Questions about magnets.

Hey all, I’m working on a little project and I was wondering if anyone in the CD community had any answers to the following questions.

Does every magnet have a north and south pole? I would assume so.

If you break a magnet, do those individual pieces now have a N&S pole each?

What is the strongest pole?

What is the most effective way to shield from magnetic fields?

Where can I buy magnets besides at the hardware store?
The stronger the better (but still relatively cheap, and also lightweight).

I know some of these.

Yea, all magnets are N & S.

Yes, breaking magnet means N & S pole

Every magnet has a north and south pole no matter how large or small. (think really small on the surface of a magnetic floppy disk) When you break a magnet you get two magnets and they each have two poles. The poles are generally balanced so neither is stronger. Magnetic fields suscribe to the inverse square law so you can modify a little by putting non magnetic materials between the pole and magnet. Round magnets get a little complicated and some motor magnets have multiple NS pairs but are really individual pole pairs in the same material. Ferrous metals are good for magnetic shields especially those that have high nickel and carbon concentrations. They are more bending the field away than actually shielding. Check out science stores like “the American Science Center” or industrial supply houses like McMaster-Carr. The cheapest are those you salvage out of dead devices, like DC motors, VCRs, disk drives, etc.

Why wouldn’t the two poles be equal in strength?

Neither is stronger. The strength of a magnet is based on the material, the size and somewhat the age. If you took a bar magnet and a Guassmeter (measures magnetic field intensity. 10,000 Gauss=1 Tesla=very strong magnet. 500 is fairly strong), you will see that the N end will have approx the same value as the S end (but one will be negative). Around the middle, you will have nearly 0. The sensor I was using for a science project regularly read about -1.7 Gauss (I guess Earth’s magnetic field) and went up to about -+ 100 gauss with the really weak, old magnets the school had.

If you can find an old microwave, then you could find some very strong ring magnets in the “MAGNETRON” (that is really what they are called). I measured one and was getting about -+800 gauss

You can get really strong magnets out of hard drives. I would recommend using old broken ones as working hard drives are expensive.

Try here for strong magnets: http://search-desc.ebay.com/Neodymium_Industrial-Supply-MRO_W0QQcatrefZC12QQfmcZ1QQfromZR10QQfsooZ1QQfsopZ1QQftrtZ1QQftrvZ1QQftsZ2QQsacatZ1266

You can make an electromagnet with some sort of magnetically attractable metal core and a coil of wire plus an electric current to boot. They’re usually weaker though. For some really strong magnets, look in any bad trashed audio speaker (not internal to a TV set).

Oh yeah, they do have N&S poles as others have mentioned, when split in two they indeed make two separate magnets. There is no such thing as the strongest pole and the best shielding is space or some seriously magnetic resistant material. I could not name one for you though.

-Joe

I second that. Neodymium magnets are as strong as they get, and eBay is always a fantastic place for good deals. :wink:

rubs hands together Boy are you in for it…

http://www.amazingmagnets.com
http://www.engconcepts.net/
http://www.wondermagnet.com
The last one also has ferro-fluid.

Also, in response to to one pole of a magnet being “stronger” than another… If a magnet is symmetrical on a plane perpendicular to the poles, then they’re the same strength. You can increase the surface gauss at one end by tapering it to a point, however. It doesn’t matter if this end is a N or S pole, however.

For lots of info on magnets and stuff, see:
http://www.dansdata.com/magnets.htm
http://www.dansdata.com/magnets2.htm

That stuff is really cool. I saw on TV something sort of like that which they were developing to use as fluid in off-road racing shocks to essentially change the viscosity in a matter of miliseconds.

High permeability material, sold under the trade name “mu metal”
is used to shield magnetic fields.

Gene,
Do you have a source for mumetal, we are almost out of the sheet stock and I need to replenish but have only been half hearted about looking.

Ferro fluid is not a magnet it is ferrous metal particles suspended in a viscous liquid. It is most commonly used for speaker voice coil cooling. A few drops squirted into the gap between the voice coil and the magnet structure will do a great job of transferring heat to the magnet body and basket assy without changing the damping of the speaker.

That stuff is really cool. I saw on TV something sort of like that which they were developing to use as fluid in off-road racing shocks to essentially change the viscosity in a matter of miliseconds.

Lord Corporation (http://www.lord.com/Default.aspx?tabid=718) has been developing magnetorheological fluid applications for several years. The site has lots of interesting information and applications.

There must be a way to make a continuously variable transmission for a FIRST robot using this stuff, but I haven’t come up with one yet.

NeoWorld Magnets is a good site for lots of different kinds of really strong magnets and magnetic jewelry.

Use some kinda of ferrous casing around what ever you want to protect. This way the magnetic field will go around the outside of the case. This is how hard drives are protected. If you don’t know what I am talking about, go look at he magnetics field of a ring magnet. Or go put a magnet up to a steel washer and sprinkle iron filings on it to visualize the magnetic field.

we are studying this in physics right now, so someone please correct me if I am wrong. This is just what I have been taught, No experience with this.

Well, Lord is located near us, and at a career fair I got to play around a bit with it. I cannot imagine a way that you could make a CVT with it, but a magnetic clutch is very feasible. FIRST or otherwise, it’d make for some nice shifting tranmissions!

As already said, every magnet has a north and south pole. One of the major searches underway in nuclear physics is for “the magnetic monopole”: If one can be found, it will tie up some loose ends in the grand unification theory, helping equate gravity to magnetism (and therefore electricity). Can you say anti-gravity??

Strongest means magnetic flux density. The absolute strength is the same at both ends, kind of like the current in a simple DC circuit, but you can modify the flux density (measured in Gauss) by physically changing the shape of the magnet, or channeling the magnetic flux to a point. (For example, make an electromagnet from a nail. The pointy end will appear ‘stronger’ than the flat end)

Mu metal is indeed a great magnetic shield. For cheap, find a TV and take the metal shielding near the picture tube*. The better a material ‘conducts’ magnetism, the better of a magnetic shield it is - just like electrical shielding. “Shielding” just gives the magnetic (or electrical, or whatever) field an easier place to travel, and so you direct it around whatever object you are shielding. Steel is not too bad a shield, for the cost.

  • Warning: The picture tube has high voltage, and acts like a capacitor to store it for quite a long time. Stay the heck away from the high voltage wire to the picture tube (which looks kind of like a spark plug wire) and any connectors to same.

Buying magnets: American Science & Surplus, plus those already mentioned.

Don

I have to throw in my 2 cents here. It is almost impossible to remove the degaussing shield from a picture tube without removing the anode lead. The picture tube can hold a charge for months after it has been unplugged. Depending on the design that voltage can be up to 40kV. In addition, the tube has an extreme vacuum and if handled improperly, can launch tube internals at lethal speeds should the tube break.

**Al’s comment on the High Voltage needs to be taken seriously, **and I perhaps underestimated it in my post above.

While I do not disagree with Al’s warning on tube vacuum, I have some comments:

It’s not that the tube is under such a high vacuum, but that there is such a huge volume of it. A regular light bulb has the same grade of vacuum, but in just a tiny volume.

On the other hand, in a former job, we used to break picture tubes all the time for testing, and we never had a problem like that. Yes, raw phosphor material blasted into the air (Warning: Poison!) , glass shards at surprisingly high velocities, loud noise… but no internals.

Don