How Can You Weaken the Strength of a Standard Magnet?

[RestlessMind]

Is there any way to dampen the strength of a standard magnet? Some kind of material you could put on it to make it weaker?

Thanks!

 

[Gear300]

You can demagnetize a material through successive hysteresis loops. I'm not too knowledgeable about it but you can look up "demagnetization" and "magnetic hysteresis."

 

[RestlessMind]

Hmm... I see, but is there a way to have some kind of covering that would stop it from being as strong, that could be placed on one of the poles of a magnet, making, say, the north pole stronger than the south?

 

[Born2bwire]

You can't make one pole weaker than the other, a magnet is an inherent dipole.

 

[RestlessMind]

That's good to know, thanks!

 

[Born2bwire]

It's rather difficult to shield against magnetic fields. One solution is to apply another magnetic field that cancels out the original field. This is what they do with many so called magnetically shielded speakers. The other option is to surround the magnet with a high-mu material. That is a material with a high permeability. A very large block of iron could work but more specialized materials are more effective. The high-mu material does not really attenuate the fields, but rather concentrates most of the field into the material as opposed to out in space.

 

[RestlessMind]

Ah, so that's something. And you couldn't place this material on just one pole of a magnet? What would happen if you did?

I bet this stuff is expensive, though, huh?

 

[Born2bwire]

Not much would happen. The fields would be distorted inside and immediately around the high-mu material but for the most part you would not notice too much of difference in the behavior of the field. The material does not destroy the field, it only tries to bend and distort it such that most of the field travels through the material and then back to the other pole. Without the high-mu, the fields would travel far out into space before returning to the other pole. It's really just a technique to help minimize stray fields.

 

[RestlessMind]

Ah I see! Thanks for the info. ^_^

 

[ford2go]

Actually, if you shield one pole of a magnet, you might get what you want. Shielding is a misleading term. The shielding material ( which can be anything ferrous -- depending on the effieciency that you want) essentially provides a 'path of low resistance' for the magnetic field.

This being the case, if you shield one end of a magnet, it will pack the field tighter around that end. Thus, that pole will seem 'weaker', if you measure it outside of the shield area.

If you think of water flow, it might make more sense. If you added a bypass pipe to a section of a stream, the stream in that area would not have as much flow because some would be diverted by the pipe. The same concept applies to magnetic shields - some of the field is diverted into the shield material.

The only caveat is that shield materials do saturate. At some level of magnetic intensity, the shield material will be saturated. Any magnetic field in excess of the saturation level will not be affected by the shield. (the pipe is full of water)

Hope that helps.

 

[RestlessMind]

Now that is interesting. What kind of actual ferrous material could be "wrapped" around a magnet, though?

Also, another question: If I had two standard bar magnets, and I took the two south poles and held them together when they would normally fly apart, would this change the field at all?

 

[Born2bwire]

Iron would work, but you may need a large amount of it to be effective. It really depends on the strength of the magnet and how far away you want to suppress the fields.

Magnetic fields, like electric fields, follow linear superposition in classical physics. If you have two magnetic fields, the total field is just the summation of the two. Two bar magnets would just produce a magnetic field that is the summation of their individual fields for the most part. The presence of the actual magnet will affect the other magnet's field to some extent, mainly because most bar magnets are ferrous materials to begin with. This may be mostly localized to the area immediately surrounding and containing the magnets though. Again, it all depends.

It is like placing a dielectric scatterer inside a static electric field. The field distorts because of the dielectric's presence, but the distortion drops off as you observe the fields farther away from the dielectric. It would be the same with the two magnets, at some distance away the fields will be like the summation of the two fields, but close to the actual magnets there will probably be some distortion due to the presence of the other magnet's ferromagnetic body.

 

[RestlessMind]

Interesting. So is there any practical way to dampen one pole of a magnet at all? Because just tossing on a massive block of iron isn't exactly easy. :P

 

[Phrak]

How much weaker do you want it? Put a thin metal plate on one end. Say 2mm thick of soft iron. It will be weaker

 

[RestlessMind]

How about... 50% weaker. How much would that take?

 

[Bob S]

To reduce the field by about 50%, one solution would be to estimate the cross section of the magnet (e.g., 1" diameter is 0.78 square inches), and "short" one pole to the other by using perhaps 0.25 square inches of soft iron. Several layers of thin sheet metal is better than a single piece of thick steel..

 

[RestlessMind]

Hm, that shouldn't be so hard.

 

[Phrak]

To reduce the field by about 50%, one solution would be to estimate the cross section of the magnet (e.g., 1" diameter is 0.78 square inches), and "short" one pole to the other by using perhaps 0.25 square inches of soft iron. Several layers of thin sheet metal is better than a single piece of thick steel..

