What is Magnetism? – Part 1 (Electron Spin)

We'll start our exploration into magnetism by looking at bar magnets, which most people will be familiar with, because they have the ability to show us what magnetic fields look like.

So, to begin with, we are looking at permanent magnets that never loose their magnetism under normal conditions and it's all associated with the magnetic field that permanent magnets produce.

The Magnetic Field

All magnets, permanent magnets and electro-magnets, produce a magnetic field. Magnetic fields consist of lines of magnetic force that radiate out from one end of a magnet and loop back to the opposite end of the magnet. In short, magnets produce a magnet force field.

Bar magnets consist of a rectangular piece of magnetic material that has a north-pole and a south-pole. Magnetic lines of force, also referred to as magnetic flux, emanate out of the north-pole into the surrounding space and loop back to the south-pole. You never find a single line of magnetic force that doesn't have both a source (north-pole of a magnet) and a sink (south-pole of a magnet) and the two poles don't have to be on the same magnet as we'll discover.

It is possible to see the effects of these lines of magnetic force.

If you take a small bar magnet, lay it flat and cover it with a sheet of paper and sprinkle the paper with iron fillings (tiny pieces of iron), the iron fillings line up along the lines of magnetic force. This effect is illustrated in figure 1.

Figure 1

The outline of the bar magnet has been drawn on the image to show its position under the paper. The north-pole is shown on the left and the south-pole on the right. What you see are the iron fillings lining up along the loops of the magnetic field lines. Some can be clearly seen looping back round to the south pole, but some extend far beyond the paper but must loop back to the south-pole.

You should appreciate that this experiment only shows a 2D depiction of the magnetic field. In reality, it extends out from the magnet in all directions in 3D space and loops back to the opposite pole. It's simply convention that dictates the field lines emanate from the north-pole and return to the south pole; it could just as easily have been the other way round.

Figure 2 shows the same situation depicted in diagrammatic form. The magnetic flux is shown running through the magnet as well as out into the surrounding space.

Figure 2

The diagram shows just a few lines of force, but each indicates the direction of the force line, always heading out from the north-pole and returning back to the south-pole.

Magnets can apply a force to other magnets, which can be attractive or repulsive, depending on how the poles of the two magnets are orientated. How the magnetic fields behave can be observed using iron fillings and two bar magnets.

Magnetic Repulsion

For this experiment, two bar magnets are set up so that their north-poles are set against each other. As you can see in figure 3, the magnetic flux emanating from each magnet is pushing against the other and a force of repulsion becomes evident, strong enough to push the two bar magnets away from each other. To avoid this, the magnets are clamped in place and kept close to each other.

The iron fillings clearly show the repulsion of the two fields in the centre of the image. Once again, remember that this is only a 2D depiction of a 3D situation.

Figure 3

Repulsion only occurs where the two magnetic fields meet and oppose one another. If the clamps are released, the magnets will move away from each other until the repulsive force diminishes as the distance between the two north-poles increases and friction prevents any further movement from the greatly reduced fields.

So, what happens if one of the magnets has its orientation reversed and a north-pole faces a south-pole?

Magnetic Attraction

In figure 4 you see the two bar magnets re-orientated so that a north-pole faces a south-pole. The magnetic flux emanating from the north-pole uses the second magnet's south-pole as its sink. Magnets aren't bothered where they sink their magnetic flux, as long as it's closer than their own south-pole.

Figure 4

We now have a force of attraction between the two magnets and if they weren't being restrained under the paper, they would be attracted with considerable force and become one long bar magnet, where the two ends represent the poles as the centre becomes a conjoined section of the two magnets.

Both the force of repulsion and attraction are actually quite strong, making the magnets jump dramatically in new spatial configurations if not clamped down.

Flux Density

The flux density (lines of force per square cm, for example) is a vector quantity having both direction and magnitude at any point in 3D space and is measured in tesla or gauss, depending on which measurement system you are working in. The two systems are the SI system (International system of units, also known as the metric system) or the CGS system (Centimetre-gram-second system, a variant of the metric system). These two measurement systems are the main ones used in physics.

Flux density is designated as the vector quantity B. Magnetic fields are also measured by their magnetic field intensity, another vector quantity, designated H. The equation that relates the magnetic flux density to the intensity or strength of the field, is;

B represents the number of magnetic field lines per unit area (square metre or centimetre, for example) at a point in the magnetic field.  H is the intensity of the magnetic field at a point in space and mu, the Greek letter, represents the ability of the medium, in which the field exists, to support a magnetic field.

Both H and B are measures of how strong a magnetic field is, related by mu. If you recall, the speed of propagation of electromagnetic waves in a vacuum is defined as a constant from Maxwell's equations as;

where c is the velocity of propagation of the wave, and the two items on the bottom of the equation represent the ability of the vacuum of space to support an electric field (the epsilon character on the left) and a magnetic field (the mu character on the right). The only difference between mu with a subscript of 0 and without, is that with a subscript of 0, it relates to the vacuum of space.

