Magnet size vs strength

[Njguy]

If two magnets have the same Gauss on identical faces in surface area, but one of the magnets is thicker, is the thicker one stronger? Will the Gauss of the thicker magnet have more room to increase as a ferrous material is brought close to it?

 

[MrAl]

Hello there,

If one magnet is thicker than the other but one has a higher Gauss reading at the same point in front of the magnet face then that would have to mean that the materials they are made of are different or somehow one is more magnetized than the other assuming both materials are homogeneous. But in any case, the reading obtained at a given point in space is due to every microscopic contribution from within the magnet, so to get that reading the entire bulk of the magnet has to have already been taken into account which means nothing is going to make it stronger other than doing something to the material like subjecting it to a stronger magnetic field to magnetize it or adding more material to the bulk with the same level of magnetization.

Some materials have higher residual magnetism than others so they make stronger magnets. Of course a stronger magnet can be smaller than a weaker magnet and have the same pull strength.

 

[Njguy]

Thanks. Just to clarify because i may not have stated it correctly, gauss is the measure of the strength of a magnet. So if you have two magnets if equal gauss but one is thicker, they are still the same in strength?

 

[MrAl]

Hi,


Well for a given point in space if they have the same field there then they have the same 'strength' there. But when we have different shapes it does get a little tricky because we have to take every point inside the magnet into account, and that means that as an object is approaching the front face of the magnet a different thickness magnet will have a slightly different effect on all the points of the object. So we have to at least specify a distance for the object. The strength is measured as the pull force, and as an identical object nears the front face of both magnets we'll see different forces, and that's because a larger part of the thicker magnet is 'behind' the front face while most of the thinner magnet mass is very close to the face.

So an approximate model would be to model the thin magnet as a single thin surface with some given strength as measured at a known distance, and model the thick magnet as two thin surfaces one slightly behind the other. In order for both composite 'magnets' to have the same strength at a given point out in front of them, the two surface magnet would have to have a material that is not as magnetized as the single surface magnet. That's because the two surfaces 'strengths' add up at the point of interest differently.

For example, lets say that at a distance of 1 inch we have equal force at a point on the object with both the single surface magnet and the dual surface magnet. If we then move the object closer to the single magnet we see a change in force of about 1.235, while we see a change in force of the two surface magnet of about 1.223 (same small distance moved about 0.1 inch). Not much difference, but now the single surface magnet looks stronger when before they exerted the same force on the object. We again assumed that the magnetization of the dual magnet is less than the single magnet as that is how we got them both to exert a pull force that was equal at a distance of 1 inch.

But if in a given application if both magnets exert the same force, then they have the same strength.

Did you have some specific reason you needed this information?

 

[dr pepper]

They use lifting magnets on a jib here at work, they can lift 4 tonnes at a minimum thickness, a graph on the side shows lifting capacity versus thickness, capacity is derated right down at the thinner end.

 

[MrAl]

Hi there dr pepper,


You brought up an interesting point. If we have two magnets made out of the same material and the material is magnetized the same, yet one magnet is thicker than the other, the thicker magnet will be stronger. That's of course because there is more material there to act on an object out in front of the magnet.
But there's a limit to the effectiveness of this because the particles at the back of the magnet are farther away from the object than the particles at the front of the magnet, so if the magnet is too thick the effect from the particles on the back will be much less than those on the front, and that of course means that the strength does not increase as much. So a magnet that is 2 inch thick compared to a magnet that is 1 inch thick, the 2 inch magnet will be stronger. But comparing a 10 inch thick magnet to an 11 inch thick magnet (same difference in thickness) the 11 inch magnet is only slightly stronger than the 10 inch magnet along the same direction (object out in front).

 

[dr pepper]

I had to concentrate reading that, but yes I agree, the effectiveness of the forces does not drop off linearly, increasing thickness is effective up to a point, then increasing by magnitudes makes little difference.