towards the magnet? and 2: is it the hydrogen in the water, that makes it magnetic? and if so, does that mean everything containing hydrogen is potentially magnetic?
- joel (age 14)
st. catharines ontario canada
A:
Joel-
Things that move away from magnetic fields are called diamagnetic. Many ordinary materials (including water, it sounds like) are weakly diamagnetic. Superconductors are very strongly diamagnetic. To answer your specific questions:

1. No, changing the polarity of the field won't matter. The weak magnetism induced in the water by the magnetic field will also change polarity, and thus will still repel the field.

2. No, it's not specifically the hydrogen that gives this effect. Nearly all materials have some background diamagnetism (it's caused by the way the quantum mechanical states of the electrons are changed by magnetic fields ) but some materials also have stronger magnetic effects which cause them to be pulled into fields (paramagnetism). The paramagnetic effects come from little magnetic parts (often, electrons) which line up in a field to lower their energies, and thus get sucked into the field. In most materials, the electrons are in pairs whose magnetism cancels, eliminating that source of paramagnetism. The hydrogen nuclei (protons) are actually very weakly paramagnetic at room temperature, so they slightly reduce the net diamagnetism.

Mike W.

(published on 10/22/2007)

Follow-Up #1: why ferromagnetism?

Q:
OK. so when i ask someone that knows, "What FUNDAMENTALLY makes an atom/molecule/thing ferro/dia/para magnetic, they always say something along the lines of, "The arrangement of the electrons around blah blah blah" but they were NEVER specific enough to give me a straight forward answer. I mean, if the magnetic dipoles in iron align so easily, why not copper, or water? why are some things so diamagnetic, like quarters (the coin, i forgot what there made of? can you help me out.
- Tristan Johnson (age 13)
Atlanta GA, Mars
A:

This is really a nice question. I can't think of how to answer most of it, but will at least give an excuse for why so many of us have trouble answering.  The excuse is that when there are any spins free to line up, the net interaction between the spins on different atoms really does depend on details. 

One source of that sensitivity to details is that there are two different but similar length scales that matter. One is the typical wavelength of the electrons that are free to line up (the "Fermi wavelength"). The other is the spacing between the atoms. The way the phase of the wave changes from one atom to the next depends on the ratio of those lengths, and is important for whether the electrons can lower their energy by aligning or not. For example, this paper (, abstract is free) describes how in amorphous iron the magnetic state changes qualitatively depending on the precise density, which can be adjusted by applying pressure.

Some parts of your question are easier to answer. In water, each molecule has an even number of electrons. They settle into low-energy wave-functions. Each such "orbital" can hold only two electrons, of opposite spins, because electrons are fermions. No two fermions can be in the same quantum state. The low-energy orbitals fill up, so the spins cancel, leaving no spins free to line up. The weak diamagnetism is a subtle quantum mechanical effect shared by everything which doesn't have unpaired electrons.

Mike W.


(published on 05/30/2013)

Follow-up on this answer.