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Q & A: Magnetism of iron ions

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Most recent answer: 08/09/2011
Q:
If iron is magnetic and there is iron dissolved in sea water Fe2+ would a magnet not attract it? negative thin metal case on magnet then surely the Fe2+ attracted to the magnet would deposit selectively? does the Fe concentration increase at magnetic poles of earth?
- Bob (age 15)
jersey
A:
Fe++ is usually found in the spin state S=2, which is indeed magnetic. However, even in an extremely large field (one Tesla) the energy of interaction of the spin with the field is only about 1% of the thermal energy scale at room temperature. (The thermal energy scale is roughly the typical amount of energy available to each little magnet or moving particle, just due to thermal jiggling.) That means that the spin is almost randomly aligned, so that its energy is on the average scarcely lowered at all, compared to the thermal energy scale.

So yes, a strong permanent magnet will attract the iron ions, but not enough to be of much use. The earth's magnetic field is under 0.0001 Tesla, so its effect on the ions is really small. To get an effect you need to get thousands of iron molecules together as a single magnetic domain. Magnetotactic bacteria do just that.

Mike W.

(published on 08/09/2011)

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