I thought he wanted to reduce the field strength at one end, over the other. (Has the question evolved?) Shorting wouldn't do this. Spreading the field over a larger area at one end with a plate making a 'T' does it.

 

[RestlessMind]

Er, well, I just want one side of a typical bar magnet to be weaker than the other side. So if the N side had a pull force of "1" if 2 cm (for example) from the S side of another magnet, then the S side of this same former magnet with the dampener applied would only have "0.5" pull force (or something) with the N side of said latter magnet if the same distance (2 cm) away.

I hope that made sense.

 

[Phrak]

Er, well, I just want one side of a typical bar magnet to be weaker than the other side. So if the N side had a pull force of "1" if 2 cm (for example) from the S side of another magnet, then the S side of this same former magnet with the dampener applied would only have "0.5" pull force (or something) with the N side of said latter magnet if the same distance (2 cm) away.

I hope that made sense.

Do you have a magnet with some dimensions you can give us?

 

[RestlessMind]

How about this one? http://www.kjmagnetics.com/proddetail.asp?prod=B448

 

[Phrak]

OK. You can buy Neodymium magnets that are shaped like wedges They are made for electric motors. At one end the field strength will be less than the other. Try other suppliers too. I didn't see any at K&J.

 

[RestlessMind]

Oh awesome! Does anyone have any idea where I might get some?

 

[RestlessMind]

Wait... are these the wedges you speak of? I found some at K&J.

http://www.kjmagnetics.com/proddetail.asp?prod=AY0X030-N

 

[Phrak]

Wait... are these the wedges you speak of? I found some at K&J.

http://www.kjmagnetics.com/proddetail.asp?prod=AY0X030-N

That's it, but 25 dollars a piece at grade N50. Come to think of it they're only an inch long, so, even though the field strength is 2:1 at the surface, the ratio should diminish greatly an inch away.

 

[RestlessMind]

That's it, but 25 dollars a piece at grade N50.

I prefer to have a strong one. Sure is expensive though. o_0 This place has them cheaper though: http://www.magnet4less.com/product_info.php?cPath=15&products_id=457 at grade N42, and I don't even need that strong.

Come to think of it they're only an inch long, so, even though the field strength is 2:1 at the surface, the ratio should diminish greatly an inch away.

That's perfect really.

 

[Phrak]

I prefer to have a strong one. Sure is expensive though. o_0 This place has them cheaper though: http://www.magnet4less.com/product_info.php?cPath=15&products_id=457 at grade N42, and I don't even need that strong.


That's perfect really.

OK, the respective areas are 2:1 for magnets in the URL, so 2:1 field stength. Good luck with the purpetual motion machine, or whatever it is.

 

[Cyrus]

Is there any way to dampen the strength of a standard magnet? Some kind of material you could put on it to make it weaker?

Thanks!

The word 'dampen' implies a rate dependency. You want to reduce the field strength of a magnet.

 

[RestlessMind]

OK, the respective areas are 2:1 for magnets in the URL, so 2:1 field stength. Good luck with the purpetual motion machine, or whatever it is.

Hey, I contacted K&J magnetics and they tell me that this magnet: http://www.kjmagnetics.com/proddetail.asp?prod=AY0X030-N has a field density on the outer radius of 6420 Gauss but 6850 on the inner radius. I hope I'm not getting "density" and "strength" mixed up here... or at least, I want to know if on this magnet the inner radius will not attract an object with as much force as the outer radius if they were both placed at the same distance from the same object...

Oh, it's not a perpetual motion machine, but thanks. ^^

The word 'dampen' implies a rate dependency. You want to reduce the field strength of a magnet.

You're right.

 

[Phrak]

Hey, I contacted K&J magnetics and they tell me that this magnet: http://www.kjmagnetics.com/proddetail.asp?prod=AY0X030-N has a field density on the outer radius of 6420 Gauss but 6850 on the inner radius. I hope I'm not getting "density" and "strength" mixed up here... or at least, I want to know if on this magnet the inner radius will not attract an object with as much force as the outer radius if they were both placed at the same distance from the same object...

Hmm. Very strange. Field density is the magnetic field strength. Surface field strength is very iffy. Off-surface measurements are more dependable. If their numbers are accurate, the magnets don't seem to be optimally magnetized for their intended use in motors. It's good you called.

I may have assumed too much. You could always drop back and stick some iron or soft steel on one end. If you could obtain use of a Gauss meter, you could experiment until you obtained the field stength you want.

If you have the opportunity to experiment you mught try this as a first attempt. For a pole face that's nominally square and one unit on a side, use a disk of soft iron or steal 1/2 unit thick and 3 units in diameter. My best guess.