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Q & A: magnets under water

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Q:
i have to write an essay about "do magnets work in water" and i did a little experiment to see if they did, and they did...but i need to knw why they worked in water......so please help me!
- taylor (age 13)
Watertown Middle School, MA, USA
A:
That's a nice question. maybe I can turn it around and ask : why wouldn't they work under water?

Basically, the force between two magnets depends on how far apart they are and what angles they're turned at. Of course, if there is something else around exerting forces on the magnets, you might lose track of the direct force between them. That happens if there's, say, a piece of iron between them. They magnetize the iron, so there's now another magnet around. Whether it makes the net force on each magnet bigger or smaller depends on how they are arranged.

Since water is almost completely non-magnetic, it just doesn't make any significant extra magnetic force on the magnets. All you get is the same force there would have been if they were in air or in a vacuum.

Mike W

(republished on 07/13/06)

Follow-Up #1: magnet in water

Q:
what if the magnet is small or large doues it make a difference then?
- david (age 13)
pasadena,CA,USA
A:
not really. The magnetization of the water is too small to make a significant difference in the magnetic forces. However, friction forces with the water can be important.
Mike W.

(published on 03/03/07)

Follow-Up #2: magnets in hot water

Q:
What if the tempature of the water varied? Would it make a difference?
- Andrew (age 12)
Boston, MA, United States
A:
It wouldn’t make much difference because water has very little magnetism whether hot or cold. If the water were hot enough, however, it could cause the ’permanent’ magnets themselves to weaken. That’s because when heated the magnetic domains can rearrange, and they tend to cancel each other out. When very hot, the magnetism of the magnetic material collapses altogether, but for ordinary permanent magnets that takes heating far above the boiling point of water.

Mike W.

(published on 01/30/08)

Follow-Up #3: water magnetism

Q:
First, you said that water was non-magnetic. But, then you said that water was slightly magnetic (or however you worded it). I am very confused, is water magnetic, yes or no?
- Aleasha (age 14)
A:
Actually, what we said was consistent in all three answers. Water is "almost completely non-magnetic," which means the same thing as "too small to make a significant difference"or as "very little magnetism."

There's a nice Wikipedia article on "diamagnetism" http://en.wikipedia.org/wiki/Diamagnetism, which mentions the diamagnetic strength of water. It affects the strength of the magnetic field by about 0.001%, which is a very small effect.

Mike W.

(published on 04/11/08)

Follow-Up #4: diamagnetism

Q:
I don't think I understand what wikipedia is trying to say. Does the diamagnetic property decrease the magnetic strength?
- Augh (age 13)
A:
That's right, the diamagnetic material polarizes a little in the direction that reduces the magnetic field in it. In the most extreme case of diamagnetism, simple (Type I) superconductors completely expel magnetic fields. Around the edges of a diamagnetic region, the field can go up a little- you can picture a few magnetic field lines expelled from the diamagnet piled up outside it. In the case of superconductors, the field outside the superconductor can go up a lot, while the field inside is zero.

Mike W.

(published on 04/19/08)

Follow-Up #5: magnet in water

Q:
OK, I am confused. I know that magnets work under water, but does the water have any impact on the force, and is it visible or is it so small that I shouldn't even worry about it?
- Margarita (age 12)
NY
A:
The effect of the water is too small to notice unless you use very precise instruments. You don't need to worry about it.

Mike W

(published on 05/03/08)

Follow-Up #6: magnets in hot water

Q:
I am doing a experiment in science and i put a magnet in hot water regular water and in cold water. The magnet worked better in hot water then anything else. Except you said that their is nothing well almost nothing magnetized in the water.
- Brynn (age 12)
Vienna,Va,U.S
A:
I'm not sure what "worked better" means.

If you were checking how quickly a paperclip or something  like that moved toward the magnet, it might move more quickly in hot water because that is less viscous (frictiony)  than cold water.

If you were checking how far away the magnet could get and still pick up the clip, then I don't understand the result.

Mike W.

(published on 05/02/10)

Follow-Up #7: Magnets and paper clips in water

Q:
I tried to pick up as many magnets as possible in and out the water. I tried several times, with different magnets, but i kept on getting the same result. I got more clips up in the water. I can't figure out why, but I have been thinking that the clips "weighs" less in the water. Could that be true?
- Camilla (age 16)
Denmark
A:
Brava Camilla!  Good thinking about clips "weighing less" in water.   The point is that the "effective" weight of an object in water is proportional to the density of the object minus the density of water.   In the case of paper clips in water the density of steel is about 8 grams per cubic centimeter and the density of water is about 1.  So the difference is significant; about 1/8 --> 12.5 percent.   
If the density of the object is 1, then the effective weight is zero, and the object floats. You might try to see if your results with the paper clips are consistent with the 12.5% prediction.  Let us know what you find.

LeeH

(published on 06/01/10)

Follow-Up #8: Do conducting solutions affect magnetism?

Q:
I tried to ask my chemistry teacher but he was stumped. I understand that water itself is almost non-magnetic, and conducts almost no current. Also, it is understood that electricity and magnetism are different forms of the same force. Acids conduct electricity extremely well. If I placed highly concentrated acid in a container, with extremely strong magnets on opposite sides, with opposite polarities facing each other, would the acid behave differently?
- Alex Avery (age 15)
Huntsville, Alabama, United States
A:
You're right that electricity and magnetism are aspects of the same fundamental force. Nevertheless, in general placing a good electrical conductor in a magnetic field has little effect on the magnetism. That's certainly true for acids in water, salt solutions, etc.

