Magnetic attraction force Question

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• eightplusonefingers

eightplusonefingers

Other than the most simple school stuff of attraction, repulsion and "lines of force", I've never studied magnets.

Hence this question.

On the left we have a disc magnet diameter D attracted to a much larger lump of steel with force F (when there is no gap between).

If I add a second similar magnet, a website selling magnets I found tells me the force increases to about 70% of the sum of the two - shown on the right.

If I arrange the magnets as shown with the spacing a little more that the magnet diameter and a bridging steel pole piece straddling the two as shown. Note the right hand magnet has been flipped N-S

What is the force between each magnet and the lump of steel at the bottom?

Does that force vary much with the thickness of the pole piece T?

Does the pole piece even do anything useful?

Without the pole piece I assume the attraction of the two magnets in isolation would be F each but
Does the pole piece help increase this? if so how much by?

In my mind I have the idea that there is now a "magnetic circuit" via the pole piece and the steel lump at the bottom but is a magnetic circuit even a real thing.

Not looking for pages of maths just a real world idea of what it means

TIA

Unfortunately, predicting the force is difficult, requires a lot of information that we don't have, and in many cases is approximate at best. I would go with what the vendor is saying, but be prepared to have to make measurements.

vanhees71
eightplusonefingers said:
If I add a second similar magnet, a website selling magnets I found tells me the force increases to about 70% of the sum of the two - shown on the right.
That's nonsense. Making the magnet taller does not increase its force on a steel piece attracted to one side.

eightplusonefingers said:
Not looking for pages of maths just a real world idea of what it means
Generally speaking, in a magnetic circuit, the attractive force is roughly proportional to the surface area and the square of the magnetic flux density, and inversely proportional to the gap length.
Maybe you can try to use these principles to make a rough assessment.

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hutchphd, berkeman and vanhees71
berkeman said:
Making the magnet taller does not increase its force on a steel piece attracted to one side.
I disagree ...with caveats. So long as the nearer permanent magnet is not saturated then the addition of more magnets will increase the field because there is still some susceptability remaining in the lower magnet. The field strength at the lower pole will increase and the attraction is quadratic in that field strength at the pole. Its just like making a solenoidal electromagnet longer. Real magnets (particularly ones with some age on the) are seldom saturated.
Of course closing the magnetic circuit is a much better technique (or close the circuit and add magnets! )

hutchphd said:
So long as the nearer permanent magnet is not saturated then the addition of more magnets will increase the field because there is still some susceptability remaining in the lower magnet.
Interesting point; I hadn't considered that angle. I was mainly bothered by the longer magnetic path...

I once (20 yrs ago!) got a contract with some folks in Bedfordshire to design a magnet for a handheld blood coagulation meter. Having convinced them that I knew what I was doing, I had a big bunch of stuff to learn quickly. I downloaded a wonderful shareware product called Vizimag and started playing with it. I learned more practical E and M (particularly M) in a week than I thought possible.

I was hoping to be able to link to it here but it seems to be no longer extant. In any event if you can find it or something similar (it was basically 2D) , the whole concept of magnets, saturation, and magnetic circuits will get much simpler. If anyone knows of its present status please pass it along.
I did manage to design a good magnet but the parent device did not really succeed. Ah, well, I wanted them to redesign the optics, too...but got Nix'ed on that.