Interesting experiment about the attraction between two magnets

  • #1
We know that the attractive force between the opposing poles of identical permanent magnets is a product of their field strength and that the total force diminishes as the distance between them increases. If we assumed the magnets to be point entities, the formulation is the same as that for electrostatic attraction.
I thought we would find at least similar mathematics when magnets of unequal and or different shapes are placed in opposite polarity alignment.
However, when an electromagnet consisting of a coil with an air core is used as one actor and a permanent magnet as the opposing actor , surprising results occur. The force of attraction is limited by the strength of the air core electromagnet and is weaker by an order of magnitude that when a second permanent magnet of the same strength as the electromagnet is substituted for the elctromagnet
The air core field strength acts as a limit on the total force.

It is as if the nature of a magnetic field generated by a coil of charged wire is different that a magnetic field generated by the spin moments of the electrons in the permanent magnets material.

This seems illogical to me, but others have suggested the properties of electromagnet field are vastly different than that of permanent magnets.

Looking for comments and ideally a reference source to try to illuminate

Answers and Replies

  • #2
The iron core makes the magnetic field of the electromagnet/solenoid typically about 100 times stronger than with simply an air core. The reason for this is there are magnetic surface currents from the magnetization that occurs in the iron that run in the same direction (around the outer surface of the iron cylinder) as the currents of the solenoid that are typically 100 times stronger (current per unit length) than the currents of the solenoid alone. The magnetic field of the solenoid currents and/or of the magnetic surface currents can be computed by Biot-Savart's law. ## \\ ## One "link" that might be somewhat helpful (it's somewhat mathematical) is
  • #3
Thanks Charles

Nice to hear from you again

I understand that a core makes a coil much stronger

I don't think i explained my main question very well

Im saying in our experiments the hold force of the air core electronagnet against a steel plate was calculated and then a small permanent magnet was found of equal hold force on the same steel plate

Then first the aircore em was faced in opposition to a large permanent magnet and combined pull force was computed at various distances

Then the small permanent magnet was faced off against the much larger pm and combined pull force computed at the same distances,as the air core electromagnet

Results were radically different

I can't understand why they would be and seek validation..
  • #4
Without much detailed analysis involving the geometries and the relative field strengths, it is difficult to make a good assessment. The experiment with the unmagnetized steel plate that becomes magnetized by the applied field is much different than the case of another magnet used as the other part of the pair to create the force. In addition, a small magnet that has the same holding force probably is much smaller geometrically, but has a much stronger magnetic field at its surface than the air core solenoid. With the vastly different geometries, it is difficult to make good quantitative assessments, but it comes as little surprise that the results were very different. ## \\ ## editing...To give you one example of why the geometry can be such a factor in this problem, when placed in a uniform magnetic field, even a very strong one, a small permanent magnet will get rotated so that its magnetization lines up with the magnetic field, but once aligned, it will not experience any force. For a quantitative assessment of the force that is experience, it is necessary to know the geometry and even the magnetization of the magnets, as well as the spatial pattern of the magnetic field. The magnetic forces can depend upon gradients in the magnetic field as well as the magnetic field strengths.
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  • #5
I'm in a bit of a rush but here are some off-the-top-of-my-head thoughts..

When the steel plate is brought near the air cored electromagnet it's a bit like adding a core which makes it stronger.

However the permanant magnet might already be saturated? So when you bring it near the air cored electromagnet the strength isn't increased.

The steel may also be "soft" (high permeability) and the premanant magnet will be "hard".
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  • #6
Thank you CWatters and again to you Charles.
A few clarifying comments and questions:

Do we agree that two identical permanent magnets will attract each other with a pull force that is some product of the strength of the two opposing fields diminished by the square of the distance , and that such pull force is larger than that exerted by either magnet on a steel plate at the same distance from the magnet?

Charles suggests that the results with the air core electromagnet are more due to the shape of the field opposing the permanent magnet than any theoretical limit imposed by some qualitative difference between the magnetic field created by an electromagnet than that of a permanent magnet

Others elsewhere suggest that UNLIKE point charges in electrostatics the attractive pull force between magnets depends upon having ferromagnetic material in the equation and that a magnetic field generated by an air core coil is "disembodied" from ferromagnetic material and therefore contributes nothing to the combined pull force.

I find that unappealing , and I am thinking about redoing my tests with a "shaped" air core field that might provide insight into the mechanism

Looking for a source to understand first the shapes of magnetic fields of different sources and shapes, and then information about how to shape an air core electromagnet to "look like" that of a permanent magnet

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  • #7
The problems are complicated enough by the geometry that generally you don't find too many problems of the forces between two magnets in the textbooks that are used in the standard physics curriculum. Sometimes you can make approximations that will get you close to the exact answer with highly simplified formulas, but in general, most of the problems of this type can be somewhat mathematically complex.
  • #10
One thing worth mentioning that I mentioned previously is the force may depend upon the gradient in the field from the electromagnet. If you have a small permanent magnet, it essentially has both "+" and "-" poles and the "-" pole may feel almost as much attraction as the " +" repulsion. If you have a long cylindrical permanent magnet, you can treat it as a simple "+" pole at one end to a good approximation. A similar thing holds for you electromagnet=if it is long enough, it may be possible to approximate the magnetic field from it as a single pole at the nearest endface. ## \\ ## Just one suggestion that you probably already considered, and that is to make sure anything involved in your measurement apparatus to measure the forces is of non-magnetic material if it gets in close proximity of any of the magnetic fields, or it could alter the field that you are trying to keep constant.
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  • #11
In case I wasn't clear above I was suggesting that steel and a permanent magnet behave differently when brought near and electromagnet and that may effect the force between them.
  • #12
Thank you both

Very interesting ideas.

Charles , I was wondering about the very thing you mentioned above. In our test apparatus , we are using a very wide , but quite thin permanent magnet..3 inches wide , and 1 inch deep , in other words the axis of magnetization would be a line connecting the broad faces of the disc. So the "north" and south" poles are 1 inch apart.

The idea behind this had something to do with presenting the maximum possible square centimeters of surface to the opposing electromagnet, which itself had a 3 inch diameter.

Thinking bout this now , I realize that the south side of the permanent magnet is so close to the north side that the much weaker electromagnet is "an uninteresting target" for the lines of force emanating from the opposing pole of the permanent magnet. I kn ow this nomenclatture is imprecise , but I can't find the right words for the predisposition of a charge to pair with one opposite charge versus another of the same sign but different magnitude. Maybe the word gradient is better, and I suppose what I am saying is the positive gradient of the electromagnet , even though it is very very close to the negative surface of the permanent magnet is still less a net positive force than the reverse ( positive side) of the permanent magnet.

The specs for this v ery strong permanent magnet indicate a pull force of roughly 50 lbs at a 1 inch air gap, and by definition my electromagnet doesn't exert that much even at surface to surface ( zero) air gap.

I can see several ways to retest the data : 1) by using a long slender permanent magnet coupled with a possibly conical shaped electromagnet and 2) a stronger electromagnet that can compete more effectively for " connection" to the permanent magnet.

Will discuss with team , re - rig , retest and advise results.
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