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Magnetic Field "Blocker"


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#1 d22k

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Posted 4 July 2005 - 02:45 AM

I originally posted this in a different section, but i think its more appropriate here.

I was lying in bed last night when an interesting idea struck me, however, it revolves around 1 important thing; I need material capable of completely blocking a magnetic field.

Bismuth is something tht i have considered, as it is diamagnetic, albeit weakly, but any other suggestions would be greatly appreciated.

Also, is bismuth "complete" in its diamagnetism? i.e will it completely reverse the magnetic field, so even though the reverse field will be weak, there is nothing left of the previous polarity?

i hope i made that clear enough :s

heres a picture to demonstrate

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#2 calbiterol

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Posted 4 July 2005 - 03:21 AM

Are you trying to get a one-sided magnet?
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#3 DQW

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Posted 4 July 2005 - 08:17 AM

The picture you've drawn can not be achieved as proposed...and even something close to what you've drawn can be made only with a superconductor (SC). SCs are your perfectly diamagnetic materials. They have a susceptibility  \chi _m = -1 making  B_{in} = H + M = H + \chi _m H = H ( 1+ \chi _m) = 0

Bismuth, while being a relatively strong diamagnet (compared to other normal diamegnets) has a susceptibility of only about  \chi _m = -10^{-4} , so it will allow 99.99% of the field right through it.
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#4 swansont

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Posted 4 July 2005 - 10:50 AM

You can encase the magnet in mumetal and shield a large fraction of the field that way.
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#5 d22k

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Posted 4 July 2005 - 03:05 PM

yes a 1 sided magnet is exactly what i want.
is it possible to make one with this mumetal? it would seem so, from a quick google.

any ideas on the pricing?

Edit: OMG! i cant afford this!!

http://www.goodfello...AAAD&SPAGE=NI03
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#6 DQW

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Posted 4 July 2005 - 03:16 PM

You can encase the magnet in mumetal and shield a large fraction of the field that way.

This is the traditional method of shielding. However, it does not do the kind of "shielding" the OP is talking about.

Mumetal shields regions outside it by restricting a large fraction of the field to itself. It is an excellent soft ferromagnet with a susceptibility of around 105 (large, positive susceptibility). What the OP wants, is a material that will "completely reverse the magnetic field" - a perfect diamagnet (large, negative susceptibility).

The geometry described in the picture, however, can be achieved (roughly) either way. With a SC, the field lines pass through air (more likely liquid nitrogen or its cold vapor); with mumetal, the field lines pass through the metal. The first is harder to implement (no room temperature solution), but the second restricts the field lines to the inside of a metal.
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#7 d22k

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Posted 4 July 2005 - 03:47 PM

ahh, i dont think i mad emy intentions clear!
the field does not have to be reversed on the right hand side, just so long as it isnt there.

I mentioned diamagnetism because thats the only method i was aware of.

Would mumetal do this, or something like it?

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#8 DQW

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Posted 4 July 2005 - 04:08 PM

If what you are interested in the the weak field in the region indicated, then yes, mumetal will do just that. But a side effect, is that this will also considerably weaken the field on the other side, though not as much as on the side that the mumetal covers.
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#9 d22k

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Posted 4 July 2005 - 04:22 PM

interesting, it MAY work for the purposes i intend.

Are you aware of any cheaper ways to do the same thing. that works out to about $75 per square centimeter :s
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#10 calbiterol

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Posted 4 July 2005 - 04:49 PM

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A Halbach array is probably your best option.

It uses normal magnets, so it can be (more or less) as expensive or inexpensive as you want, according to your power needs, and it augments the field on one side, and pretty much cancels out the field on the other. The link is wikipedia's article on it, and the article also has some good links.
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#11 d22k

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Posted 4 July 2005 - 04:53 PM

wow! thats absolutely perfect!
thanks alot, i will let you know how the project progresses.
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#12 calbiterol

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Posted 4 July 2005 - 05:46 PM

Glad to help.
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#13 DQW

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Posted 4 July 2005 - 06:02 PM

A Halbach array is probably your best option.

It uses normal magnets, so it can be (more or less) as expensive or inexpensive as you want, according to your power needs, and it augments the field on one side, and pretty much cancels out the field on the other. The link is wikipedia's article on it, and the article also has some good links.

I may be completely off here, but it seems to me like you can only change the field along the z-direction (using a halbach array or some winding scheme with electromagnets). My z-axis points along the symmetry axis of the magnet, so I'm not sure how you get the field-variation that d22k wants along the x-direction.

I just might be missing something obvious, though.
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#14 d22k

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Posted 4 July 2005 - 06:44 PM

well, im planning on using cube rare earth magnets, so asuming i built an array, what would the field look like in this pic?

(you will have to excuse the lack of scale, accuracy, and drawing skills :D )

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#15 DQW

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Posted 4 July 2005 - 07:57 PM

If you showed us the directions of the magnetizations of each of your cubes, one might be able to make a rough guess of the field geometry.
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#16 d22k

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Posted 4 July 2005 - 08:12 PM

well i simply dont know. I know NOTHING about magnets, so i cant give u any useful information =\

I guess im just gonna have to build it an see if it works =)
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#17 calbiterol

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Posted 5 July 2005 - 12:32 AM

This page has a bunch of more technical information on Halbach arrays.

If you let us know your idea, we could tell you if it would work or not, and/or help you out with it. I understand not wanting to divulge your ideas, but it would be much easier for us to help you. Either way, good luck.
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#18 d22k

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Posted 5 July 2005 - 12:38 AM

i plan to tell all, eventually :D

im not sure if its gonna work u see... but if it does work its gonna be freakin cool!

so i just need to save up some cash for the magnets and try it out.

If it doesnt work, it least i will have some cool magnets to play with :D
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#19 calbiterol

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Posted 5 July 2005 - 01:14 AM

Incredibly fun. If it doesn't work, you should make a gauss rifle. Those are really fun.
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#20 d22k

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Posted 5 July 2005 - 01:20 AM

tell me about it!

i was sketching up some plans just yesterday for a gauss "rifle" using 4 electro magnets, i have everything i need except 4 suitable pieces of iron =)

I was reluctant t call it a rifle, because it isnt... but it just sounds cooler than "gauss gun" doesnt it :D
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