There is one general effect of conductivity on magnetism. If the magnetism is changing over time then it stirs up eddy currents in the conductor. Those eddy currents create magnetic fields which tend to cancel the changes in the field in the region of the conductor. That's how ordinary metal boxes can shield their contents from high-frequency magnetic fields. However, with the exception of superconductors, this sort of shielding doesn't work for dc fields.

What would happen if you had magnetic ions (e.g. H+or Mn++) in the solution? Even then they wouldn't change the magnetism much, because each one aligns very little in big fields at room temperature. That's true even for Mn++, with an electronic moment, and even more true for H+ with just a nuclear moment. So the magnetic fields are changed very little by these solutes. Very little is not, of course, quite the same as zero. MRI machines work by sensing tiny magnetic changes, primarily in H+ nuclei.

Mike W.

(published on 02/21/11)

Follow-Up #9: small magnets feel the earth's field

Q:
I understand, from your clear explanations, that water doesn't affect the ability of a magnet to attract objects. How big does a magnet have to be to be affected by the Earth's magnetic field? If you are using magnets in the ocean, will they be affected by the magnetic field?
- Claire
A:
I'm not 100% sure what you mean by "affected by the Earth's magnetic field", so I'll interpret the question to mean something I can answer.

You can see  that the small magnets in compasses are affected by the Earth's field. That's what lines them up. How small could a magnet be and still line up pretty well? The magnetic alignment is disrupted by random thermal jiggling, as molecules bounce off the little magnet. In order to line up pretty well, the magnetic alignment energy has to be about as big as the thermal energy scale.  At room temperature that's around 4*10-14 ergs. The magnetic alignment energy is the product of the field and the magnetic moment of the magnet. The Earth's field is around a half a Gauss. That means that you need a magnetic moment of around 10-13 emu. A single electron can contribute about 10-20 emu. So you need about 107 electrons involved, at about one electron per atom for a typical magnet. That's a really tiny magnet, only 10 million atoms.

Who would use such a tiny magnet in the ocean? Why, a magnetotactic bacterium, of course. http://en.wikipedia.org/wiki/Magnetotactic_bacteria   These bacteria use tiny magnets to help orient themselves. The smallest magnetic crystals used are only modestly bigger than the minimum size we calculated.

Mike W.

(published on 03/14/11)

Follow-Up #10: magnets in liquids

Q:
i am doing a science project on how well magnets attract through different liquids, could you give me some information on this subject?
- noah (age 14)
louisville, KY, USA
A:
Could you tell us what info you want, beyond what was in the previous series of answers?

Mike W.

(published on 05/09/11)

Follow-Up #11: Magnetic surface tension in water? Or a layer of baloney?

Q:
http://wiki.answers.com/Q/What_cause_surface_tension_in_water says water molecules are "magnetically bonded to each other".
- Mike W (age 61)
A:
The forces between water molecules have almost no magnetic component. This is an example of a common error- calling all sorts of forces "magnetic" for no particular reason.

Mike W.

(published on 05/24/11)

Follow-Up #12: liquids and magnetism

Q:
Okay, we know how water effects magnetism, but what about other liquids, like acids, bases, or salt water?
- Ben Dolder (age 16)
Dunwoody, GA
A:
The sorts of liquids you mention are just solutions of different things in water. Some of them have weak paramagnetism, meaning that they magnetize very slightly so long as they're in the presence of a magnetic field. For example, MnCl2 in solution has some Mn+2 ions, which are paramagnetic. I doubt if you'd notice this sort of effect with any sort of home measurement.

Some liquids, like liquid oxygen, are rather more strongly paramagnetic. You can easily see it getting pulled into a magnetic field. The reason is that every molecule contributes to the paramagnetism. In those aqueous solutions only a small fraction of the material is contributing.

Mike W.

(published on 11/14/11)

Follow-Up #13: magnetic elements

Q:
This is why i now love smartphones and the internet. Ok.... What other elements besides iron are affected by and can create magnetism? And im not talking about by ionizing or electrifying or inducing... Is it only iron? The answer i get might fuel another question about nuclear fusion, and/or gravity
- Vernon vouga (age 26)
Anchorage ak us
A:

All materials are at least slightly affected by magnetism. When you say "create magnetism" I think you mean ferromagnets, materials that form magnetized domains with appreciable fields at a distance. In addition to iron, there are 3 other elements that form ferromagnets at room temperature: nickel, cobalt, and (depending on how cool the room is) gadolinium. In addition, several others go ferromagnetic at lower temperatures, as described in this link: http://www.periodictable.com/Properties/A/CuriePoint.html.


In addition to these elements, there are countless alloys and compounds that are ferromagnetic.

Mike W.


(published on 06/06/12)

Follow-Up #14: magnets in cold or hot water

Q:
What happens if you put a magnet in cold water? would it be the exact opposite of hot water???
- Caleb (age 11)
FL
A:
No, not really. In cold water, the magnetism changes very little. In hot water, the magnet itself becomes weaker, although the water hardly magnetizes.

If you put a magnet in some really cold fluid (say liquid helium) its magnetism will probably go up just a tiny bit. If you put it in some very hot fluid (maybe very hot steam) it will lose its magnetism altogether.

Mike W.

(published on 04/29/13)

Follow-Up #15: Curie point of dysprosium

Q:
With respect to Answer #13 - dysprosium has a Curie point of around 87 K and is thus not ferromagnetic at room temperature.
- Gareth (age old)
Carpentersville, Illinois, USA
A:

Yipes, thanks for the correction!

Mike W.


(published on 08/10/13)

Follow-up on this answer.