The mu character qualifying H in the first equation is exactly the same permeability, but relating to the medium in which the magnetic field is found such as air or water. Also, it's worth pointing out that B and H are parameters used in Maxwell's equations.

In terms of our bar magnets, there will be one value of B inside the material of the magnet and another value of B immediately outside the magnet. The different values of B are governed by the different values of mu, one value for inside the metal and another for outside in the air.

Magnetic fields exist in the emptiness of space and also in a media such as a gas, liquid or a solid, where the strength of the magnetic field is governed by the permeability of the medium and varies accordingly.

What is the typical strength of a bar magnet?

If you type into Google, 'what is the strength of a bar magnet', you'll get lots of different answers, because there are lots of bar magnets of different shapes, sizes and compositions. In general, flux density of a bar magnet is strongest when measured at the magnet's north or south pole and gives the same value in each case.

The flux density then drops off quite rapidly as the inverse cube of distance away from the magnet. If the flux density drops off, then so too will the intensity, or strength, of the magnetic field. Both gravity and the force between charges fall off as the inverse square law. The inverse square law implies that if you double the distance between two charges, or two masses, the Coulomb or gravitational force, is diminished by 1/4.  If you double the distance from a bar magnet, the intensity of the field is diminished by 1/8, i.e., it falls off much faster than that of the inverse square law.

Generally speaking, typical bar magnets range in strength from about 0.01 Tesla to about 0.4 Tesla, depending on size and composition. This is actually much stronger than the earth's magnetic field, which is about 0.00005 Tesla at most points on the earth's surface, vastly weaker than a bar magnet. We can investigate this difference using a magnetic compass.

How does a Compass behave near a bar magnet?

A compass needle will respond to a bar magnet's field strength when close by. At a distance from the bar magnet, the influence of the bar magnet's magnetic field disappears completely and the earth's magnetic field forces the compass needle to orientate south to north and yet the earth's magnetic field is so weak on the planet's surface, about 0.00005 Tesla.

The bar magnet has a strong field close to its poles, which drops off by 1/8 every time the distance doubles. So the bar magnet's field is very localised in a small volume of space. The earth's magnetic field, on the other hand, spreads out thousands of miles into space and occupies a vast volume. So, it might be very weak at the earth's surface, but it extends across the entire planet, fluctuating in value but ever present.

Consequently, the earth's magnetic field is contained in a massive volume of space and influences a compass needle anywhere on the earth and above it.

How do you create a bar magnet?

First, you need a material that is susceptible to strong magnets. The best metal for this is iron and it's all to do with iron's outermost electrons.

An electron can have an UP-spin or a DOWN-spin, creating a tiny magnetic field, known as a magnetic dipole, as demonstrated by the famous Stern-Gerlach experiment. Spin for an electron is a quantum property, there is no actual spinning involved. You can read more about this in the article 'Do electrons spin' published on this website's blog.

The tiny magnetic field produced by electrons that are paired in an orbital lobe cancel out. So, it's only unpaired electrons in outermost orbitals that create an atom's composite magnetic field. Iron has four unpaired electrons in its outermost orbitals and represents the element with the most magnetic potential in the entire periodic table. There are other elements and composite materials that are magnetic, but iron based alloys are the kings of permanent magnets.

To make a permanent magnet, first apply a strong magnetic field along the axis of an unmagnetised bar of magnetic material that has not yet been magnetised and increase the strength of the field. There comes a time when all the unpaired electrons line up in the direction of the external field. This is known as saturation.

Next, reduce the external field back to 0 and the flux density inside the specimen remains at what's called the retention level, indicating that the metal has been permanently magnetised and has become a permanent magnet.

Why isn't iron magnetic naturally?

Iron extracted from underground is never magnetic. Iron ores formed when the earth was very young and very hot. Consequently, as the ores cooled, they took their time and what are called magnetic domains settled into configurations that produce a minimum degree of magnetic energy, resulting in no overall magnetic field.

A domain is an area of a material where all the atoms' magnetic fields are locally aligned in the same direction. Iron is made up of many of these domains and the overall magnetic field is cancelled out by the random orientation of the full collection of domains, achieved through the slow solidification of the ore in the early history of the planet.

`What happens to the domains when a permanent magnet is created?

When the collection of randomly orientated domains is subjected to a strong magnetic field, then domain boundaries are broken down and all the atoms' magnetic dipoles are forced into the same alignment. In figure 5 you can see depictions of randomly orientated domains and the result of subjecting them to a strong magnetic field.

Note that a single unpaired electron has a tiny magnetic dipole and iron has four of these. The external field forces them all into the same orientation and this is reflected in each atom's combined magnetic dipole.

The elements nickel and cobalt as well as gadolinium and dysprosium also form permanent magnets. Gadolinium and dysprosium are members of the lanthanides and are classed as rare-earth metals. Because pure iron tends to lose its magnetism easily, engineers use alloys made up from iron and these other elements to produce tough magnets that maintain their magnetism. Rare earth metals are also used to create tough alloys that retain magnetism, even in harsh environmental conditions.

Figure 5

What about Lodestones?

