# Magnetic attraction force Question

• B
• 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.

Last edited:
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.

The steel "magnetic coupling" piece will have an induced magnetization - with a "north pole" on the right, and a "south pole" on the left. This will increase the strength of the magnetic field applied to the large steel block at the bottom, creating effectively a single large "north pole" on the right (at the bottom of the magnet) and "south pole" on the left. Assuming the magnetization of the steel block is not saturated, it will be more strongly magnetized than without the "coupling piece" on top. Hence the attractive force will be (somewhat) stronger.

## What is magnetic attraction force?

Magnetic attraction force is the force exerted by a magnet when it attracts certain materials, such as iron, nickel, and cobalt. This force is a result of the magnetic field created by the magnet, which aligns the magnetic domains in the attracted material, causing them to pull towards the magnet.

## How is the strength of magnetic attraction force measured?

The strength of magnetic attraction force is typically measured in units called teslas (T) or gauss (G), where 1 tesla equals 10,000 gauss. The force can also be quantified by measuring the amount of force (in newtons) required to separate two magnetic objects.

## What factors affect the strength of magnetic attraction force?

The strength of magnetic attraction force is affected by several factors, including the distance between the magnets, the material and size of the magnets, the alignment of magnetic domains in the attracted material, and the presence of any interfering magnetic fields or materials.

## Can magnetic attraction force pass through materials?

Yes, magnetic attraction force can pass through many non-magnetic materials such as wood, plastic, and glass. However, the force can be weakened or blocked by materials that are magnetic or have high magnetic permeability, such as iron or steel.

## What are some practical applications of magnetic attraction force?

Magnetic attraction force has numerous practical applications, including in electric motors, generators, magnetic storage devices, medical imaging machines (like MRI scanners), magnetic separators in recycling processes, and everyday items like refrigerator magnets and magnetic clasps.

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