The ancients used thin slivers of naturally occurring magnetic iron to act as compass needles, either suspended from thread or floating on water. These were made from flakes of lodestones.

Lodestones are naturally occurring magnetic iron ore, but they are only found on the surface, never underground.  It's believed that lightning strikes the stone and magnetises it through the high current that the strike generates in the stone. This aligns all the domains in the same direction and they remain magnetised and helped ancients to navigate on the high seas.

What makes a good Permanent Magnet?

Pure iron is quite soft and looses its magnetism relatively easily, through mechanical shocks or high temperatures. Consequently, engineers use a mixture of metals to produce a strong alloy that can be magnetised and maintains its magnetism.

All permanent magnets are made up from one of the following combinations of elements.

1. Neodymium (NdFeB):

The strongest permanent magnets in the world are made from an alloy of Neodymium (another rare-earth lanthanide), Iron, and Boron, making a strong magnet that will maintain its magnetic properties. Magnets made from Neodymium are found in electric motors and electric generators as well as computers, sensors and magnetic locks.

2. Samarium-Cobalt (SmCo):

This results in an extremely powerful rare-earth magnet alloyed from Samarium and Cobalt, first produced in the early 1970s. Magnets made from these elements hold their magnetism at higher temperatures than Neodymium and can even withstand strong magnetic fields, making them ideal for space and military applications. Samarium-Cobalt magnets are second only to the Neodymium magnets in terms of strength, but are superior when it comes to maintaining their magnetism.

3. Alnico:

First developed in the 1930s, this is an alloy made of Aluminium, Nickel, and Cobalt together with traces of iron and even titanium. For many years permanent magnets were made up from Alnico combinations before rare-earth magnets were developed. Magnets made from Alnico are not as strong as neodymium or samarium-cobalt magnets, but although declining in use, are still found. The material is brittle and suffers in high impact situations, but can still be found in bar magnets, guitar pickups and speakers.

4. Ferrite/Ceramic Compounds:

Also developed in the 1930s, these magnets are made from a mixture of Iron Oxide mixed with either Barium or Strontium carbonates producing permanent magnets. These magnets have excellent resistance to being de-magnetised, but are very brittle. However, they can be produced very cheaply in large volumes and one of their main uses is in fridge magnets. They perform well in hot conditions, but not in the cold.

So, what is magnetism?

This can be answered for permanent magnets in that permanent magnets produce a magnetic field configuration that has a north-pole as a source and all magnetic lines of force from the source, bend back to a south-pole sink.

The heart of a permanent magnet is the magnetic field produced by the tiny dipoles of unpaired electrons in the material's outer-atomic orbitals that create each atom's dipole. These magnetic dipoles exist in domains, which for naturally occurring iron, are arranged randomly so that there is no overall alignment of atomic dipoles and no magnetism in the material, which exerts no force on other magnets.

However, existing magnets will exert a magnetic force on magnetic materials by forcing alignment of the magnetic material which can create a temporary magnet and the face adjacent to the influencing magnet results in an opposite pole. In this way, the permanent magnet exerts an attractive force on the non-magnetised material.

If you hold a bar magnet over a large number of paper clips, for example, they will be attracted to the magnet and each other as their magnetic dipoles line up along the influencing magnetic field. 

The strength of a permanent magnet depends on the materials from which it is made, iron being the best material, but to give physical strength to the source of the magnet, other metals must be used to create strong alloys.

Do all observers see the same magnetic field from a permanent magnet?

All observers, regardless of their relative velocity to a permanent magnet will detect a magnetic field. However, the magnetic-field strength that an observer measures is relative to their motion such that the value of E² - c²B² is a Lorentz invariant. Because this value is fixed, an observer can never change their speed in a way that makes a permanent magnet's field completely disappear.

How can the North pointer of a compass point North?

The needle in a compass has a north-pole and a south-pole and the north-pole of the  needle points to the geographic north-pole of the earth. The implication is that the earth's geographic north-pole must be the magnetic south-pole of the huge magnet produced deep in the earth.

Key Takeaways

1) Magnetism produced by permanent magnets is an inherent quantum phenomenon, created by the combination of magnetic dipoles of all the atoms pointing in the same direction.

2) Permanent magnetism will be detected by all observers, regardless of their relative motion to the permanent magnet and is classified as a pure quantum effect associated with the inherent quantum spin of unpaired electrons for which there is no explanation in classical physics.

3) The magnetic field produced by permanent magnets falls off swiftly as you move further from the magnet, until the earth's weak magnetic field on the surface of the earth becomes the dominant field. This is associated with the volume of space taken up by the magnetic field.

© 2026 Kieron Conway - All rights reserved.

--

In 'What is magnetism - part 2' we'll look at magnetic fields created by moving charges and how magnetic fields can be used to develop motors and generators by exerting forces on moving charges.

----

Liked this article? Check out:

modern-physics-journey.com 

where you can read all about an exciting new science series: A Journey into Modern Physics, available in three parts, from Amazon on-line shops. (Some of the above article may appear in this series).

Also, the web-site has an index of blog articles published to date, for easy access to an article that might interest you. You can access the index using the link above.

--------